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How does chromosome cross-over occur?

How does chromosome cross-over occur?


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I have heard that during meiosis, homologous chromosomes from each parent "cross-over", which enables the off-spring to inherit some alleles from the mother and some alleles from the father. The picture below illustrates this "cross-over", but of course this must occur at multiple sites, rather than just the one shown in the picture.

Now my question is what causes the chromosomes to align perfectly during cross-over so that the loci of a particular gene will substitute for the corresponding loci on the homologous chromosome, as opposed to being substituted with a completely random locus? Does each gene have a unique non coding sequence before it specifying what gene it is to enable this process to occur?


Quite simply, because chromosome pairing is sequence specific. Holliday Junctions, which are the functional structures of a cross-over, occur through a process called "strand invasion," during which a region of one chromosome physically base-pairs with that of another. Thus one locus cannot pair with a random locus, as there is generally insufficient sequence complimentarity between two random regions to form a functional Holliday Junction. One interesting consequence of this mechanism is gene duplication and deletion in repetitious regions of the chromosome. For example, genes with large repeated regions, such as the gene responsible for Huntington's disease, can expand and contract during homologous recombination due to strand invasion occurring at non-equivalent, but still homologous, sites. Wikipedia does a nice job going over homologous recombination. I also recommend looking over the relevant sections in Molecular Biology of the Cell, available on the PubMed Bookshelf.


How does the crossing-over of chromosomes that occurs during recombination result in increased genetic variability in population? THIS IS A TEST ANSWER

when homologous chromosomes cross over during meiosis, they exchange segments of DNA/swap alleles. this means that the chromosomes now have a completely unique combination of alleles, which causes variation. It is also random which of these unique chromosomes end up in which daughter cell (the gametes) and since fertilisation is a random and independent event, which unique male gamete that fertilises which unique female gamete is also random. This means the resultant zygote has a unique combination of alleles that is unlike either of their parents.

d. reduction in the number of chromosomes per cell

meiosis creates four genetically unique cells with half the number of chromosomes compared to the parent cell.

Decrease in the total number of cells per organism

im stuck on this one too but my best guess is group of genetically identical cells

The correct answer is option B, that is, reduction in the number of chromosomes per cell.

Meiosis refers to a unique kind of cell differentiation, which minimizes the number of chromosomes by half, forming four haploid cells, each genetically different from the parent cell from which they originated.

This procedure takes place in all the sexually reproducing single-celled and multicellular eukaryotes, including fungi, animals, and plants. In meiosis, the replication of DNA is succeeded by two rounds of cell differentiation to generate four daughter cells, each with half the number of chromosomes as the main parent cell.


Crossing Over of Genes: Mechanism, Theories and Types

The linkage is caused due to linked genes borne on the same chromosome. Morgan pointed out that the phenomenon of complete linkage occurs rarely because sometimes the linked genes show the tendency to separate during meiosis and new combinations are formed.

This is due to interchange of parts between two homologous chromosomes for which the term “crossing over” is used.

Thus, crossing over may be defined as a “mechanism of the recombination of the genes due to interchange of chromosomal segments at the time of pairing.”

In the linkage experiment with maize, it is seen that the genes for seed colour C and full seed S remain associated in the parental combination in about 96 per cent but break apart in about 4 per cent (see Fig. 5.8). This recombination of linked genes to interchange parts between homologous chromosomes is termed as crossing over.

Crossing over takes place in the segment of the chromosome between the loci of the genes C and S in some cells but not in others, so that about 96 per cent of the gametes contain the parental gene combination and 4 per cent contain recombination’s.

Mechanism of Crossing Over:

During the zygotene stage of the first prophase of meiosis, the homologous maternal and paternal chromosomes start pairing and lie closely side by side. This phenomenon is called synapsis. This pairing of homologous chromosomes is brought about by the mutual attraction between the allelic genes. The paired chromosomes are known as bivalent. A recent study reveals that synapsis and chiasma formation is facilitated by a highly organised structure of filaments called synaptonemal complex. Synapsis is followed by the duplication of chromosomes which change the bivalent nature of chromosome pair into tetravalent.

During this each of the homologous chromosomes in a bivalent split longitudinally into two sister chromatids attached to the undivided centromere. Thus, four chromatids are formed which remain side by side as two pairs. Later, in pachytene stage crossing over takes place during which the non-sister chromatids of homologous pair twist over each other, the point of contact of cross over chromatids being called as chiasma (Fig. 5.9).

In crossing over two or three chromatids are involved and accordingly two or more chiasmata are formed. At each chiasma the chromatid breaks and the broken segment rejoin a new chromatid (Fig. 5.10A & B). Thus exchange of parts of chromatids brings about alteration of original sequence of genes in the chromosome.

After crossing over is completed, the non-sister chromatids repel each other due to lack of attraction between them. The repulsion or separation of chromatids starts from the centromere towards the end just like a zipper and this separation process is named as terminalization. The process of terminalization continues through diplotene, diakinesis and ends in metaphase I.

At the end of terminalization the twisting chromatids separate so that the homologous chromosomes are separated completely and move to opposite poles in Anaphase I. The crossing over thus brings about alteration of the linear sequence of gene in chromosomes that produce gametes and thus add new combination of character in progeny.

Theories of Crossing Over:

(i) Contact First Theory (by Serebrovsky):

According to this theory the inner two chromatids of the homologous chromosomes undergoing crossing over first touch each other and then cross over. At the point of contact breakage occurs. The broken segments again unite to form new combinations (Fig. 5.11).

(ii) The Breakage-First Theory (By Muller):

According to this theory the chromatids under-going crossing over first of all break into two without any crossing over and after that the broken segments reunite to form the new combinations (Fig. 5.11).

(iii) Strain Theory (by Darlington):

According to this theory the breakage in chromosomes or chromatids is due to strain caused by pairing and later the breakage parts again reunite.

Types of Crossing Over:

(i) Single Crossing Over:

In this type of crossing over only one chiasma is formed all along the length of a chromosome pair. Gametes formed by this type of crossing over are called single cross over gametes (Fig. 5.10A and B).

(ii) Double Crossing Over:

In this type two chiasmata are formed along the entire length of the chromosome leading to breakage and rejoin of chromatids at two points. The gametes produced are called double cross over gametes (Fig. 5.14B).

(iii) Multiple Crossing Over:

In this type more than two chiasmata are formed and thus crossing over occurs at more than two points on the same chromosome pair. It is a rare phenomenon.

Factors Influencing Crossing Over:

In Drosophila, crossing over is completely suppressed in male but very high in female. Also there is a tendency of reduction of crossing over in male mammals.

Gowen first discovered that mutation reduces crossing over in all the chromosomes of Drosophila.

