Hh

Male phenotype horned horned hornless

Female phenotype horned hornless hornless

(b) We could carry out a number of different crosses to test our hypothesis that yellow is a recessive lethal and straight is dominant over fuzzy. For example, a cross between any two yellow individuals should always produce 2/3 yellow and 1/3 gray, and a cross between two gray individuals should produce all gray offspring. A cross between two fuzzy individuals should always produce all fuzzy offspring.

4. In some sheep, the presence of horns is produced by an autosomal allele that is dominant in males and recessive in females. A horned female is crossed with a hornless male. One of the resulting F1 females is crossed with a hornless male. What proportion of the male and female progeny from this cross will have horns?

The presence of horns in these sheep is an example of a sex-influenced characteristic. Because the phenotypes associated with the genotypes differ for the two sexes, let's begin this problem by writing out the genotypes and phenotypes for each sex. We will

In the problem, a horned female is crossed with a hornless male. From the preceding table, we see that a horned female must be homozygous for the allele for horns (HH) and a hornless male must be homozygous for the allele for hornless (H+H+); so all the F1 will be heterozygous; the F1 males will be horned and the F1 females will be hornless, as shown below:

F1 H+H H+H horned males and hornless females

A heterozygous hornless F1 female (H+H) is then crossed with a hornless male (H+H+):

H+H X H+H+ horned female hornless male

Males hornless horned

Females hornless hornless

Therefore, 1/2 of the male progeny will be horned but none of the female progeny will be horned.

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