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# Genetic Interactions Practice Problems

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By — McGraw-Hill Professional
Updated on Sep 20, 2011

Review the following concepts if needed:

## Genetic Interactions Practice Problems

#### Practice 1

Coat colors of dogs depend upon the action of at least two genes. At one locus a dominant epistatic inhibitor of coat color pigment (I-) prevents the expression of color alleles at another independently assorting locus, producing white coat color. When the recessive condition exists at the inhibitor locus (ii), the alleles of the hypostatic locus may be expressed, iiB- producing black and iibb producing brown. When dihybrid white dogs are mated together, determine (a) the phenotypic proportions expected in the progeny, (b) the chance of choosing, from among the white progeny, a genotype that is homozygous at both loci.

#### Solution 1

1. The genotypic proportions among the white progeny are as follows:
2. The only homozygous genotypes at both loci in the above list are 1/12 IIBB and 1/2 IIbb = 2/12 or 1/6 of all the white progeny. Thus there is 1 chance in 6 of choosing a homozygous genotype from among the white progeny.

#### Practice 2

Matings between black rats of identical genotype produced offspring as follows: 14 cream-colored, 47 black, and 19 albino. (a)What epistatic ratio is approximated by these offspring? (b)What are the genotypes of the parents and the offspring (use your own symbols)?

#### Solution 2

1. The total number of offspring is 80 (14 + 47 + 19) and 80/16 = 5. So 1/16 of 80 = 5. In order to figure out what the ratio is, use this factor. So, 5 × 3 = 15, which is close to 14; 5 × 4 = 20, which is close to 19; and 5 ×9 = 45, which is close to 47. Thus, the ratio is 9 : 3 : 4 (the numbers are not given in this order). This is an example of recessive epistasis.
2. This translates to a 9 black : 3 cream : 4 albino ratio; cc, the recessive condition, is epistatic to alleles at locus B.

#### Practice 3

Red color in wheat kernels is produced by the genotype R-B-, white by the double-recessive genotype (rrbb). The genotypes R-bb and rrB- produce brown kernels. Ahomozygous red variety is crossed to awhite variety. (a) What phenotypic results are expected in the F1 and F2? (b) If the brown F2 is artificially crossed at random (wheat is normally self-fertilized), what phenotypic and genotypic proportions are expected in the offspring?

#### Solution 3

1. The proportion of genotypes represented among the brown F2 must first be determined.
2. Next, the relative frequencies of the various matings may be calculated in a Punnett square.

#### Practice 4

A plant of the genus Capsella, commonly called "shepherd's purse," produces a seed capsule, the shape of which is controlled by two independently assorting genes, represented by symbols A and B. When dihybrid plants were interpollinated, 6% of the progeny were found to possess ovoid-shaped seed capsules. The other 94% of the progeny had triangular-shaped seed capsules. (a)What two-factor epistatic ratio is approximated by the progeny? (b) What is the genotype of the ovoid-shaped seed capsules?

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