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Most eukaryotes have several to many pairs of chromosomes, and we might expect that at metaphase of mitosis the chromosomes would align at the metaphase plate at random so that some containing the immortal DNA strand would go to one pole; the remainder to the other. And this is generally the case. However, there may be some exceptions.
Stem cells divide to produce two daughter cells:
- one that will continue as a stem cell and
- one that will go on to differentiate.
There is evidence that when some types of stem cells divide, for example a subset found in skeletal muscle, the chromatids containing the immortal strand all line up on one side of the metaphase plate and the daughter cell receiving this set is the one that remains a stem cell. Although the mechanism by which this occurs is unknown, one can appreciate a potential value to the organism. Errors (mutations) in DNA occur most often during its replication. By keeping the original template in the stem cell population, introduced errors (mutations) disappear when the differentiated cell dies at the end of its useful life. Another possible advantage of nonrandom segregation of parental vs. newly-synthesized DNA: it may assure that epigenetic alterations of their respective DNA strands are transmitted to the appropriate daughter cells.
(The figure represents a haploid cell with n = 2. Each bar represents one strand of the DNA double helix.)
However, other experiments, with other types of stem cells, find that
- only certain chromosomes (e.g., the X and Y in Drosophila male germline stem cells) preferentially segregate the parental chromatids to the cell that will remain a stem cell;
- and for others, the distribution of immortal strands at metaphase is random, and thus the drawing on the right does not reflect what happens in those cases.
Genetic mapping of high caries experience on human chromosome 13
Background: Our previous genome-wide linkage scan mapped five loci for caries experience. The purpose of this study was to fine map one of these loci, the locus 13q31.1, in order to identify genetic contributors to caries.
Methods: Seventy-two pedigrees from the Philippines were studied. Caries experience was recorded and DNA was extracted from blood samples obtained from all subjects. Sixty-one single nucleotide polymorphisms (SNPs) in 13q31.1 were genotyped. Association between caries experience and alleles was tested. We also studied 1,481 DNA samples obtained from saliva of subjects from the USA, 918 children from Brazil, and 275 children from Turkey, in order to follow up the results found in the Filipino families. We used the AliBaba2.1 software to determine if the nucleotide changes of the associated SNPs changed the prediction of the presence of transcription-binding site sequences and we also analyzed the gene expression of the genes selected based on binding predictions. Mutation analysis was also performed in 33 Filipino individuals of a segment of 13q31.1 that is highly conserved in mammals.
Results: Statistically significant association with high caries experience was found for 11 markers in 13q31.1 in the Filipino families. Haplotype analysis also confirmed these results. In the populations used for follow-up purposes, associations were found between high caries experience and a subset of these markers. Regarding the prediction of the transcription-binding site, the base change of the SNP rs17074565 was found to change the predicted-binding of genes that could be involved in the pathogenesis of caries. When the sequence has the allele C of rs17074565, the potential transcription factors binding the sequence are GR and GATA1. When the subject carries the G allele of rs17074565, the potential transcription factor predicted to bind to the sequence is GATA3. The expression of GR in whole saliva was higher in individuals with low caries experience when compared to individuals with high caries experience (p = 0.046). No mutations were found in the highly conserved sequence.
Conclusions: Genetic factors contributing to caries experience may exist in 13q31.1. The rs17074565 is located in an intergenic region and is predicted to disrupt the binding sites of two different transcription factors that might be involved with caries experience. GR expression in saliva may be a biomarker for caries risk and should be further explored.
T cell acute lymphoblastic leukemia (T ALL) is associated with a normal karyotype in 25 to 40% of patients, a higher percentage than in B cell lineage ALL. 1,2,3,4 Fluorescence in situ hybridization (FISH) techniques have improved the detection of subtle chromosome abnormalities, allowing the identification of the B ALL specific t(1221)(p13q22), 5 but spectral karyotype analysis of T ALL samples did not uncover new recurrent chromosomal abnormalities. 6 Since abnormalities of chromosome 14, most of them affecting the TCR genes, are common in T ALL, we searched for abnormalities of this chromosome in patients with T ALL using FISH techniques. We now report the identification and characterization of a previously undescribed recurrent chromosomal translocation.
