12.2F: Lethal Inheritance Patterns - Biology

12.2F: Lethal Inheritance Patterns - Biology

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Inheriting two copies of mutated genes that are nonfunctional can have lethal consequences.

Learning Objectives

  • Describe recessive and dominant lethal inheritance patterns

Key Points

  • An inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype is referred to as recessive lethal.
  • The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age.
  • Dominant lethal alleles are very rare because the allele only lasts one generation and is, therefore, not usually transmitted.
  • In the case where dominant lethal alleles might not be expressed until adulthood, the allele may be unknowingly passed on, resulting in a delayed death in both generations.

Key Terms

  • mutation: any heritable change of the base-pair sequence of genetic material
  • recessive lethal: an inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype
  • dominant lethal: an inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age

Lethal Inheritance Patterns

A large proportion of genes in an individual’s genome are essential for survival. Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to sustain life and is, therefore, considered to be dominant over the nonfunctional allele. However, consider two heterozygous parents that have a genotype of wild-type/nonfunctional mutant for a hypothetical essential gene. In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die in utero, or die later in life, depending on what life stage requires this gene. An inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered non-lethal phenotype is referred to as recessive lethal.

For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed. The genotypic ratio would therefore be 2:1. In other instances, the recessive lethal allele might also exhibit a dominant (but not lethal) phenotype in the heterozygote. For instance, the recessive lethal Curly allele in Drosophila affects wing shape in the heterozygote form, but is lethal in the homozygote.

Dominant Lethal Alleles

A single copy of the wild-type allele is not always sufficient for normal functioning or even survival. The dominant lethal inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age. Individuals with mutations that result in dominant lethal alleles fail to survive even in the heterozygote form. Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However, just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles also might not be expressed until adulthood. Once the individual reaches reproductive age, the allele may be unknowingly passed on, resulting in a delayed death in both generations. An example of this in humans is Huntington’s disease in which the nervous system gradually wastes away. People who are heterozygous for the dominant Huntington allele (Hh) will inevitably develop the fatal disease. However, the onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50 percent of their offspring.

Phenotype and Genotype

The phenotype includes all observable characteristics of an individual. Although Mendel's studies were restricted to the outward traits of pea plants, such as flower color and plant height, phenotype can include characteristics observable only under certain circumstances or with specialized tools and technology. For example, a human phenotype certainly includes eye and skin color, but it also includes characteristics such as blood type and bone density.

A genotype is the complete genetic makeup of an individual. Nonetheless, as it has been impractical to consider the entire genetic makeup of an individual, the reference to genotype is usually restricted to those genes influencing the aspect of phenotype being studied at the time. In other words, if scientists are interested in studying the coat color trait in Labrador retrievers, they focus their attention on the gene(s) identified with influencing coat color.

Chapter Summary

Working with garden pea plants, Mendel found that crosses between parents that differed by one trait produced F1 offspring that all expressed the traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P0 parent. Reciprocal crosses generated identical F1 and F2 offspring ratios. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

Two rules in probability can be used to find the expected proportions of offspring of different traits from different crosses. To find the probability of two or more independent events occurring together, apply the product rule and multiply the probabilities of the individual events. The use of the word “and” suggests the appropriate application of the product rule. To find the probability of two or more events occurring in combination, apply the sum rule and add their individual probabilities together. The use of the word “or” suggests the appropriate application of the sum rule.

12.2 Characteristics and Traits

When true-breeding or homozygous individuals that differ for a certain trait are crossed, all of the offspring will be heterozygotes for that trait. If the traits are inherited as dominant and recessive, the F1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F2 offspring will exhibit a ratio of three dominant to one recessive.

Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two. Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant lethal alleles are fatal in heterozygotes as well.

12.3 Laws of Inheritance

Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome. For crosses involving more than two genes, use the forked line or probability methods to predict offspring genotypes and phenotypes rather than a Punnett square.

Although chromosomes sort independently into gametes during meiosis, Mendel’s law of independent assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1,000 genes. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel's law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space.

Whether or not they are sorting independently, genes may interact at the level of gene products such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis.

A Special Case: Lethal Alleles

When a genetic defect causes 100% mortality in the offspring it is called a lethal allele. When a lethal allele is present, we don’t “see” any offspring result from the cross (they die before birth) so the proportions in the offspring appear off compared to what we expect from a Punnett square.