Inversion is an intersegmental change in the chromosome. In a given segment of chromosome crossing over is suppressed due to inversions.

Plough has experimentally shown that when Drosophila is subjected to high and low temperature variations, the percentage of crossing over in certain parts of the chromosome is increased.

Muller demonstrated that X-ray irradiations increase crossing over near centromere. Similarly Hanson has shown that radium increases crossing over.

Bridges has demonstrated that the age also influences the rate of crossing over in Drosophila. When the female becomes older the rate of crossing over increases.

High calcium diet in young Drosophila decreases crossing over rate where as diet deficient of metallic ions increases crossing over.

8. The frequency of crossing over is less at the ends of the chromosome and also near the centromere in comparison to other parts.

Significance of Crossing Over:

1. Crossing over provides direct proof for the linear arrangement of genes.

2. Through crossing over segments of homologous chromosomes are interchanged and hence provide origin of new characters and genetic variations.

3. Crossing over has led to the construction of linkage map or genetic maps of chromosomes.

4. Linkage group and linear order of the genes help to reveal the mechanism and nature of the genes.

5. Crossing over plays a very important role in the field of breeding to improve the varieties of plants and animals.


What is Crossing Over

The exchange of DNA segments between non-sister chromatids during the synapsis is known as the crossing over. The crossing over occurs during the prophase 1 of meiosis 1. It facilitates the genetic recombination by exchanging the genetic information and producing new combinations of alleles.

Synapsis of a homologous chromosome pair is achieved by the formation of two synaptonemal complexes between the two p arms and q arms of each chromosome. This tight holding of the two homologous chromosomes allows the exchange of genetic information between the two non-sister chromatids. The non-sister chromatids contain matching DNA regions, which can be exchanged through chiasmata regions. The chiasma is an X like region, where the two non-sister chromatids are joined together during crossing over. The formation of the chiasma stabilizes the bivalents or the chromosomes until their segregation at the metaphase 1.

Crossing over is initiated by the breaking down of similar DNA regions that occur within the homologous chromosome pair. Double-strand breaks can be introduced to the DNA molecule either by Spo11 protein or DNA damaging agents. Then, the 5’ ends of DNA edges are digested by exonucleases. This digestion introduces 3’ overhangs into the DNA edges of the DNA strands. The single-stranded 3’ overhangs are coated by recombinases, Dmc 1 and Rad51, producing nucleoprotein filaments. The invasion of this 3’ overhang into the non-sister chromatid is catalyzed by recombinases. This invaded 3’ overhang primes the DNA synthesis, using the non-sister chromatid’s DNA strand as the template. The resulting structure is known as the cross-strand exchange or the Holliday junction. This Holliday junction is pulled along the chiasma by recombinases.

Figure 2: A Holliday junction


Crossing Over: Meaning, Significance and Factors | Genetics

In this article we will discuss about:- 1. Meaning of Crossing Over 2. Frequency of Crossing Over 3. Cytological Basis 4. Cytological Proof 5. Significance 6. Factors.

  1. Meaning of Crossing Over
  2. Frequency of Crossing Over
  3. Cytological Basis of Crossing Over
  4. Cytological Proof of Crossing Over
  5. Significance of Crossing Over
  6. Factors affecting Crossing Over

1. Meaning of Crossing Over:

If linkage is complete, there should be all parental combinations only and no recombination. But actually there is no absolute linkage, thus allowing for some recombination. How does recombination take place? At the beginning of the 20th century, Janssens had made cytological observations on meiotic chromosomes in salamanders.

He found that chromosomes showed cross-shaped configurations and suggested that they represented a break and exchange of chromosome segments. A few years later, Morgan supplemented his genetical studies on Drosophila with cytological observations and explained linkage on the basis of breakage and exchange in synapsed chromosomes.

He could thus account for the greater frequency of parental combinations and also why linkage was not absolute so that recombinant types occurred in F2 progeny. Morgan termed the cross-shaped configuration observed by Janssens as chiasma. The term crossing over referred to the actual exchange of segments between homologous chromosomes and could take place due to breakage and reunion in the paired homologues.

Recombination is a genetic outcome of breakage and exchange of segments. It cannot be observed cytologically, but can by inferred genetically from experiments. Crossing over is the process of exchange of genetic segments which cannot be observed cytologically but can be estimated genetically from the frequency of recombinants in the F2 progeny.

Chiasma Frequency:

Every pair of homologous chromosomes usually forms at least one chiasma somewhere along its length. There is a characteristic average number of chiasmata for each type of chromosome. In general, the longer the chromosome the greater the number of chiasmata. Moreover, the further apart two genes are located on a chromosome, the greater the chance for a chiasma to occur between them.

The percentage of crossover (recombinant) gametes formed by a given genotype indicates the frequency of chiasma formation between the genes in question. When chiasma forms in one cell between two gene loci, only half of meiotic products will be of crossover type. Therefore, chiasma frequency is twice the frequency of crossover products.

In other words, if chiasma forms between the loci of genes A and B in 30 % of the tetrads (paired homologous chromosomes) of an individual of genotype AB/ab, then 15% of the gametes will be recombinant (Ab or aB), and 85% will be parental (AB or ab). Further, the map distance between A and B would be 15 units apart.

It is noteworthy that the Drosophila male shows complete linkage due to absence of crossing over. In the cross between a normal red-eyed long-winged fly and purple vestigial, the F1 hybrids are all red-eyed and long-winged.

If heterozygotes from the F1 progeny are used as male parents and backcrossed with purple vestigial females, only two phenotypes appear in the progeny: the homozygous purple vestigial and the heterozygous red-eyed long-winged.

The re-combinations are absent demonstrating absence of cross over and presence of complete linkage in Drosophila male. If a reciprocal testcross is performed using F1 heterozygotes as females and purple vestigial as male, recombinants appear in the progeny.

2. Frequency of Crossing Over:

In experiments on linkage, the proportions of parental phenotypes and the new combinations can be counted. From the percentage of recombinants the amount of crossing over can be calculated.

In the cross between purple long and red round sweet peas described earlier, the sum of the new combinations (106 + 117) = 223 when divided by total progeny (1528 + 106 + 117 + 381) = 2132 and multiplied by 100 indicates 10.4% recombination or frequency of crossing over in F1 gametes. In this cross the parental types equal 89.6%.

The fact that recombinants occurred in F2 indicates that the distance between genes for flower colour and pollen shape allowed crossing over to take place between one parental chromosome and its homologue from the other parent.

Now if distance between two linked genes is more, there are greater chances of chiasma formation between them resulting in higher percentage of recombinants in the progeny. Contrarily, if distance between linked genes is less, chiasma formation between them will be less, and corresponding reduction in the number of recombinants in the progeny.