5 Special Types of Chromosomes | Botany
The following points highlights the five special types of chromosomes. The types are: 1. Polytene Chromosome or Giant Chromosome 2. B-Chromosome or Supernumerary Chromosome 3. Chimaera 4. SAT-Chromosome.
Type # 1. Polytene Chromosome or Giant Chromosome:
To prepare acetocarmine squash of giant chromosome or polytene chromosome from salivary glands of Drosophila.
Drosophila melanogaster larvae. NaCl, acetocarmine solution (1%), slides, coverslips. glacial acetic acid, rubber fitted glass rod. filter paper microscope, spirit lamp, dissection box
1. Pick out the largest larva of Drosophila with a fine forceps, place it on a clean slide and put drop of 0.78% saline water (NaCl).
2 Dissect out the salivary gland of the larva which is a Y-shaped, bilobed structure with thin strips of whitish fat.
3. Put the salivary gland in 1% acetocarmine stain for about 10-15 minutes, and transfer the stained gland on a slide.
4. Put a drop of glacial acetic acid (10%) on the stained salivary gland and then put the coverslip.
5. Press it by a rubber-fitted glass rod or by thumb, heat gently and squash.
6. Use a small piece of filter paper to absorb extra fluid. Observe the slide under microscope.
Giant chromosomes of salivary gland (Fig. 322B) are now clearly visible. These are also called polytene chromosomes.
Distinct transverse bands are present on these chromosomes which also show following characteristics:
1. These are commonly known as Giant chromosomes or salivary gland chromosomes or polytene chromosomes.
2. They were first observed by Balbiani in 1881.
3. Each chromosome is made up of two homologous chromosomes which are loosely twisted around each other.
4. These are much larger than the somatic chromosome at metaphase.
5. Each chromosome is made up of dark and light bands.
6. Dark bands are rich and DNA. RNA and proteins are, however, also found.
7. Inter bands show no staining with basic dyes. They contain small amount of nucleic acid and proteins.
8. Puffs and Balbiani rings are present in these chromosomes.
9. Drosophila salivary gland contains 4 haploid pair of giant chromosomes.
10. Functions of polytene chromosome include the following:
(i) They help in nucleic acid synthesis.
(ii) They help in the protein synthesis indirectly.
(iii) They help in the formation of nuclear material from heterochromatin.
Type # 2. B-Chromosome or Supernumerary Chromosome:
1. The nuclei of some animals or plants contain one or more chromosomes in addition to normal chromosomes. These additional chromosomes are called B-chromosomes or supernumerary chromosomes.
2. First described by Wilson in 1905 in Metapodius, an insect, these chromosomes have now been reported in several plants and animals.
3. These chromosomes are generally smaller than other members of the chromosomal complement.
4. Structurally, these chromosomes are mostly heterochromatic in nature. But in Tradeschantia B-chromosomes are completely euchromatic while in maize they are partly heterochromatic and partly euchromatic (Fig. 323).
5. In most of the grases, B-chromosomes are smaller than normal ones and they can be distinguished easily.
Type # 3. Chimaera:
1. This is a twig of Bougainvillea bearing flowers of two different colours (pink as well as white) on the same axis.
2. It is due to chimaera. Chimaera are a type of plants whose tissues are of more than one genetic kind. This can happen due to mutations in a cell of a very young plant, or can be caused by grafting.
3. White flowers are produced by the tissues responsible for white flowers, and pink flowers on the same branch are produced by the tissues responsible for pink flowers.
Type # 4. Inversion Bridge:
1. The slide shows a meiotically dividing cell at early telophase stage.
2. A dicentric chromosome bridge and an acentric fragment in the form of a dot in between two poles are also visible in the slide.
3. Because of the presence of acentric fragment the bridge is due to paracentric inversion.
4. In the formation of the inversion bridge,
(i) Crossing over is seen in between two chromosomes having heterozygous paracen­tric inversion (Fig. 324A)
(ii) Diplotene stage shows a chiasma within the inverted segment (Fig. 324B) and
(iii) At anaphase I stage is seen a chromatidal bridge and an acentric fragment (Fig. 324C).