Lethal alleles can be dominant or recessive. Recessive lethal alleles cause death in a recessive homozygote (aa). Dominant lethal alleles cause death in a dominant homozygote (AA).

Examples of dominant lethal alleles include Huntington’s disease or achondroplasia (a type of dwarfism). In achondroplasia, individuals with an homozygous dominant genotype die before or shortly after birth. Heterozygotes (Aa) show the dwarf phenotype, and homozygous recessives are of average stature (aa).

Stop and Think: Examine the picture below showing the inheritance of coat color in mice. It shows an example of a dominant lethal allele. What are the genotype and phenotype ratios you would see in the actual mouse population? (answer: P-R is 1 dead, 2 yellow, 1 white )

Mendelian inheritance of endogenous viral elements (EVE) of white spot syndrome virus (WSSV) in shrimp

Previous work has shown that non-retroviral endogenous viral elements (EVE) are common in crustaceans, including penaeid shrimp. So far, they have been reported for infectious hypodermal and hematopoietic necrosis virus (IHHNV) and white spot syndrome virus (WSSV). For the latter, it was shown that shrimp sperm were positive for an EVE of WSSV called EVE366, suggesting that it was heritable, since shrimp sperm (non-motile) do not contain mitochondria. However, to prove this hypothesis that EVE366 was heritable and located in chromosomal DNA, it was necessary to carry out mating tests to show that EVE366 could be detected in parental shrimp and distributed in their offspring in a Mendelian fashion. To do this, we analyzed two shrimp crosses using polyacrylamide gels with a multiple-allele, microsatellite marker Pmo11 as a quality control for single allele detection. In both crosses, all of the shrimp (parents and siblings) were positive for 2 Pmo11 alleles as expected. In Cross 1, the female was PCR-positive for EVE366 while the male was negative, and in Cross 2, both the female and male were PCR-positive for EVE366. Individual analysis of the offspring of Cross 1 revealed a distribution of 1:1 for EVE366, indicating that the EVE366-positive female parent was heterozygous for EVE366. In the second cross, the distribution of EVE366 in the offspring was 3:1, indicating that both PCR-positive parents were heterozygous for EVE366. These results supported the hypothesis that EVE366 was present in shrimp chromosomal DNA and was heritable in a Mendelian fashion. This work provides a model to screen for heritable EVE in shrimp and shows that selection of one parent heterozygous for an EVE and the other negative for it can result in approximately half of the siblings positive and half negative for that EVE as expected. Dividing the siblings of such a cross into an EVE positive group and an EVE negative group followed by challenge with the originating lethal virus should reveal whether or not possession of that specific EVE results in any significant protection against disease caused by the homologous virus.

Keywords: EVE Endogenous viral elements Mendelian inheritance WSSV White spot syndrome virus.

Dominance Between Alleles

The path from allele to phenotype is complex in most organisms, since more than one allele of a given gene is usually present. In diploid organisms such as humans, an individual carries two alleles of each gene. If the individual carries two identical alleles (a homozygote), then the phenotype necessarily will reflect the only version present. However, if an individual carries two different alleles (a heterozygote), each encoding a slightly different characteristic, what will the phenotype show? For example, if a diploid plant carries one allele encoding red flowers and one allele encoding white flowers, will the flowers be red or white? The answer depends on the molecular behavior of the encoded proteins.

Imagine that the red flower allele encodes a functional enzyme essential for the synthesis of the chemical compound leading to red pigmentation. If the white flower allele is a loss-of-function mutation, the enzyme encoded by this allele will not be functional and consequently will not contribute toward the synthesis of red pigment the absence of red pigment leads to white flowers. In the heterozygote, however, enough functional enzyme may be produced by one allele to result in pigmented flowers.

In this case, geneticists would describe the red allele as dominant and the white allele as recessive. If the allele encoding red flowers is dominant, then this allele will phenotypically mask expression of the recessive allele (in this case encoding white flowers), resulting in the expression of red flowers. Thus, in order for a recessive allele to be expressed in the phenotype, only the recessive alleles can be present.

Master Non-Mendelian Patterns of Inheritance

There are many types of inheritance that do not follow the Mendelian pattern. Notable ones include: multiple alleles, gene interactions (complementary genes, epistasis and quantitative or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance.

Pleiotropy, the lack of dominance and lethal genes cannot be classified as variations of inheritance since genes can have these behaviors and at the same time obey Mendelian laws.