Thus it is possible to locate genes on chromosomes on the basis of their crossover frequency. The distance between genes is measured in map units. According to Sturtevant one map unit is equal to 1% crossing over. In other words, if one gamete out of 100 gametes carries a crossover chromosome for two linked genes, we say that the two genes are one map unit apart.

The correlation between crossing over and distances between genes may not be true for all genes on a chromosome. This is because chiasma formation does not occur at random throughout the length of a chromosome. It occurs with different frequency in different parts of the chromosome.

In the two large chromosomes of Drosophila there is less crossing over near the centromere and towards the ends, but more in the middle of the two arms. This would suggest that genes near the centromere are closer when actually they are further apart than the crossover percentage indicates. After determining positions of several linked genes in Drosophila, Morgan hypothesized that genes occurred in linear order along the length of the chromosome. The position of each gene was called the locus.

The Three-Point Cross:

When two genes are mapped by performing a cross, it is called a two-point cross. The three- point cross Involves three pairs of linked genes, is a valuable method for determining positions of genes in relation to each other and for mapping distances between genes.

In 1926, Bridges and Olbrycht used this method for mapping three recessive sex-linked genes in Drosophila: scute sc (without bristles), echinus ec (rough eyes), and crossveinless wings cv (absence of transverse veins). The cross involved mating of a scute crossveinless fly with an echinus fly, and then testcrossing female F1 heterozygotes with the recessive hemizygous male showing all three recessive phenotypes (Fig. 8.4).

In the data of Bridges and Olbrycht clearly the first group of progeny represents the parental combinations, and the second two groups and are re-combinations.

Since chiasma formation takes place between linked genes, in order to determine crossover percentage in a three point cross, the genes must be analysed two at a time, ignoring the third gene each time. Thus we can examine three combinations: cv – ec, cv – sc, and ec – sc in each group of progeny. Considering genes cv – ec, these are found to be present in progeny of the first group in the same way as in the parent.

That is cv is present in its normal wild form (+) on one homologue along with ec in the other homologue cv is present along with the wild form of ec. Thus in the first group of progeny cv and ec are present as cv + and ec +. Since this is the way they are present in the parent, we can infer that there is no recombination between cv and ec.

The second group of progeny again shows cv + on one homologue and ec + on the other as in the parents. Therefore, the second group of progeny also does not represent recombination between genes cv and ec. In the third group of progeny both cv and ec are present on one homologous chromosome whereas the other homologue has wild forms (+ +) of both cv and ec. This arrangement of cv and ec i.e., + and + is different from that in the parental combination and has arisen due to crossing over between the loci for cv and ec.

The data in Fig. 8.4 shows that there were 192 flies in the third group of progeny. We can calculate the percentage recombination between cv and ec by dividing 192 by the total progeny (1638 + 150 + 192 = 1980) and multiplying by 100. The amount of crossing over between cv and ec is thus found to be 9.7%.

Let us now consider genes sc and cv. In the first group of progeny they are present as in the parental combinations, that is ++ on one chromosome and as sc cv on the other. In the second group of progeny their arrangements are different, sc + being together in one chromosome and cv + together on the other homologue.

Their changed positions indicate recombination between sc and cv. In the third group of progeny also sc and cv are present differently than in parental combinations. Thus all the 150 members of second group of progeny and 192 of the third group represent recombination between sc and cv. The recombination percentage between sc and cv is calculated as described, i.e.,

Analysing the genes ec-sc in the same way we find that in the first group of progeny they are present as in the parental combinations. In the second group they are present as ++ on one chromosome, and as sc ec on the other i.e. they represent recombination. In the third group of progeny they are present as in the parental combinations.

Therefore, percentage recombination between sc-ec is (150 X 100)/1980 = 7.6%. We have thus found that percentage recombination between scute and crossveinless is 17.3, between scute and echinus 7.6 and between crossveinless and echinus 9.7. The recombination percentage also represents crossover percentage and map distance between the genes.

With the data available, it is now possible to map the genes. The chromosome is drawn as a line and the two genes showing lowest recombination frequency are marked first (Fig. 8.5a). In this case they are scute and echinus 7.6 map units apart. Next mark cv and ec which are 9.7 units apart by indicating cv either on the left or right of ec.

If we mark cv 9.7 map units on the left of ec, then as seen in Fig. 8.5b, the distance between cv and sc would equal 9.7 minus 7.6 equal to 2.1 units. This does not agree with the data on recombination percentage found experimentally. But if we mark cv on the right of ec 9.7 units apart, then it indicates a map distance between sc and ev equal to 9.7 plus 7.6 i.e., 17.3 which is identical to the experimental data.

Double Crossing Over:

In a three-point cross involving three genes A, B and C there are eight possible combinations of genes, namely ABC, AbC, ABc, abC, aBc, Abe, abc. Sometimes one or more of the expected combinations do not appear in the progeny. This is due to two crossovers occurring simultaneously in two regions (Fig. 8.7).

When a single crossover occurs, two genes are exchanged resulting in the formation of a crossover gamete. But if at the same time a second crossover also takes place between the next two genes, the original combination of genes is restored on each chromatid resulting in a parental combination. It happens then that in such a cross a double crossover is represented as a non-crossover, giving a recombination frequency lower than the actual.

This also reveals that Sturtevant’s statement that 1% crossing over equals one map unit is not always justified. Clearly, crossover percentage is not always equal to recombination percentage. When there are double crossovers between same two chromatids, the number of recombinants in the progeny is less than the number of crossover gametes.

The crossover percentages are important for mapping genes accurately. Some geneticists prefer to use Morgan for map units, one Morgan being equal to one per cent recombination frequency and one centimorgan equal to 0.01 Morgan.

Double crossovers cannot occur between genes that are located close to each other. In Drosophila it has been found that double crossovers cannot occur between genes closer than 10 or 15 map units apart. Moreover, the class of progeny that occurs least frequently represents the double crossovers. It also indicates greater map distance between two genes.

Maximum frequency of double crossovers can occur between gene loci at each end of the chromosome. In any case, more than 50% recombination cannot be expected between two genes because only two of the four chromatids in a paired meiotic bivalent are involved in a crossover.

Sturtevant pointed out that certain parts of chromosomes were more liable to exchange segments than others. Thus if we consider three hypothetical genes A, B and C the probability of a crossover occurring between A and B may be 10%, and between B and C may be 15%.

But what is the probability that two crossovers between A and B and between B and C should occur simultaneously? We know that the probability of two chance events occurring simultaneously is equal to the product of the individual probabilities.

In the hypothetical cross stated above, the probability that two crossovers occur between A and B, and B and C would be 10% x 15% or 0.1 x 0.15 = 0.015 = 1.5%. It has also been found experimentally that the actual percentage of double crossovers is a little less than that expected theoretically. This is due to interference, a term coined by Muller. Accordingly, the occurrence of one crossover reduces the chance of a second crossover in its neighbourhood.