Type # 5. SAT-Chromosome:
1. This is a chromosome possessing a satellite, and hence called SAT-chromosome.
2. Heitz (1930) mentioned that the word ‘SAT stands for Sine-Acido-Thymonucleinico (without thymonucleic acid).
3. In these chromosomes, the secondary constriction marks the formation of a round or elongated body called satellite.
4. A thin chromatin filament separates the satellite from the remaining chromosome.
5. In diameter, the satellite may be smaller or similar to that of chromosome.
6. For each particular chromosome, the satellite and filament are always constant in form and size.
7. This is clearly seen in the mitotic metaphase stage because in such slides nuclear membrane and nucleolus are absent and the chromosomes are thick and short.
14.5 DNA Replication in Eukaryotes
In this section, you will explore the following questions:
- What are the similarities and differences between DNA replication in eukaryotes and prokaryotes?
- What is the role of telomerase in DNA replication?
Connection for AP ® Courses
Concepts and examples described in this section are not in scope for AP. However, the roles of telomeres and telomerase in aging and cancer are informative and build on your knowledge of DNA replication in prokaryotes.
Contrast eukaryotic DNA replication with prokaryotic replication. Table 14.2 is useful. Obtain illustrations of the process in eukaryotic cells that allow students to view the details.
Combine these topics in a discussion of telomeres, aging, and cancer. Students might think that telomere length explains differences in life spans among different animals, such as humans and dogs. Explain that this might be a tempting conclusion, but some long-lived species, such as humans, have shorter telomeres than mice, which live only a few years.
Eukaryotic genomes are much more complex and larger in size than prokaryotic genomes. The human genome has three billion base pairs per haploid set of chromosomes, and 6 billion base pairs are replicated during the S phase of the cell cycle. There are multiple origins of replication on the eukaryotic chromosome humans can have up to 100,000 origins of replication. The rate of replication is approximately 100 nucleotides per second, much slower than prokaryotic replication. In yeast, which is a eukaryote, special sequences known as Autonomously Replicating Sequences (ARS) are found on the chromosomes. These are equivalent to the origin of replication in E. coli.
The number of DNA polymerases in eukaryotes is much more than prokaryotes: 14 are known, of which five are known to have major roles during replication and have been well studied. They are known as pol α, pol β, pol γ, pol δ, and pol ε.
The essential steps of replication are the same as in prokaryotes. Before replication can start, the DNA has to be made available as template. Eukaryotic DNA is bound to basic proteins known as histones to form structures called nucleosomes. The chromatin (the complex between DNA and proteins) may undergo some chemical modifications, so that the DNA may be able to slide off the proteins or be accessible to the enzymes of the DNA replication machinery. At the origin of replication, a pre-replication complex is made with other initiator proteins. Other proteins are then recruited to start the replication process (Table 14.2).
A helicase using the energy from ATP hydrolysis opens up the DNA helix. Replication forks are formed at each replication origin as the DNA unwinds. The opening of the double helix causes over-winding, or supercoiling, in the DNA ahead of the replication fork. These are resolved with the action of topoisomerases. Primers are formed by the enzyme primase, and using the primer, DNA pol can start synthesis. While the leading strand is continuously synthesized by the enzyme pol δ, the lagging strand is synthesized by pol ε. A sliding clamp protein known as PCNA (Proliferating Cell Nuclear Antigen) holds the DNA pol in place so that it does not slide off the DNA. RNase H removes the RNA primer, which is then replaced with DNA nucleotides. The Okazaki fragments in the lagging strand are joined together after the replacement of the RNA primers with DNA. The gaps that remain are sealed by DNA ligase, which forms the phosphodiester bond.
Unlike prokaryotic chromosomes, eukaryotic chromosomes are linear. As you’ve learned, the enzyme DNA pol can add nucleotides only in the 5' to 3' direction. In the leading strand, synthesis continues until the end of the chromosome is reached. On the lagging strand, DNA is synthesized in short stretches, each of which is initiated by a separate primer. When the replication fork reaches the end of the linear chromosome, there is no place for a primer to be made for the DNA fragment to be copied at the end of the chromosome. These ends thus remain unpaired, and over time these ends may get progressively shorter as cells continue to divide.
