If a cross between two mice produce 10 black mice and 3 white mice. which skin color is dominant?

Mendel's First Law

Variations to Mendel's First Law

Pedigree Analysis

Mendel's Second Law

Chi-Square Test

Pleiotropy

Epistasis

Modifier Genes

Penetrance and Expressivity

Study Questions

Mendelian Genetics Overheads

Mendelian Genetics WWW Links

Genetic Topics

Pleiotropic Effects and Lethal Genes

During the first years after the rediscovery of Mendel's laws, a number of experiments were performed that gave results that at first glance did not coincide with the laws. In 1904, a cross was made between a yellow-coated mouse and a mouse with a gray coat. The gray- coated mouse was extensively inbred and therefore was considered to be pure bred.

What allelic relationship do we have here? We know that the gray mouse is homozygous (because it is a pure line). If gray coat was dominant then we would see all gray mouse. Since we obtain both yellow and gray mice, yellow must be dominant to gray. So what are the genotypes of the two mice populations? First, let's provide gene symbols.

Gene Symbols:

gray = y yellow = Y

From the above discussion, the genotype of the gray mouse must be yy. What is the genotype of the yellow mouse? If the mouse was homozygous we would not see any gray mice from the cross, therefore the genotype must be heterozygous or Yy.

Next a cross was made between two yellow mice. What genetic ratio would we expect to see? Yy x Yy should give a ratio of 3 yellow:1 gray. The result, though, was a ratio of 2 yellow to 1 gray mice. How can this result be explained? Let's first set up a Punnett Square.

Expected Punnett Square

		Female Gametes
		Y	y
Male Gametes	Y	YY (yellow)	Yy (yellow)
y	Yy (yellow)	yy (gray)

As we can see, we should get a 3:1 ratio of yellow to gray mice. Could some genotype be absent from the progeny. How can we test the genotypes of the yellow mice, since we already know the genotypes of the gray mice are yy. Testcross!! All testcross data with the yellow mice give a 1:1 ratio. This ratio is typical of what is seen with heterozygous individuals. Therefore, all of the yellow mice from the cross of two heterozygous yellow mice are genotypically Yy. Somehow the YY genotype is lethal. The 2:1 ratio is the typical ratio for a lethal gene.

Coat Color in Mice

Lethal Gene - a gene that leads to the death of an individual; these can be either dominant or recessive in nature

An important question is how can a gene controlling coat color cause death in an organism? Possibly in a single dose the allele causes a yellowing of the coat, but when expressed in two doses, the gene product kills the animal. Thus, this gene actually has an effect on two phenotypes.

Pleiotropic gene - a gene that affects more than one phenotype

In this example the gene that causes yellowing of the coat also affects viability and is termed a pleiotropic gene.

Copyright © 2000. Phillip McClean

Learning Outcomes

• Explain how epistasis impacts trait expression

Mendel’s studies in pea plants implied that the sum of an individual’s phenotype was controlled by genes (or as he called them, unit factors), such that every characteristic was distinctly and completely controlled by a single gene. In fact, single observable characteristics are almost always under the influence of multiple genes (each with two or more alleles) acting in unison. For example, at least eight genes contribute to eye color in humans.

Eye color in humans is determined by multiple genes. Use the Eye Color Calculator to predict the eye color of children from parental eye color.

In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other, with one gene modifying the expression of another.

In epistasis, the interaction between genes is antagonistic, such that one gene masks or interferes with the expression of another. “Epistasis” is a word composed of Greek roots that mean “standing upon.” The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking. Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA), is dominant to solid-colored fur (aa). However, a separate gene (C) is necessary for pigment production. A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A (Figure 1). Therefore, the genotypes AAcc, Aacc, and aacc all produce the same albino phenotype. A cross between heterozygotes for both genes (AaCc x AaCc) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino (Figure 1). In this case, the C gene is epistatic to the A gene.

Figure 1. In mice, the mottled agouti coat color (A) is dominant to a solid coloration, such as black or gray. A gene at a separate locus (C) is responsible for pigment production. The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc enotype is albino regardless of the allele present at the A locus. Thus, the C gene is epistatic to the A gene.

Epistasis can also occur when a dominant allele masks expression at a separate gene. Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the W gene (ww) coupled with homozygous dominant or heterozygous expression of the Y gene (YY or Yy) generates yellow fruit, and the wwyy genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. A cross between white heterozygotes for both genes (WwYy × WwYy) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.

Finally, epistasis can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype. In the shepherd’s purse plant (Capsella bursa-pastoris), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes A and B are both homozygous recessive (aabb), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than aabb results in triangular seeds, and a cross between heterozygotes for both genes (AaBb x AaBb) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.

As you work through genetics problems, keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. Recall the phenotypic inheritance pattern for Mendel’s dihybrid cross, which considered two non-interacting genes—9:3:3:1. Similarly, we would expect interacting gene pairs to also exhibit ratios expressed as 16 parts. Note that we are assuming the interacting genes are not linked; they are still assorting independently into gametes.

For an excellent review of Mendel’s experiments and to perform your own crosses and identify patterns of inheritance, visit the Mendel’s Peas web lab.

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.

In Summary: Epistasis

Epistasis is a form on non-Mendelian inheritance in which one gene is capable of interfering with expression of another. This is often found associated with gene pathways where the expression of one gene is directly dependent on the presence or absence of another gene product within the pathway.

Watch this video for a quick review of epistasis:

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