Although some explanations have been put forward for interference, both at the cytological and molecular levels, none is considered satisfactory. Muller further proposed the terms coefficient of coincidence to describe the strength or degree of interference. The coefficient of coincidence is equal to the ratio of the observed percentage of double crossovers to the expected percentage of double crossovers.

The extent of interference is different between different pairs of genes. The value of coincidence falls and the value of interference rises when the distance between genes decreases. Based on the coefficient of coincidence interference can be described to range from absolute (no double crossovers) to partial (doubles less frequent than expected), none (doubles equal to expected frequency) or negative (doubles more frequent than expected).

3. Cytological Basis for Crossing Over:

At the zygotene stage of meiosis homologous chromosomes come together and start pairing. By pachytene pairing is stabilized, and each ribbon-like chromosome actually consists of two homologues paired (synapsed) close to each other called bivalents. Each homologue in a bivalent consists of two identical sister chromatids.

Chromatids belonging to two different homologues in a bivalent are called non-sister chromatids. Due to presence of four chromatids, the pachytene bivalent is sometimes called a tetrad.

Crossing over takes place between non-sister chromatids and involves breakage and reunion of only two of the four chromatids at a given point on the chromosomes. Figure 8.8 illustrates how one homologous pair of chromosomes goes through meiosis to form four gametes.

Two of the gametes receive a chromosome with genes linked in the same way as in the parental chromosomes (ABC and abc). These gametes represent non-crossovers or parental types and are produced from chromatids that were not involved in crossing over. The other two gametes, (ABc, abC) represent the recombinant or crossover types and were produced after crossing over and recombination between the originally linked genes.

4. Cytological Proof of Crossing Over:

In 1917 Goldschmidt proposed that recombination takes place due to exchange of alleles with­out exchange of chromosome segments. He assumed that at metaphase genes get detached from the chromosome. Later during meiosis the genes get reabsorbed on the chromosome either in the same or in a different place.

In 1930 Winkler put forth his “gene conversion” hypothesis. Accordingly, if gene replication occurs in closely synapsed homologous chromosomes, the wrong allele may get replicated. When this occurs at only one locus it appears like a crossover. Both the above theories presented difficulties in understanding.

In 1931 John Belling proposed a theory of crossing over based on exchange of chromosome segments in lily plants. Belling studied the morphology of bead-like chromomeres which are arranged linearly on the chromonema. Since the structure and arrangement of chromomeres is identical in a pair of homologous chromosomes, Belling thought they might represent genes.

He explained recombination by assuming that the chromomeres were synthesised first, and the chromonemata which were synthesised later, became connected to the chromomeres.

Wherever the strands of chromonemata did not get connected with chromomeres in the original linear order, they crossed over and passed through another chromomere resulting in recombination and genetic exchange. The name copy choice was later given to Belling’s mechanism for recombination. However, the idea was disproved due to lack of evidence from genetic tests.

5. Significance of Crossing Over:

Crossing over occurs in living organisms ranging from viruses to man. It constitutes evidence for sexual reproduction in an organism. Its widespread occurrence in organisms ensures exchange of genes and production of new types which increase genetic diversity.

This increases phenotypic diversity, which at the species level is responsible for genetic polymorphism. The occurrence of polymorphism is of advantage to a species because it leads to groups of individuals becoming adapted to a wider range of habitats. This increases the potential for evolutionary success.

6. Factors Affecting Crossing Over:

The following external factors can affect the frequency of crossing over:

1. Bridges showed that in Drosophila, as maternal age increases, crossing over decreases.

2. H.H. Plough, a student of Morgan found that both low and high temperatures changed the frequency of crossing over.

3. The existence of crossing over factors has been shown in the cytoplasm of females. Consequently, females with reduced recombination frequencies can pass on this trait to their daughters.

4. Calcium and magnesium ions affect crossover frequency. Antibiotics such as mitomycin-C and actinomycin-D increase crossing over. Similarly X-ray irradiation can increase crossover frequency in Drosophila females and induce it in males.


Answers and Replies

Short answer is the Y chromosome does not crossover. There are haplotypes (genes) that are passed directly from father to son. These genes along with mitochondrial DNA are the least affected by meiosis. So they are important in determining family lineages - ancestry. com or 23and me.com

For homologous chromosomes to pair correctly during meiosis I, the homologous chromosome need to crossover with eachother. This obviously presents a problem in males because the X and Y chromosomes need to pair up and crossover, yet they have very different sequences.

The solution to this problem is that the X and Y chromosomes contain regions called pseudoautosomal regions at the ends of the X and Y chromosomes that allow the X and Y chromosomes to cross over and pair only at these regions. Crossing over does not exchange much genetic information between the X and Y chromosomes (since the PAR regions are at the ends), but they do allow pairing so that meiosis can go forward.

That's what I thought. I read somewhere that fertilization is not possible even if one pair doesn't crossover.

Okay. So, if X and Y do crossover somehow, what makes the resultant chromosome X or Y?
I am assuming the absence of the SRY gene on the amalgamated chromosome makes it a X chromosome. Is that a correct assumption?

Because crossover cannot occur within the larger bodies of the X and Y chromosomes, most of the important genes on the X chromosome cannot move to the Y chromosome and vice versa. The X and Y chromosomes can recombine genes within the PAR regions (listed in the figure above), but recombination is limited just to those genes. Because the PAR regions on the X and Y chromosomes encode the same genes, recombination of these genes proceeds like recombination would in the other 22 autosomal chromosomes (hence why these regions are called pseudoautosomal regions. Despite being on sex chromosomes, genes on the pseudoautosomal regions can recombine and their inheritance patterns resemble those of normal autosomal genes).

Do you have a picture of a recombined 23rd gene?
What makes a recombined gene either X or Y, once parts of the PAR regions are swapped?


Chromosomes and Genes

Each species has a characteristic number of chromosomes. Chromosomes are coiled structures made of DNA and proteins called histones (Figure below). Chromosomes are the form of the genetic material of a cell during cell division. See the "Chromosomes" section for additional information.

The human genome has 23 pairs of chromosomes located in the nucleus of somatic cells. Each chromosome is composed of genes and other DNA wound around histones (proteins) into a tightly coiled molecule.

The human species is characterized by 23 pairs of chromosomes, as shown in Figure below. You can watch a short animation about human chromosomes at this link:http://www.dnalc.org/view/15520-DNA-is-organized-into-46-chromosomes-including-sex-chromosomes-3D-animation.html.

Human Chromosomes. Humans have 23 pairs of chromosomes. Pairs 1-22 are autosomes. Females have two X chromosomes, and males have an X and a Y chromosome.