The ends of the linear chromosomes are known as telomeres , which have repetitive sequences that code for no particular gene. In a way, these telomeres protect the genes from getting deleted as cells continue to divide. In humans, a six base pair sequence, TTAGGG, is repeated 100 to 1000 times. The discovery of the enzyme telomerase (Figure 14.16) helped in the understanding of how chromosome ends are maintained. The telomerase enzyme contains a catalytic part and a built-in RNA template. It attaches to the end of the chromosome, and complementary bases to the RNA template are added on the 3' end of the DNA strand. Once the 3' end of the lagging strand template is sufficiently elongated, DNA polymerase can add the nucleotides complementary to the ends of the chromosomes. Thus, the ends of the chromosomes are replicated.
Telomerase is typically active in germ cells and adult stem cells. It is not active in adult somatic cells. For her discovery of telomerase and its action, Elizabeth Blackburn (Figure 14.16) received the Nobel Prize for Medicine and Physiology in 2009.
Telomerase and Aging
Cells that undergo cell division continue to have their telomeres shortened because most somatic cells do not make telomerase. This essentially means that telomere shortening is associated with aging. With the advent of modern medicine, preventative health care, and healthier lifestyles, the human life span has increased, and there is an increasing demand for people to look younger and have a better quality of life as they grow older.
In 2010, scientists found that telomerase can reverse some age-related conditions in mice. This may have potential in regenerative medicine. 2 Telomerase-deficient mice were used in these studies these mice have tissue atrophy, stem cell depletion, organ system failure, and impaired tissue injury responses. Telomerase reactivation in these mice caused extension of telomeres, reduced DNA damage, reversed neurodegeneration, and improved the function of the testes, spleen, and intestines. Thus, telomere reactivation may have potential for treating age-related diseases in humans.
Cancer is characterized by uncontrolled cell division of abnormal cells. The cells accumulate mutations, proliferate uncontrollably, and can migrate to different parts of the body through a process called metastasis. Scientists have observed that cancerous cells have considerably shortened telomeres and that telomerase is active in these cells. Interestingly, only after the telomeres were shortened in the cancer cells did the telomerase become active. If the action of telomerase in these cells can be inhibited by drugs during cancer therapy, then the cancerous cells could potentially be stopped from further division.
Download CBSE class 12th revision notes for chapter 5 Principles of Inheritance and Variation in PDF format for free. Download revision notes for Principles of Inheritance and Variation class 12 Notes and score high in exams. These are the Principles of Inheritance and Variation class 12 Notes prepared by team of expert teachers. The revision notes help you revise the whole chapter 5 in minutes. Revision notes in exam days is one of the best tips recommended by teachers during exam days.
Genetics is the study of principles and mechanism of heredity and variation. Gregor Johann Mendel is known as ‘father of Genetics’.
- Inheritance is the process by which characters are passed on from parent to progeny. It is the basis of heredity.
- Variation is the degree by which progeny differ from their parents. Variation may be in terms of morphology, physiology, cytology and behavioristic traits of individual belonging to same species.
- Variation arise due to
- Reshuffling of gene/chromosomes.
- Crossing over or recombination
- Mutation and effect of environment.
Mendel’s Law of Inheritance: Mendel conducted hybridization experiments on garden pea (Pisum sativum) for seven years and proposed the law of inheritance in living organisms.
Selection of pea plant: The main reasons for adopting garden pea (Pisum sativum) for experiments by Mendel were –
- Pea has many distinct contrasting characters.
- Life span of pea plant is short.
- Flowers show self-pollination, reproductive whorls being enclosed by corolla.
- It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.
Working method: Mendel’s success was also due to his meticulous planning and method of work –
- He studied only one character at a time.
- He used all available techniques to avoid cross-pollination by undesirable pollen grains.
- He applied mathematics and statistics to analyse the results obtained by him.
- Mendel selected 7 contrasting characters of garden pea for his hybridization experimentsContrasting Characters Studied by Mendel in Pea
Character Contrasting character (Dominant/Recessive) Stem height Tall/Dwarf Flower colour Violet/White Flower position Axial/Terminal Pod shape Inflated/Constricted Pod colour Green/Yellow Seed shape Round/wrinkled Seed colour Yellow/Green
- Mendel conducted artificial hybridization/cross pollination using true breeding pea lines. True breeding lines are those that undergo continuous self-pollination and shows stable trait inheritance.
- Hybridization experiment includes emasculation (removal of anther) and transfer of pollen (pollination).