Mutations and aneuploidies are abnormalities that alter the Mendelian pattern of inheritance as well as mitochondrial inheritance (the passage of mitochondrial DNA from the mother through the cytoplasm of the egg cell to the offspring).

Lack of Dominance

More Bite-Sized Q&As Below

2. What is the genetic condition in which the heterozygous individual has a different phenotype from the homozygous individual?

This condition is called lack of dominance and it can happen in two ways: incomplete dominance or codominance.

In incomplete dominance, the heterozygous individual presents an intermediate phenotype between the two types of homozygous ones, such as in sickle cell anemia, in which the heterozygous individual produces some sick red blood cells and some normal red blood cells. Codominance occurs, for example, in the genetic determination of the MN blood group system, in which the heterozygous individual has a phenotype totally different from the homozygous one, and not an intermediate form.

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3. What is pleiotropy?

Pleiotropy (or pliotropy) is the phenomenon in which a single gene conditions several different phenotypical traits.

Some phenotypical traits may be sensitive to the pleiotropic effects (for example, inhibition) of other genes, even when conditioned by a pair of alleles in simple dominance. A mixture of pleiotropy and gene interaction is characteristic of these cases.

Lethal Genes

4. What are lethal genes?

Lethal genes are genes with at least one allele that, when present in the genotype of an individual, cause death. There are recessive lethal alleles and dominant lethal alleles. (There are also genes with alleles that are dominant when in heterozygosity but lethal when in homozygosity, meaning that the dominance related to the phenotype does not correspond to the dominance related to lethality.)

Multiple Alleles

5. What are multiple alleles? Is there dominance in multiple alleles?

Multiple alleles is the phenomenon in which the same gene has more than two different alleles (in normal Mendelian inheritance, the gene only has two alleles). Obviously, these alleles combine in pairs to form genotypes.

In multiple alleles, relative dominance among the alleles may exist. A typical example of multiple alleles is the inheritance of the ABO blood group system, in which there are three alleles (A, B or O, or IA, IB and i). IA is dominant over i, which is recessive in relation to the other IB allele. IA and IB lack dominance between themselves.

Another example is the color of rabbit fur, which is conditioned by four different alleles (C, Cch, Ch and c). In this case, the dominance relations are C > Cch > Ch > c (the symbol > means “is dominant over”).

Complementary Genes

6. What are gene interactions? What are the three main types of gene interactions?

Gene interactions are the phenomenon in which a given phenotypic trait is conditioned਋y two or more genes (do not confuse this with multiple alleles, in which there is a single gene with three or more alleles).

The three main types of gene interaction are: complementary genes, epistasis and polygenic inheritance (or quantitative inheritance).

7. What are complementary genes? Does this inheritance pattern obey Mendel’s second law?

Complementary genes are different genes that act together to determine a given phenotypic trait.

For example, consider a phenotypical trait conditioned by 2 complementary genes whose alleles are respectively X, x, Y and y. Performing hybridization in F2, 4 different phenotypic forms are obtained: X_Y_ (double dominant), X_yy (dominant for the first pair, recessive for the second), xxY_ (recessive for the first pair, dominant for the second) and xxyy (double recessive). This is what happens, for example, in the color of budgie feathers, in which the double dominant interaction results in green feathers the interaction that is dominant for the first pair and recessive for the second results in yellow feathers the interaction that is recessive for the first pair and dominant for the second leads to blue feathers and the double recessive interaction leads to white feathers.

Each complementary gene segregates independently from the others since they are located in different chromosomes. Therefore, the pattern follows Mendel’s second law (although it does not obey Mendel’s first law).


8. What is epistasis? What is the difference between dominant epistasis and recessive epistasis?

Epistasis is the gene interaction in which a gene (the epistatic gene) can disallow the phenotypical manifestation of another gene (the hypostatic gene). In dominant epistasis, the inhibitor allele is the dominant allele (for example, I) of the epistatic gene and, as result, inhibition occurs in dominant homozygosity (II) or in heterozygosity (Ii). In recessive epistasis, the inhibitor allele is the recessive allele of the epistatic gene (i) and, as a result, inhibition occurs only in recessive homozygosity (ii).

9. In the hybridization of 2 genes (4 different alleles, 2 of each pair), how does epistasis affect the proportion of phenotypic forms in the F2 generation?

In dihybridism without epistasis, double heterozygous parents cross-breed and ਄ phenotypical forms appear in F2. The proportion is 9 individuals double dominant, 3 individuals dominant for the first pair and recessive for the second pair, 3 individuals recessive for the first pair and dominant for the second pair, and 1 individual double recessive (9:3:3:1).