Autosomes

Of the 23 pairs of human chromosomes, 22 pairs are autosomes (numbers 1&ndash22 in Figureabove). Autosomes are chromosomes that contain genes for characteristics that are unrelated to sex. These chromosomes are the same in males and females. The great majority of human genes are located on autosomes. At the link below, you can click on any human chromosome to see which traits its genes control.http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/chooser.shtml

Sex Chromosomes

The remaining pair of human chromosomes consists of the sex chromosomes, X and Y. Females have two X chromosomes, and males have one X and one Y chromosome. In females, one of the X chromosomes in each cell is inactivated and known as a Barr body. This ensures that females, like males, have only one functioning copy of the X chromosome in each cell.

As you can see from Figure above and Figure above, the X chromosome is much larger than the Y chromosome. The X chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100, none of which are essential to survival. (For comparison, the smallest autosome, chromosome 22, has over 500 genes.) Virtually all of the X chromosome genes are unrelated to sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene, called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into a male. Without a Y chromosome, an individual develops into a female, so you can think of female as the default sex of the human species. Can you think of a reason why the Y chromosome is so much smaller than the X chromosome? At the link that follows, you can watch an animation that explains why:www.hhmi.org/biointeractive/g. evolution.html.

Human Genes

Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn&rsquot. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Of the 3 billion base pairs in the human genome, only about 25 percent make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear. To learn more about the coding and noncoding sequences of human DNA, watch the animation at this link: www.hhmi.org/biointeractive/d. sequences.html.

The majority of human genes have two or more possible alleles, which are alternative forms of a gene. Differences in alleles account for the considerable genetic variation among people. In fact, most human genetic variation is the result of differences in individual DNA bases within alleles.


What is the purpose of crossing over in meiosis?

Likewise, people ask, what is the purpose of crossover in meiosis?

Crossing over is the swapping of genetic material that occurs in the germ line. During the formation of egg and sperm cells, also known as meiosis, paired chromosomes from each parent align so that similar DNA sequences from the paired chromosomes cross over one another.

Also, when in meiosis does crossing over occur? Further genetic variation comes from crossing over, which may occur during prophase I of meiosis. In prophase I of meiosis, the replicated homologous pair of chromosomes comes together in the process called synapsis, and sections of the chromosomes are exchanged.

Similarly, what is the importance of crossing over?

Crossing over is the process by which homologous chromosomes exchange portions of their sequence. It is important because it is a source of genetic variation.

What happens if there is no crossing over in meiosis?

Without crossing over, each chromosome would be either maternal or paternal, greatly reducing the number of possible genetic combinations, which would greatly reduce the amount of genetic variation between related individuals and within a species.


Karyotypes

The isolation and microscopic observation of chromosomes forms the basis of cytogenetics and is the primary method by which clinicians detect chromosomal abnormalities in humans. A karyotype is the number and appearance of chromosomes, and includes their length, banding pattern, and centromere position. To obtain a view of an individual’s karyotype, cytologists photograph the chromosomes and then cut and paste each chromosome into a chart, or karyogram, also known as an ideogram (Figure 1).

Figure 1. This karyotype is of a female human. Notice that homologous chromosomes are the same size, and have the same centromere positions and banding patterns. A human male would have an XY chromosome pair instead of the XX pair shown. (credit: Andreas Blozer et al)

In a given species, chromosomes can be identified by their number, size, centromere position, and banding pattern. In a human karyotype, autosomes or “body chromosomes” (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22). The X and Y chromosomes are not autosomes. However, chromosome 21 is actually shorter than chromosome 22. This was discovered after the naming of Down syndrome as trisomy 21, reflecting how this disease results from possessing one extra chromosome 21 (three total). Not wanting to change the name of this important disease, chromosome 21 retained its numbering, despite describing the shortest set of chromosomes. The chromosome “arms” projecting from either end of the centromere may be designated as short or long, depending on their relative lengths. The short arm is abbreviated p (for “petite”), whereas the long arm is abbreviated q (because it follows “p” alphabetically). Each arm is further subdivided and denoted by a number. Using this naming system, locations on chromosomes can be described consistently in the scientific literature.

Geneticists Use Karyograms to Identify Chromosomal Aberrations

Although Mendel is referred to as the “father of modern genetics,” he performed his experiments with none of the tools that the geneticists of today routinely employ. One such powerful cytological technique is karyotyping, a method in which traits characterized by chromosomal abnormalities can be identified from a single cell. To observe an individual’s karyotype, a person’s cells (like white blood cells) are first collected from a blood sample or other tissue. In the laboratory, the isolated cells are stimulated to begin actively dividing. A chemical called colchicine is then applied to cells to arrest condensed chromosomes in metaphase. Cells are then made to swell using a hypotonic solution so the chromosomes spread apart. Finally, the sample is preserved in a fixative and applied to a slide.

The geneticist then stains chromosomes with one of several dyes to better visualize the distinct and reproducible banding patterns of each chromosome pair. Following staining, the chromosomes are viewed using bright-field microscopy. A common stain choice is the Giemsa stain. Giemsa staining results in approximately 400–800 bands (of tightly coiled DNA and condensed proteins) arranged along all of the 23 chromosome pairs an experienced geneticist can identify each band. In addition to the banding patterns, chromosomes are further identified on the basis of size and centromere location. To obtain the classic depiction of the karyotype in which homologous pairs of chromosomes are aligned in numerical order from longest to shortest, the geneticist obtains a digital image, identifies each chromosome, and manually arranges the chromosomes into this pattern (Figure 1).

At its most basic, the karyogram may reveal genetic abnormalities in which an individual has too many or too few chromosomes per cell. Examples of this are Down Syndrome, which is identified by a third copy of chromosome 21, and Turner Syndrome, which is characterized by the presence of only one X chromosome in women instead of the normal two. Geneticists can also identify large deletions or insertions of DNA. For instance, Jacobsen Syndrome—which involves distinctive facial features as well as heart and bleeding defects—is identified by a deletion on chromosome 11. Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome. Translocations are implicated in certain cancers, including chronic myelogenous leukemia.

During Mendel’s lifetime, inheritance was an abstract concept that could only be inferred by performing crosses and observing the traits expressed by offspring. By observing a karyogram, today’s geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring, even before birth.