Inheritance of one gene (Monohybrid cross)
Mendel crossed tall and dwarf pea plant and collected all the seeds obtained from this cross. He grew all the seeds to generate plants of first hybrid generation called F1 generation. He observed that all the plants are tall. Similar observation was also found in other pair of traits.
Mendel self-pollinated the F1 plants and found that in F2 generation some plants are also dwarf. The proportion of dwarf plants is 1/4th and tall plants of 3/4th.
- Mendel called the ‘factors’ that passes through gametes from one generation to next generation. Now a day it is called as genes (unit of inheritance).
- Genes that code for a pair of contrasting traits are known as alleles.
- Alphabetical symbols are used to represent each gene, capital letter (TT) for gene expressed in F1 generation and small letter (tt) for other gene.
- Mendel also proposed that in true breeding tall and dwarf variety allelic pair of genes for height is homozygous (TT or tt). TT, Tt or tt are called genotype and tall and dwarf are called phenotype.
- The hybrids which contain alleles which express contrasting traits are called heterozygous (Tt).
- The monohybrid ratio of F2 hybrid is 3:1(phenotypic) and 1:2:1(genotypic).
Test cross is the cross between an individual with dominant trait and a recessive organism in order to know whether the dominant trait is homozygous or heterozygous.
Principle or Law of Inheritance
Based on observations of monohybrid cross, Mendel proposed two law of inheritance-
1. Law of dominance– states that –
a. Characters are controlled by discrete units called factors.
b. Factors always occur in pair.
c. In a dissimilar pair of factors one member of pair dominate the other.
Dominance Recessive (i) When a factor (allele) expresses itself in the presence or absence of its dominant factor called dominance. It can only express itself in the absence of or its recessive factor allele. (ii) It forms a complete functional enzyme that perfectly express it. It forms a incomplete defective enzyme which fails to express itself when present with its dominant allele, i.e., in heterozygous condition.
2. Law of Segregation- alleles do not blends and both the characters are recovered during gametes formation as in F2 generation. During gametes formation traits segregate (separate) from each other and passes to different gametes. Homozygous produce similar kinds of gametes but heterozygous produce to different kinds of gametes with different traits.
- It is a post Mendelian discovery. Incomplete dominance is the phenomenon of neither of the two alleles being dominant so that expression in the hybrid is a fine mixture or intermediate between the expressions of two alleles.
- In snapdragon (Mirabilis jalapa), there are two types of pure breeding plants, red flowered and white flowered. On crossing the two, F1 plants possess pink flowers. On selfing them, F2 generation has 1red: 2 pink: 1white. The pink flower is due to incomplete dominance.
- It is the phenomenon of two alleles lacking dominance-recessive relationship and both expressing themselves in the organism.
- Human beings, ABO blood grouping are controlled by gene I. The gene has three alleles I A, I B and i. Any person contains any two of three allele I A, I B are dominant over i.
- The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.
- When I A and I B are present together, both express their own types of sugars because of co-dominance.
They are multiple forms of a medelian factor or gene which occur on the same gene locus distributed in different organisms in the gene pool with an organism carrying only two alleles and a gamete only one allele. ABO blood grouping also provides a good example of multiple alleles.
Inheritance of Two genes (Dihybrid Cross)
A cross made to study simultaneous inheritance of two pairs of mendelian factors of genes.
Law of independent Assortment – The law states that ‘when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters’.In Dihybrid cross two new combinations, round green & wrinkled yellow are formed due to independent assortment of traits for seed shape i.e round, wrinkled and seed color i.e , yellow and green.
The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round : 1 wrinkled. This derivation can be written as follows: (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow: 3 Round, Green : 1 Wrinkled, Green
Chromosomal Theory of Inheritance
- Chromosome as well as gene both occurs in pair. The two alleles of a gene pair are located on the same locus on homologous chromosomes.
- Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to segregation of a pair of factors (gene) they carried.
- Sutton united the knowledge of chromosomal segregation with mendelian principles and called it the chromosomal theory of inheritance.
Linkage and Recombination
- When two genes in a Dihybrid cross were situated on same chromosome, the proportion of parental gene combination was much higher than the non-parental type. Morgan attributed this due to the physical association or the linkage of the two genes and coined the linkage to describe the physical association of genes on same chromosome.