Considering that the epistatic gene is the second pair and that the recessive genotype of the hypostatic gene implies the lack of the characteristic, in the F2 generation of dominant epistasis, the following phenotypic forms would emerge: 13 individuals dominant for the second pair or recessive for the first pair, meaning that, the characteristic is not manifest 3 individuals dominant for the first pair and recessive for the second pair, meaning that the characteristic is manifest. The phenotypical proportion would be 13:3. In recessive epistasis, the phenotypical forms that would emerge in F2 are: 9 individuals double dominant (the characteristic is manifest) and 7 individuals recessive for the first pair or recessive for the second pair, meaning that the characteristic is not manifest. Therefore, the phenotypical proportion would be 9:7.

These examples show how epistasis changes phenotypical forms and proportions, from the normal 9:3:3:1 in F2 to 13:3 in dominant epistasis or to 9:7 in recessive epistasis (note that some forms have even disappeared).

(If the recessive genotype of the hypostatic gene is active, not only meaning that the dominant allele is not manifest, the number of phenotypic forms in F2 changes.) 

Polygenic Inheritance

10. What is polygenic inheritance? How does it work?

Polygenic inheritance, also known as quantitative inheritance, is the gene interaction in which a given trait is conditioned by several different genes with alleles that may or may not contribute to increasing the intensity of the phenotype. These alleles may be contributing or non-contributing and there is no dominance among them. Polygenic inheritance is the type of inheritance, for example, of skin color and stature in humans.

Considering a given species of animal in which the length of the individual is conditioned by the polygenic inheritance of three genes, for the genotype with only non-contributing alleles (aabbcc), a basal phenotype, for example, 30 cm, would emerge. Also considering that, for each contributing allele, a 5 cm increase in the length of the animal is added, in the genotype with only contributing alleles (AABBCC), the animal would present the basal phenotype (30 cm) plus 30 cm more added for each contributing allele, that is, its length would be 60 cm. In the case of triple heterozygosity, for example, the length of the animal would be 45 cm. That is the way polygenic inheritance works.

11. What is the most likely inheritance pattern of a trait with Gaussian proportional distribution of phenotypic forms?

If a trait statistically has a normal (Gaussian, bell curve) distribution of its phenotypical forms, it is probable that it is conditioned by polygenic inheritance (quantitative inheritance).

In quantitative inheritance, the effects of several genes are added to others, making it possible to represent the trait variation of a given population in a Gaussian curve with the heterozygous genotypes in the center, that is, those that appear in larger number, and the homozygous ones on the ends.

12. How can you find the number of pairs of alleles involved in polygenic inheritance by using the number of phenotypic forms of the trait they condition?

Considering “p” the number of phenotypicਏorms and “a” the number of alleles involved in the polygenic inheritance, the formula p = 2a + 1 applies.

(Often, it is not possible to precisely determine the number of phenotypic forms, p, due to the multigenic nature of inheritance, since the observed variation of phenotypes often seems to be a continuum or the trait may suffer from environmental influences.)

Sex-linked Inheritance

13. Why is sex-linked inheritance an example of non-Mendelian inheritance?

Sex-linked inheritance is a type of non-Mendelian inheritance because it opposes Mendel’s first law, which postulates that each trait is always conditioned by two factors (alleles). In non-homologous regions of sex chromosomes, the genotypes of the genes contain only one allele (even in the case of the XX karyotype, in women, one of the X chromosomes is inactive).

Mitochondrial Inheritance

14. What is mitochondrial inheritance?

Mitochondrial inheritance is the passing down of mitochondrial DNA molecules (mtDNA) to the offspring. An individual's entire stock of mtDNA must come from the mother, the maternal grandmother, the maternal great grandmother and so on, since mitochondria are inherited from the cytoplasm of the egg cell (that later composes the cytoplasm of the zygote).

There are several genetic diseases caused by mitochondrial inheritance, such as Leber's hereditary optic neuropathy, which leads to loss of the central vision of both eyes, and Kearns-Sayre syndrome, a neuromuscular disease that causes ophthalmoplegia and muscle fatigue.

Mitochondrial inheritance is an excellent means for the genetic analysis of maternal lineage (just like the Y chromosome is an excellent means of studying paternal lineage).