Contents

Physical traits Edit

People with the 47,XYY karyotype have an increased growth rate from early childhood, with an average final height approximately 7 cm (3") above expected final height. [5] In Edinburgh, Scotland, eight 47, XYY boys born 1967–1972 and identified in a newborn screening programme had an average height of 188.1 cm (6'2") at age 18—their fathers' average height was 174.1 cm (5'8½"), their mothers' average height was 162.8 cm (5'4"). [6] [7] The increased gene dosage of three X/Y chromosome pseudoautosomal region (PAR1) SHOX genes has been postulated as a cause of the increased stature seen in all three sex chromosome trisomies: 47,XXX, 47,XXY, and 47,XYY. [8] Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47,XYY. [9]

Prenatal testosterone levels are normal in 47,XYY males. [10] Most 47,XYY males have normal sexual development and have normal fertility. [6] [11] [12] [13]

Cognitive and behavioral traits Edit

In contrast to the other common sex chromosome aneuploidies—47,XXX and 47,XXY (Klinefelter syndrome)—the average IQ scores of 47,XYY boys identified by newborn screening programs were not reduced compared to the general population. [14] [15] In a summary of six prospective studies of 47,XYY boys identified by newborn screening programmes, twenty-eight 47,XYY boys had an average 100.76 verbal IQ, 108.79 performance IQ, and 105.00 full-scale IQ. [16] In a systematic review including two prospective studies of 47,XYY boys identified by newborn screening programs and one retrospective study of 47,XYY men identified by screening men over 184 cm (6'½") in height, forty-two 47,XYY boys and men had an average 99.5 verbal IQ and 106.4 performance IQ. [15] [17] [18] [19]

In prospective studies of 47,XYY boys identified by newborn screening programs, the IQ scores of 47,XYY boys were usually slightly lower than those of their siblings. [6] [20] In Edinburgh, fifteen 47,XYY boys with siblings identified in a newborn screening program had an average 104.0 verbal IQ and 106.7 performance IQ, while their siblings had an average 112.9 verbal IQ and 114.6 performance IQ. [17]

Approximately half of 47,XYY boys identified by newborn screening programs had learning difficulties—a higher proportion than found among siblings and above-average-IQ control groups. [6] [13] In Edinburgh, 54% of 47,XYY boys (7 of 13) identified in a newborn screening program received remedial reading teaching compared to 18% (4 of 22) in an above-average-IQ control group of 46,XY boys matched by their father's social class. [17] In Boston, USA 55% of 47,XYY boys (6 of 11) identified in a newborn screening program had learning difficulties and received part-time resource room help compared to 11% (1 of 9) in an above-average-IQ control group of 46,XY boys with familial balanced autosomal chromosome translocations. [18]

Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47,XYY and are managed no differently from in 46,XY males. [11] Aggression is not seen more frequently in 47,XYY males. [6] [11]

47,XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An incident in chromosome separation during anaphase II (of meiosis II) called nondisjunction can result in sperm cells with an extra copy of the Y-chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y-chromosome in each of the body's cells. [21]

In some cases, the addition of an extra Y-chromosome results from nondisjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics. [21]

47,XYY syndrome is not usually diagnosed until learning issues are present. The syndrome is diagnosed in an increasing number of children prenatally by amniocentesis and chorionic villus sampling [22] in order to obtain a chromosome karyotype, where the abnormality can be observed.

It is estimated that only 15–20% of children with 47,XYY syndrome are ever diagnosed. Of these, approximately 30% are diagnosed prenatally. For the rest of those diagnosed after birth, around half are diagnosed during childhood or adolescence after developmental delays are observed. The rest are diagnosed after any of a variety of symptoms, including fertility problems (5%) [23] have been seen.

Around 1 in 1,000 boys are born with a 47,XYY karyotype. [6] [11] The incidence of 47,XYY is not known to be affected by the parents' ages. [6] [11]

1960s Edit

In April 1956, Hereditas published the discovery by cytogeneticists Joe Hin Tjio and Albert Levan at Lund University in Sweden that the normal number of chromosomes in diploid human cells was 46—not 48 as had been believed for the preceding thirty years. [24] In the wake of the establishment of the normal number of human chromosomes, 47,XYY was the last of the common sex chromosome aneuploidies to be discovered, two years after the discoveries of 47,XXY, [25] 45,X, [26] and 47,XXX [27] in 1959. Even the much less common 48,XXYY [28] had been discovered in 1960, a year before 47,XYY.

Screening for those X chromosome aneuploidies was possible by noting the presence or absence of "female" sex chromatin bodies (Barr bodies) in the nuclei of interphase cells in buccal smears, a technique developed a decade before the first reported sex chromosome aneuploidy. [29] An analogous technique to screen for Y-chromosome aneuploidies by noting supernumerary "male" sex chromatin bodies was not developed until 1970, a decade after the first reported sex chromosome aneuploidy. [30]

The first published report of a man with a 47,XYY karyotype was by internist and cytogeneticist Avery Sandberg and colleagues at Roswell Park Comprehensive Cancer Center (then known as Roswell Park Memorial Institute) in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence who was karyotyped because he had a daughter with Down syndrome. [31] Only a dozen isolated 47,XYY cases were reported in the medical literature in the four years following the first report by Sandberg. [32]

Then, in December 1965 and March 1966, Nature and The Lancet published the first preliminary reports by British cytogeneticist Patricia Jacobs and colleagues at the MRC Human Genetics Unit at Western General Hospital in Edinburgh of a chromosome survey of 315 male patients at State Hospital in Carstairs, Lanarkshire—Scotland's only special security hospital for the developmentally disabled—that found nine patients, ages 17 to 36, averaging almost 6 ft. in height (avg. 5'11", range: 5'7" to 6'2"), had a 47,XYY karyotype, and mischaracterized them as aggressive and violent criminals. [32] [33] [34] [35] Over the next decade, almost all published XYY studies were on height-selected, institutionalized XYY males. [11]

In January 1968 and March 1968, The Lancet and Science published the first U.S. reports of tall, institutionalized XYY males by Mary Telfer, a biochemist, and colleagues at the Elwyn Institute. [36] Telfer found five tall, developmentally disabled XYY boys and men in hospitals and penal institutions in Pennsylvania, and since four of the five had at least moderate facial acne, reached the erroneous conclusion that acne was a defining characteristic of XYY males. [36] After learning that convicted mass murderer Richard Speck had been karyotyped, Telfer not only incorrectly assumed the acne-scarred Speck was XYY, but reached the false conclusion that Speck was the archetypical XYY male—or "supermale" as Telfer referred to XYY males outside of peer-reviewed scientific journals. [37]

In April 1968, The New York Times—using Telfer as a main source—introduced the XYY genetic condition to the general public in a three-part series on consecutive days that began with a Sunday front-page story about the planned use of the condition as a mitigating factor in two murder trials in Paris [38] and Melbourne [39] —and falsely reported that Richard Speck was an XYY male and that the condition would be used in an appeal of his murder conviction. [34] [40] The series was echoed the following week by articles—again using Telfer as a main source—in Time and Newsweek, [41] and six months later in The New York Times Magazine. [42]