- The generation of non-parental gene combination during Dihybrid cross is called recombination. When genes are located on same chromosome, they are tightly linked and show very low recombination.Difference between crossing over and linkage
Crossing over Linkage 1. It leads to separation of linked genes 1. keeps the genes together 2. It involves exchange of segments between non-sister chromatics of homologous chromosomes. 2. It involves individual chromosomes. 3. The frequency of crossing over can never exceed 50%. 3. The number of linkage group can never be more than haploid Chromosome number. 4. It increases variability by forming new gene combinations. 4. It reduces variability.
- Henking in 1891 observed a trace of specific nuclear structure in few insects. He also observed that this specific nuclear structure is located on 50% of sperms only. He called this x body. He was not able to explain its significance.
- Latter it was observed that the ovum that receive the sperms with x body become female and those not becomes males, so this x body was called as sex chromosome and other chromosomes are called autosomes.
- In humans and other organisms XY types of sex determination is seen but in some insects like Drosophila XO type of sex determination is present.
- In both types of sex determination, male produce two different types of gametes either with or without X chromosome or some with X chromosome and some with Y chromosomes. Such types of sex determination are called male heterogamety.
- In birds ZW type of sex determination is present., two different types of gametes are produced by females in terms of sex chromosomes this type of sex determination is called female heterogamety.
- Sex determination in human beings XY type. Out of 23 pairs of chromosomes, 22 pairs are exactly same in male and female called autosomes. A pair of X chromosome is present in female and XY in male. During spermatogenesis, male produce two type of gametes (sperms), 50% carries Y chromosome and remaining 50% contain X chromosome. Female, produce only one kind of gamete (ovum) having X chromosomes only.
- When sperm having Y chromosome the sex of baby is male and when sperm carrying X chromosome fertilse the egg, the sex of baby is female.
Mutationis a phenomenon which results in alternation of DNA sequence and consequently results in the change in the genotype and phenotype of an organism. The mutations that arise due to due to change in single base pair of DNA are called point mutation e.g Sickle cell anaemia.
- The analysis of traits in several of generation of a family is called the pedigree analysis. The inheritance of a particular trait is represented in family tree over several generations. It is used to trace the inheritance of particular trait, abnormality and disease.
Broadly, genetic disorders may be grouped into two categories – Mendelian disorders and
They are transmitted as the affected individual is sterile.This is always dominant in nature.
Mendelian Disorders Chromosomal disorders These are due to alteration in a single gene. These are caused due to absence or excess of one or more chromosomes or abnormal arrangement of one/more chromosomes. They are transmitted into generations through Mendelian principles of inheritance. They may be recessive or dominant in nature. Examples: Colour blindness Pheffykenonia. Examples: Downs syndrome, Turner’s syndrome
Medelian disorder includes-
a. Haemophilia- sex linked recessive disease in which, in an infected individual, a minor cut leads to non-stop bleeding. Heterozygous female (carrier) can transmit the disease to their son. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at least carrier and the father should be haemophilic (unviable in the later stage of life).
b. Sickle cell anemia- an autosome linked recessive trait in which mutant haemoglobin molecules undergo polymerization under low oxygen tension causing change in shape of the RBC from biconvex disc to elongated sickle like structure. The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule. The substitution of amino acid in the globin protein results due to the single base substitution at the sixth codon of the beta globin gene from GAG to GUG
c. Phenylketonuria- inborn error of metabolism inherited as autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acids phenylalanine to tyrosine . . As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives that results into mental retardation.
Chromosomal Disorders-Failure of segregation of chromatids during cell division results in loss or gain of chromosome called aneuploidy. The failure of cytokinesis leads to two sets of chromosome called polyploidy.
a. Down’s Syndrome– is due to presence of additional copy of the chromosome number 21. The affected individual is short statured with small rounded head, furrowed tongue and partially opened mouth. Mental development is retarded.
b. Klinefleter’s Syndrome– due to presence of an additional copy of X-chromosome (XXY). Such persons have overall masculine development however, the feminine development (development of breast, i.e., Gynaecomastia) is also expressed. They are sterile.
c. Turner’s Syndrome– caused due to the absence of one of the X chromosome. 45 with XO, such females are sterile as ovaries are rudimentary. They lack secondary sexual characters.