Now that you have finished studying Non-Mendelian Inheritance, these are your options:

What are the different ways a genetic condition can be inherited?

Some genetic conditions are caused by variants (also known as mutations) in a single gene. These conditions are usually inherited in one of several patterns, depending on the gene involved:

One altered copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. In some cases, an affected person inherits the condition from an affected parent . In others, the condition may result from a new variant in the gene and occur in people with no history of the disorder in their family.

In autosomal recessive inheritance , variants occur in both copies of the gene in each cell. The parents of an individual with an autosomal recessive condition each carry one copy of the altered gene, but they typically do not show signs and symptoms of the condition. Autosomal recessive disorders are typically not seen in every generation of an affected family.

X-linked dominant disorders are caused by variants in genes on the X chromosome. In males (who have only one X chromosome), a variant in the only copy of the gene in each cell causes the disorder. In females (who have two X chromosomes), a variant in one of the two copies of the gene in each cell is sufficient to cause the disorder. Females may experience less severe symptoms of the disorder than males. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

X-linked recessive disorders are also caused by variants in genes on the X chromosome. In males (who have only one X chromosome), one altered copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), a variant would have to occur in both copies of the gene to cause the disorder. Because it is unlikely that females will have two altered copies of this gene, males are affected by X-linked recessive disorders much more frequently than females. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

Because the inheritance pattern of many X-linked disorders is not clearly dominant or recessive, some experts suggest that conditions be considered X-linked rather than X-linked dominant or X-linked recessive. X-linked disorders are caused by variants in genes on the X chromosome , one of the two sex chromosomes in each cell. In males (who have only one X chromosome), an alteration in the only copy of the gene in each cell is sufficient to cause the condition. In females (who have two X chromosomes), one altered copy of the gene usually leads to less severe health problems than those in affected males, or it may cause no signs or symptoms at all. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).

A condition is considered Y-linked if the altered gene that causes the disorder is located on the Y chromosome, one of the two sex chromosomes in each of a male's cells. Because only males have a Y chromosome, in Y-linked inheritance, a variant can only be passed from father to son.

In codominant inheritance , two different versions (alleles) of a gene are expressed, and each version makes a slightly different protein. Both alleles influence the genetic trait or determine the characteristics of the genetic condition.

Mitochondrial inheritance , also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondria, which are structures in each cell that convert molecules into energy, each contain a small amount of DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial variants to their children. Conditions resulting from variants in mitochondrial DNA can appear in every generation of a family and can affect both males and females, but fathers do not pass these disorders to their daughters or sons.

Many health conditions are caused by the combined effects of multiple genes (described as polygenic) or by interactions between genes and the environment. Such disorders usually do not follow the patterns of inheritance listed above. Examples of conditions caused by variants in multiple genes or gene/environment interactions include heart disease, type 2 diabetes, schizophrenia, and certain types of cancer. For more information, please see What are complex or multifactorial disorders?

Disorders caused by changes in the number or structure of chromosomes also do not follow the straightforward patterns of inheritance listed above. To read about how chromosomal conditions occur, please see Are chromosomal disorders inherited?

Other genetic factors sometimes influence how a disorder is inherited. For an example, please see What are genomic imprinting and uniparental disomy?

Inheritance Patterns for a Lethal Gene

1. A short-tailed mutant of mouse was discovered. Multiple crosses of this mouse to normal mice produced 27 normal, long-tailed mice and 25 short-tailed mice. A series of crosses among short tailed mice were made and 21 short-tailed mice and 11 long-tailed mice were produced. Determine which phenotype is dominant and explain the ratios observed with regards to the genotypes of the parents in each cross

2. In a species of beetle, the wing covers can be either green, blue or turquoise. From an interbreeding, mixed, laboratory stock population, individual virgin beetles were selected and mated in specific controlled crosses to determine the inheritance of wing-cover color. The results were as follows:
Cross Parents Progeny
1 bluexgreen all blue
2 bluexblue 3/4 blue, 1/4 turquoise
3 green x green 3/4 green, 1/4 turquoise
4 blue x turquoise 1/2 blue, 1/2 turquoise
5 blue x blue 3/4 blue 1/4 green
6 blue x green 1/2 blue,1/2 green
7 blue x green 1/2 blue, 1/4 green, 1/4 turquoise
8 turquoise x turquoise all turquoise
Suggest a hypothesis how these traits are determined which would explain all results. Write the genotypes of the parents and the progeny
Cross Parents Progeny


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