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. In this case, 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, while 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. Finally, epistasis can be reciprocal: either gene, when present in the dominant or recessive form, expresses the same phenotype. 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; a cross between heterozygotes for both genes AaBb x AaBb would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.
Keep in mind that any single characteristic that results in a phenotypic ratio that totals 16 is typical of a two-gene interaction. What could explain this variation from Mendelian ratios? Bateson set out to answer this question in a report, in which he first proposed what he called the ability of one "allelomorphic pair" pair of gene alleles to mask the affects of the alleles for another gene.
To rephrase this in terms of Bateson and Punnett's pea experiment, it seemed that two recessive alleles at one flower locus could mask the effects of the alleles at the other flower locus. Let's designate the first locus as the C locus, and the second as the P locus.
If Bateson's theory held true, it meant that any flower with the cc genotype would be white, no matter what alleles were present at its P locus. Similarly, any flower with the pp genotype would also be white, no matter what alleles were present at its C locus. Bateson later used the word "epistasis," which translates as "standing upon," to define the masking action of one gene by another. Since then, scientists have come to understand that genes can interact in more ways than just masking.
Many years after Bateson first described this phenotypic ratio in pea plants, researchers were finally able to determine the two genes responsible for it Dooner et al.
These genes control flower color by controlling pea plant biochemistry, in particular that related to pigment compounds called anthocyanins. In peas, there is a two-step chemical reaction that forms anthocyanins; gene C is responsible for the first step, and gene P is responsible for the second Figure 2. If either step is nonfunctional, then no purple pigment is produced, and the affected pea plant bears only white flowers.
The dominant C and P alleles code for functional steps in anthocyanin production, whereas the recessive c and p alleles code for nonfunctional steps. Thus, if two recessive alleles occur for either gene, white flowers will result.
Table 2 shows in detail how the ratio is a modification of phenotypic but not genotypic Mendelian ratios. Note that the C and P genes independently assort, and remember that the presence of a recessive genotype at one locus i. Note also that there are nine combinations of alleles in the F 1 generation that feature at least one dominant C and one dominant P allele, which would yield a purple flower phenotype indicated within the table by purple shading.
Conversely, there are seven combinations that result in either a cc or a pp , which would yield the white flower phenotype-hence, the ratio of purple to white flowers.
So, if two plants with genotype KkDd are crossed with each other, what is the ratio of blue offspring to nonblue offspring? The results of such a cross are detailed in Table 3. In Table 3, all of the shaded boxes contain genotypes that indicate an absence of malvidin.
In particular, the yellow boxes represent genotypes that feature at least one D allele, and the presence of the D allele suppresses the production of malvidin. Meanwhile, the light turquoise box represents a genotype with no malvidin suppression dd , but also no malvidin production kk.
The phenotypic ratio is therefore This type of epistasis is sometimes called dominant suppression, because the deviation from is caused by a single allele that produces a dominant phenotype, and the action of this allele is to suppress the expression of some other gene. The mechanism by which wheat kernel color is determined is an example of duplicate gene action.
In wheat, kernel color is dependent upon a biochemical reaction that converts a precursor substance into a pigment, and this reaction can be performed with the product of either gene A or gene B Figure 4. Thus, having either an A allele or a B allele produces color in the kernel, but a lack of either allele will produce a white kernel that is devoid of color.
Table 4 depicts the results of a dihybrid cross between two plants with genotype AaBb. Note that among the offspring of this cross, the phenotype is nearly uniform. In this cross, whenever a dominant allele is present at either locus, the biochemical conversion occurs, and a colored kernel results.
Thus, only the double homozygous recessive genotype produces a phenotype with no color, and the resulting phenotypic ratio of color to noncolor is In the malvidin example in Table 3, gene D is epistatic to gene K because one allelic pair masks the expression of an allele at the second locus. In the case of duplicate gene action , as in Table 4, the outcome is less variable, but it is still derived from multiple gene interactions.
Here, if wheat kernel color is controlled by genes A and B , then A is epistatic to allele B or allele b , and B is epistatic to allele A or allele a. Yet another type of epistasis occurs when one gene interacts with another to modify—but not mask—a phenotype.
For example, in horses, the extension gene determines whether an animal's coat color will be red or black; here, the dominant allele E produces black pigment in the coat, while the recessive allele e produces red pigment.
All horses with genotype ee are therefore red, yet there are many different types of red horses. These differences exist because of the action of epistatic modifier genes.
One such modifier gene is called cream dilution. The cream dilution gene has two alleles: C Cr and C. The C Cr allele is semidominant; it dilutes red to yellow in the heterozygous state and red to pale cream in the homozygous state. On the other hand, the C allele has no diluting effect on coat color.
Thus, horses with genotype eeCC are chestnut colored, and they have reddish-brown coats, tails, and manes. Today, scientists know that Mendel's predictions about inheritance depended on the genes he chose to study. One of the genes that's needed to make color is tyrosinase, and if you don't have tyrosinase, if it's mutated, it results in a condition call albinism.
So the fur or the skin's completely white because you can't make the pigment. If there's another gene involved in making pigment--for example, making brown pigment--if you're wild-type for tyrosinase, you can see the brown mutation, and you have a brown animal.
But if you have a tyrosinase mutation and a brown mutation, it still looks like tyrosinase mutation; it's all white. You don't see whether it has the brown mutation or not. 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.
Finally, epistasis can be reciprocal such that either gene, when present in the dominant or recessive form, expresses the same phenotype. 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. 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.
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