If an underlying cause of Harlequin syndrome is identified, treatment should be directed to the cause of the syndrome. Surgery may be possible to repair a lesion that is causing Harlequin syndrome. If there is no known cause of the symptoms and the symptoms are not affecting a person’s daily life, some people may choose not to pursue any treatment. 
In cases where an individual has symptoms of Harlequin syndrome and wishes to receive treatment, injection with botulinum toxin (Botox) or a procedure called contralateral sympathectomy is possible.    In contralateral sympathectomy, the nerve bundles that are responsible for causing flushing in the face are interrupted. Therefore, this procedure causes both sides of the face to no longer flush or sweat. Because the symptoms of Harlequin syndrome are not typically associated with affecting a person’s daily life, this treatment is only recommended if a person is very uncomfortable with the flushing and sweating associated with the syndrome. 
chromosome set, the aggregate features of the chromosomes (numaber, size, shape, details of microscopic structure) in the cells of an organism of a given species.
The concept of the karyotype was introduced by the Soviet geneticist G. A. Levitskii in 1924. The karyotype is one of the most important genetic characteristics of a species, since every species has a particular karyotype that is different from that of related species (karyosystematics, a new branch of systematics, is based on this phenomenon). The fixed nature of the karyotype in the cells of a given organism is ensured by mitosis and, within a given species, by meiosis. The karyotype of an organism may change if the gametes are altered by mutation. The karyotype of individual cells sometimes differs from the species karyotype because of chromosomal or genomic somatic mutations. The karyotype of diploid cells consists of two haploid sets (genomes) from each parent each chromosome of such a set has a homologue from the other set. The karyotype of males may differ from that of females in the shape (sometimes also in number) of the sex chromosomes, in which case they are described separately.
The chromosomes in a karyotype are studied during the metaphase stage of mitosis. The description of a karyotype must be accompanied by a microphotograph or sketch. In systematizing karyotypes, the pairs of homologous chromosomes are arranged (for example) in order of decreasing length, beginning with the longest pair. The pairs of sex chromosomes are put at the end of the series. Pairs of chromosomes of equal length are identified by the position of the centromere (primary constriction), which divides the chromosome into two arms, by the position of the nucleolar organizer (secondary constriction), and by the shape of the satellite. The karyotypes of several thousand species of plants (wild and cultivated) and animals and man have been studied.
Representing Chromosome Set of Species
Photomicrographs of the chromosomes of a single representative somatic metaphase cell are clipped out and arranged in homologous pairs according to their size. If the chromosomes are small and there is difficulty in identifying the individual chromosomes, they are arranged in groups of similar chromosomes.
For example, in human, the 23 pairs of chromosomes had been divided into 7 groups represented by the letters from A to G (Denver System) and numbers (London System) the seven groups are, A (l, 2, 3), B, (4, 5), C (X, 6, 7, 8, 9, 10 11, 12), D (13, 14, 15), E (16, 17, 18), F (19, 20), and G (Y, 21, 22).
Thus the X chromosome is placed in the C group, while the Y chromosome is placed in the G group. However, it is now possible to unambiguously identify each of the 23 chromosomes, and even individual chromosome arms, with the help of chromosome banding.
Representing Chromosome: Way # 2. Idiogram or Idiotype:
It is the graphical representation of the karyotype (Fig. 6.5). Generally, the idiogram is prepared to show the haploid chromosome complement of a species it is prepared from the measurement of somatic metaphase chromosomes.
Individual chromosomes must be identified for this purpose. There are techniques by which chromosomes or even specific chromosome segments can be identified. These techniques are fluorescent staining, pulse labelling, chromosome banding, and studying the tertiary constrictions and chromomeres.
Chromomere pattern can be studied easily and clearly in pacyhtene stage in many species and in polytene giant chromosomes of several members of Diptera.
Symmetry and Asymmetry of Karyotype:
Karyotypes may be symmetrical or asymmetrical this concept was developed by Levitzky in 1931. When all the chromosomes of a species are of approximately the same size and have median or sub-median centromeres, the karyotype is said to be symmetrical (Fig. 6.5).
When the chromosomes of an individual differ in size and position of centromere, the karyotype is called asymmetrical or heterogeneous. The symmetrical karyotype represents a primitive state from which asymmetrical karyotypes have evolved through structural chromosome changes.
