Because 9/16 of the progeny from the corn cross are purple, purple must be produced by genotypes A_B_; in other words, individual kernels that have at least one dominant allele at the first locus and at least one dominant allele at the second locus are purple. The proportions of all the other genotypes (A bb, aaB , and aabb) sum to 7/16, which is the proportion of the progeny in the corn cross that are yellow, so any individual kernel that does not have a dominant allele at both the first and the second locus is yellow.
(b) Kernel color is an example of duplicate recessive epistasis, where the presence of two recessive alleles at either the first locus or the second locus or both suppresses the production of purple pigment.
3. A geneticist crosses two yellow mice with straight hair and obtains the following progeny:
1/2 yellow, straight 1/6 yellow, fuzzy 1/4 gray, straight 1/12 gray, fuzzy
(a) Provide a genetic explanation for the results and assign genotypes to the parents and progeny of this cross.
(b) What additional crosses might be carried out to determine if your explanation is correct?
(a) This cross concerns two separate characteristics — color and type of hair; so we should begin by examining the results for each characteristic separately. First, let's look at the inheritance of color. Two yellow mice are crossed producing 1/2 + /6 = 3/6 + /6 = 4/6 = 2/3 yellow mice and V4 + = ^ + 1/12 = 4/12 = 1/3 gray mice. We learned in this chapter that a 2:1 ratio is often produced when a recessive lethal gene is present:
The probability associated with the chi-square value is greater than .05, indicating that there is a relatively good fit between the observed results and a 9:7 ratio.
We now need to determine how a dihybrid cross can produce a 9:7 ratio and what genotypes correspond to the two phenotypes. A dihybrid cross without epistasis produces a 9:3:3:1 ratio:
AaBb X AaBb
YY 1/4 die
Yy 1/2 yellow, becomes 2/3 yy 1/4 gray, becomes 1/3
Now, let's examine the inheritance of the hair type. Two mice with straight hair are crossed, producing 1/2 + 1/4 = 2/4 + 1/4 = 3/4 mice with straight hair and 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4 mice with fuzzy hair. We learned in Chapter 3 that a
3:1 ratio is usually produced by a cross between two individuals heterozygous for a simple dominant allele:
Ss X Ss
1/4 straight 1/2 straight
3/4 straight ss 1/4 fuzzy
We can now combine both loci and assign genotypes to all the individuals in the cross:
P yellow, straight X yellow, straight YySs YySs
Phenotype yellow, straight yellow, fuzzy gray, straight gray, fuzzy
YyS_ Yyss yyS_ yyss
Probability at each locus
let H represent the allele that codes for horns and H+ represent the allele for hornless. In males, the allele for horns is dominant over the allele for hornless, which means that males homozygous (HH) and heterozygous (H+H) for this gene are horned. Only males homozygous for the recessive hornless allele (H+H+) will be hornless. In females, the allele for horns is recessive, which means that only females homozygous for this allele (HH) will be horned; females heterozygous (H+H) and homozygous (H+H+) for the hornless allele will be hornless. The following table summarizes genotypes and associated phenotypes:
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