In December 1968, the Journal of Medical Genetics published the first XYY review article—by Michael Court Brown, [43] director of the MRC Human Genetics Unit—which reported no overrepresentation of XYY males in nationwide chromosome surveys of prisons and hospitals for the developmentally disabled and mentally ill in Scotland, and concluded that studies confined to institutionalized XYY males may be guilty of selection bias, and that long-term longitudinal prospective studies of newborn XYY boys were needed. [32]

In May 1969, at the annual meeting of the American Psychiatric Association, Telfer and her Elwyn Institute colleagues reported that case studies of the institutionalized XYY and XXY males they had found convinced them that XYY males had been falsely stigmatized and that their behavior may not be significantly different from chromosomally normal 46,XY males. [44]

In June 1969, the National Institute of Mental Health (NIMH) Center for Studies of Crime and Delinquency held a two-day XYY conference in Chevy Chase, Maryland. [45] In December 1969, with a grant from the NIMH Center for Studies of Crime and Delinquency, cytogeneticist Digamber Borgaonkar at Johns Hopkins Hospital began a chromosome survey of (predominantly African-American) boys ages 8 to 18 in all Maryland institutions for delinquent, neglected, or mentally ill juveniles, which was suspended from February–May 1970 due to an American Civil Liberties Union (ACLU) lawsuit about the lack of informed consent. [46] [47] Concurrently, through 1974, psychologist John Money at Johns Hopkins Hospital experimented on thirteen XYY boys and men (ages 15 to 37) in an unsuccessful attempt to treat their history of behavior problems by chemical castration using high-dose Depo-Provera—with side-effects of weight gain (avg. 26 lbs.) and suicide. [46] [48]

In the late 1960s and early 1970s, screening of consecutive newborns for sex chromosome abnormalities was undertaken at seven centers worldwide: in Denver (Jan 1964–1974), Edinburgh (Apr 1967–Jun 1979), New Haven (Oct 1967–Sep 1968), Toronto (Oct 1967–Sep 1971), Aarhus (Oct 1969–Jan 1974, Oct 1980–Jan 1989), Winnipeg (Feb 1970–Sep 1973), and Boston (Apr 1970–Nov 1974). [49] The Boston study, led by Harvard Medical School child psychiatrist Stanley Walzer at Children's Hospital, was unique among the seven newborn screening studies in that it only screened newborn boys (non-private-ward newborn boys at the Boston Hospital for Women) and was funded in part by grants from the NIMH Center for Studies of Crime and Delinquency. [50] The Edinburgh study was led by Shirley Ratcliffe who focused her career on it and published the results in 1999. [51] [52]

1970s Edit

In December 1969, Lore Zech at the Karolinska Institute in Stockholm first reported intense fluorescence of the A T-rich distal half of the long arm of the Y chromosome in the nuclei of metaphase cells treated with quinacrine mustard. [53] In April 1970, Peter Pearson and Martin Bobrow at the MRC Population Genetics Unit in Oxford and Canino Vosa at the University of Oxford reported fluorescent "male" sex chromatin bodies in the nuclei of interphase cells in buccal smears treated with quinacrine dihydrochloride, which could be used to screen for Y chromosome aneuploidies like 47,XYY. [54]

In June 1970, The XYY Man was published—the first of seven Kenneth Royce spy novels whose fictional tall, intelligent, nonviolent XYY hero was a reformed expert cat burglar recruited by British intelligence for dangerous assignments—and later adapted into a thirteen-episode British summer television series broadcast in 1976 and 1977. [55] In other fictional television works, a January 1971 episode "By the Pricking of My Thumbs . " of the British science fiction TV series Doomwatch featured an XYY boy expelled from school because his genetic condition led him to be falsely accused of nearly blinding another boy, [56] a November 1993 episode "Born Bad" of the American police procedural TV series Law & Order portrayed a 14-year-old XYY sociopathic murderer, [57] and the May 2007 season finale episode "Born To Kill" of the American police procedural TV series CSI: Miami depicted a 34-year-old XYY serial killer. [58] The false stereotype of XYY boys and men as violent criminals has also been used as a plot device in the horror films Il gatto a nove code in February 1971 (dubbed into English as The Cat o' Nine Tails in May 1971) and Alien 3 in May 1992. [34] [35]

In December 1970, at the annual meeting of the American Association for the Advancement of Science (AAAS), its retiring president, geneticist H. Bentley Glass, cheered by the legalization of abortion in New York, [59] envisioned a future where pregnant women would be required by the government to abort XYY "sex deviants". [46] [60] Mischaracterization of the XYY genetic condition was quickly incorporated into high school biology textbooks [46] [61] and medical school psychiatry textbooks, [46] [62] where misinformation still persists decades later. [35]

In 1973, child psychiatrist Herbert Schreier at Children's Hospital told Harvard Medical School microbiologist Jon Beckwith of Science for the People that he thought Walzer's Boston XYY study was unethical Science for the People investigated the study and filed a complaint with Harvard Medical School about the study in March 1974. [35] In November 1974, Science for the People went public with their objections to the Boston XYY study in a press conference and a New Scientist article alleging inadequate informed consent, a lack of benefit (since no specific treatment was available) but substantial risk (by stigmatization with a false stereotype) to the subjects, and that the unblinded experimental design could not produce meaningful results regarding the subjects' behavior. [50] In December 1974, the Harvard Standing Committee on Medical Research issued a report supporting the Boston XYY study and in March 1975, the faculty voted 199–35 to allow continuation of the study. [50] After April 1975, screening of newborns was discontinued—changes to informed consent procedures and pressure from additional advocacy groups, including the Children's Defense Fund, having led to the discontinuation of the last active U.S. newborn screening programs for sex chromosome abnormalities in Boston and Denver. [50]

In August 1976, Science published a retrospective cohort study by Educational Testing Service psychologist Herman Witkin and colleagues that screened the tallest 16% of men (over 184 cm (6'0") in height) born in Copenhagen from 1944 to 1947 for XXY and XYY karyotypes, and found an increased rate of minor criminal convictions for property crimes among sixteen XXY and twelve XYY men may be related to the lower intelligence of those with criminal convictions, but found no evidence that XXY or XYY men were inclined to be aggressive or violent. [63]

1980s and later Edit

The March of Dimes sponsored five international conferences in June 1974, November 1977, May 1981, June 1984, and June 1989 and published articles from the conferences in book form in 1979, 1982, 1986, and 1991 from seven longitudinal prospective cohort studies on the development of over 300 children and young adults with sex chromosome abnormalities identified in the screening of almost 200,000 consecutive births in hospitals in Denver, Edinburgh, New Haven, Toronto, Aarhus, Winnipeg, and Boston from 1964 to 1975. [49] [64] These seven studies—the only unbiased studies of unselected individuals with sex chromosome abnormalities—have replaced the older, biased studies of institutionalized individuals in understanding the development of individuals with sex chromosome abnormalities. [11] [65]