Pericentric inversions and unequal translocations change the position of centromere. Thus a metacentric chromosome may be converted into an acrocentric chromosome (Fig. 6.6, 15.5). However, reversion may also occur so that an acrocentric chromosome would become a meta- or sub-metacentric by the same process.
The term bimodal karyotype refers to an symmetrical karyotype that is composed of two distinct classes of chromosomes as determined from their size. Examples of such karyotypes are found in certain genera of Liliales, such as, Aloe, Gasteria, Yucca, Agave etc.
The species of Aloe and Gasteria have 7 pairs of chromosomes (x = 7) of which four are large and acrocentric, while three are short (Fig. 6.5). The species of Yucca and Agave have 30 chromosome pairs (x = 30) of which 5 are medium sized, strongly acrocentric chromosomes, while 25 are very small chromosomes.
The origin of bimodal karyotypes can be explained on the basis of pericentric inversions, unequal translocations and addition of centric fragments.
Karyotypic variations among different species of the same genus may be observed in several herbaceous genera possessing medium to large chromosomes in size.
A well known example is the genus Crepis (Compositae) where the degree of karyotype symmetry and chromosome number are negatively associated. C. kashmirica (x = 6), C. sibirica (x = 5), C. conyzaefolia (x = 4) and C. capillaris (x = 3) have larger chromosomes while C. mungieri (x = 6), C. leontodontoides (x = 5), C. suffreniana (x = 4) and C. fuliginosa (x = 3) have smaller chromosomes.
The species having smaller chromosomes exhibit a greater degree of karyotype asymmetry. However, in certain cases, such as, Clarkia and Cephalaira-Succisa, the degree of asymmetry increases with an increase in chromosome number. There are several factors which generate variation in the karyotype during evolution.
For a comparison of karyotypes of related species or genera, the following characteristics are considered:
(a) Absolute size of chromosomes:
Duplications cause differences in the absolute size of chromosomes.
(b) Centromere position:
The position of centromere changes due to unequal translocations and pericentric inversion in which the broken segments on the two sides of the centromere are not equal (Fig. 6.6, 15.5). A metacentric chromosome may be changed to become a sub-metacentric or sub-telocentric chromosome.
(c) Relative size chromosomes:
Segmental interchange involving translocation of unequal size is responsible for change in relative chromosome size. Two chromosomes of equal length may change into one smaller and one larger chromosomes (Fig. 6.6).
Basic chromosome number may be reduced due to unequal translocation and accompanied with a loss of the centromere. Increase in basic chromosome number may occur by addition of centric fragments and translocation of essential gene loci to them (Fig. 6.7).
Example of reduction in basic chromosome number can be well understood by studying the multiple sex chromosomes. The XY mechanism evolved into XY1Y2 mechanism of sex determination by translocation between the sex chromosomes and an autosome pair, as in Rumex (plant) and certain insects.
The texas race of Rumex hastatulus has 2n = 10 chromosomes (8A + XY, ♂ and 8A + XX ♀). The North Carolina race of this species has 2n = 8 chromosomes in females (6A + XX) and 2n = 9 chromosomes in males (6A + XY1Y2).
During male meiosis, both arms of X chromosome pair with the two Y chromosomes, resulting in the X-Y1Y2segregation. The evolution of such karyotype is shown diagrammatically in Fig. 6.8.
The X1X2Y mechanism is considered to have evolved from the XO type by translocation between an autosome and the X chromosome, as in marrtids (Orthopteran insect). The two trans-located chromosomes become X1 and X2 while the non-trans-located homologue of the autosome pair becomes the “neo-Y chromosome” (Fig. 6.9).
The nee-Y chromosome is oriented towards one pole, while the Xt and X2 chromosomes are oriented towards the other-pole at metaphase I. However, there is no change in the chromosome number. The female possesses X1X1X2X2, while male possesses YX1X2 chromosomes in such condition.
(e) Number and position of satellites:
Location and size of the nucleolar organizer regions may differ.
(f) Heterochromatic regions:
Heterochromatic regions may be scattered or localized at different positions in the chromosomes.
Fig. 6.9 Diagram showing of X1X2Y mechanism of sex determination from OX type of male, by means of translocation between the X chromosome and an autosome (A). (Δ indicates the break position).
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