In May 1997, Nature Genetics published the discovery by Ercole Rao and colleagues of the X/Y chromosome pseudoautosomal region (PAR1) SHOX gene, haploinsufficiency of which leads to short stature in Turner syndrome (45,X). [66] It was subsequently postulated that the increased gene dosage of three SHOX genes leads to tall stature in the sex chromosome trisomies 47,XXX, 47,XXY, and 47,XYY. [8]

In July 1999, Psychological Medicine published a case-control study by Royal Edinburgh Hospital psychiatrist Michael Götz and colleagues that found an increased rate of criminal convictions among seventeen XYY men identified in the Edinburgh newborn screening study compared to an above-average-IQ control group of sixty XY men, which multiple logistic regression analysis indicated was mediated mainly through lowered intelligence. [67]

In June 2002, the American Journal of Medical Genetics published results from a longitudinal prospective cohort Denver Family Development Study led by pediatrician and geneticist Arthur Robinson, [68] which found that in fourteen prenatally diagnosed 47,XYY boys (from high socioeconomic status families), IQ scores available for six boys ranged from 100 to 147 with a mean of 120. [69] For the eleven of fourteen boys with siblings, in nine instances their siblings were stronger academically, but in one case the subject was performing equal to, and in another case superior to, his siblings. [69]

Some medical geneticists question whether the term "syndrome" is appropriate for this condition [6] because many people with this karyotype appear normal. [6] [11]

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    Pubertal development, testicular histology, and spermatogenesis are most often normal.
    …it appears that XY pairing and recombination occur normally in 47,XYY, the extra Y chromosome being lost during spermatogenesis, so that many XYY men have fathered chromosomally normal children. It has generally been observed that reproductive risks for males with 47,XYY are no higher than for euploid males, despite the fact that in situ hybridization studies demonstrated a lower frequency of single Y-bearing sperm than expected and a variably higher rate of disomic XX, XY and YY spermatozoa in males with 47,XYY.
    Population-based studies have demonstrated that intellectual abilities tend to be slightly lower than those of siblings and matched controls and that boys with an extra Y chromosome are more likely to require educational help. However, intelligence is usually well within the normal range.
    During school age, learning disabilities requiring educational intervention are present in approximately 50% and are as responsive to therapy as they are in children with normal chromosomes. Expressive and receptive language delays and reading disorders are common.
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    Pubertal development is normal and these men are usually fertile.
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    To our knowledge, there is no report of a discernibly increased risk for the XYY male to have chromosomally abnormal children. A slight increase in gonosomal imbalances in sperm (Table 12-1) might nevertheless lead some to choose prenatal diagnosis.
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100), Controls and SCA Mosaics (mean

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  • Telfer, Mary A. Baker, David Clark, Gerald R. Richardson, Claude E. (March 15, 1968). "Incidence of gross chromosomal errors among tall criminal American males". Science. 159 (3820): 1249–50. Bibcode:1968Sci. 159.1249T. doi:10.1126/science.159.3820.1249. JSTOR1723887. PMID5715587. S2CID27416349.

Why do men commit crimes of violence? For some, the urge to violence may be inborn—traced to something called the Y chromosome.
Once in every 500 male births, for example, the sex chromosome complement is XXY rather than XY, thus erring in the direction of femaleness. The resulting individual, called a Klinefelter male, is usually retarded, unusually tall and sterile.
Erring in the other direction, however, is the XYY complement resulting in the "supermale." He is also unusually tall and somewhat retarded, but appears to be highly, perhaps too highly, sexually motivated.
We were intrigued by Dr. Jacobs' contention that an extra Y chromosome results in tall stature, mild mental retardation, and severely disordered personality characterized by violent, aggressive behavior. We therefore planned to confirm and extend her studies.

Syndrome Status for the XYY
The XXY male has long been thought to display a constellation of symptoms that makes him diagnosable that is, he has achieved syndrome status. It would seem that the XYY male is fast achieving similar status. His symptoms, as we and other laboratories tend to think of them, are: extremely tall stature, long limbs and strikingly long arm span, facial acne, mild mental retardation, severe mental illness (including psychosis) and aggressive, antisocial behavior with a long history of arrests, frequently beginning at an early age.
On reading newspaper accounts of Richard Speck, who murdered eight Chicago student nurses in 1966, we noted all these traits and therefore concluded that Speck was a likely candidate for the XYY disorder. Independently, a cytogenetic laboratory in Chicago confirmed this hunch, reinforcing our inclination to believe that the XYY syndrome is really coming of age. It seems quite possible that in the XYY male, exemplified by Speck, biologists are describing in genetic terms a certain type of defective criminal who has long been explicitly recognized by the forensic psychiatrist.

  • Garrison, Lloyd (October 15, 1968). "French murder jury rejects chromosome defect as defense". The New York Times. p. 5.
  • . (October 25, 1968). "Criminal law: Question of Y". Time. Vol. 92 no. 17. p. 76. CS1 maint: numeric names: authors list (link)
  • . (October 10, 1968). "Extra chromosome brings acquittal on murder charge". The New York Times. p. 94. CS1 maint: numeric names: authors list (link)
  • Auerbach, Stuart (October 10, 1968). "Genetic abnormality is basis for acquittal". The Washington Post. p. A1.
  • Getze, George (February 3, 1969). "Australia precedent for XYY syndrome case held dubious". Los Angeles Times. p. C1. An Australian murder case that was reported to have been decided on the basis of the so-called XYY syndrome actually was not concerned with chromosome counts at all.

Dr. Pergament said he and Dr. Sato, a research fellow, had absolutely no connection with the Speck case and never examined Speck. The report was also denied by Speck's attorney, Public Defender Gerald W. Getty. "I never knew those doctors existed before I read about them in the paper," Getty said. Getty did say that a chromosomal test was performed on Speck, before Speck's trial, by a geneticist from outside the Chicago area. He declined to identify the geneticist, and he said the results of the test never have been disclosed. "It was agreed," he said, "that the results would not be disclosed unless I wished them disclosed. And I still don't." In any case, Getty said, the results could not be used in an appeal—since they were not part of the trial evidence. If anything, he said, they could only be used in connection with a new trial.

At the same time he made public reports from Vanderbilt University showing no abnormal makeup of Speck's chromosomes.
Getty displayed a letter of Sept. 26, 1966, relating that photographic evidence of 18 cells from Speck's blood showed no chromosome abnormality. He also exhibited a letter of last July 3, indicating that 100 of 101 cells in a sample of Speck's blood studied after the original tests showed the normal 46 chromosomes. The other cell had 45, regarded by the Vanderbilt investigators as having no significance.



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