60. In some sheep, horns are produced by an autosomal allele, ‘H’ that is dominant in males and recessive in females. H⁺H⁺ individuals are hornless. 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 of F1 will have horns?
(1) 50% of male and 50% of female progeny will be horned
(2) 50% of male progeny but none of the female progeny will be horned
(3) 25% of male and 25% of female progeny will be horned
(4) 100% of male progeny and 50% of female progeny will be horned

In some sheep, horns are produced by an autosomal allele H that is dominant in males and recessive in females. Here, H+H+ individuals are hornless. In this problem, a horned female (who must be HH because H is recessive in females) is crossed with a hornless male (H+H+ because hornless males are homozygous for the recessive allele). The F1 offspring of this cross will be heterozygous H+H.

When one of the resulting F1 females (genotype H+H) is crossed with a hornless male (H+H+), the progeny genotypes and phenotypes can be analyzed based on sex and mode of dominance:

• Males: Since H is dominant in males, the heterozygous genotype H+H will be horned, and the homozygous H+H+ will be hornless. The cross produces 50% H+H (horned) and 50% H+H+ (hornless) males.

• Females: Since H is recessive in females, only HH genotypes will be horned. All females from the cross will inherit H+ from the female parent and H+ from the male (H+H+), so all females will be hornless (H+H or H+H+), 0% horned.

Therefore, the proportion of horned progeny among males is 50%, and among females is 0%. This corresponds to option (2).

Explanation of other options:

• (1) 50% male and 50% female horned: Incorrect because females require HH for horns, which is absent.

• (3) 25% male and 25% female horned: Incorrect percentage calculations based on genotype.

• (4) 100% male and 50% female horned: Incorrect, as not all males will be horned and no females will be horned.

Introduction

The inheritance of horns in some sheep breeds is controlled by an autosomal allele that behaves differently depending on the sex of the animal. This allele, known as “H,” is dominant in males but recessive in females, resulting in sex-influenced horn production. Understanding this unique pattern is crucial for breeders and geneticists studying sheep phenotypes. This article explains a typical genetic cross involving horned and hornless sheep to clarify the proportions of horned progeny among males and females.

Genetic Basis of Horn Inheritance

The autosomal allele H produces horns with sex-dependent dominance:

• In males: H is dominant over the hornless allele H+.

• In females: H is recessive, so only homozygous HH females have horns; heterozygotes and H+H+ females are hornless.

When crossing a horned female (HH) with a hornless male (H+H+), all offspring inherit H+ from the male and H from the female, resulting in heterozygous H+H genotype.

Cross Analysis and Progeny Outcomes

1. First Cross: Horned Female (HH) x Hornless Male (H+H+)

   • All F1 offspring are H+H.

   • Phenotypically: males are horned (due to dominance), females are hornless (due to recessive expression).

2. Second Cross: F1 Female (H+H) x Hornless Male (H+H+)

   • Male progeny: 50% H+H (horned), 50% H+H+ (hornless).

   • Female progeny: all hornless, as no female inherits HH genotype.

Conclusion

The detailed genetic cross shows that 50% of male progeny will have horns while none of the female progeny will have horns. This pattern highlights the sex-influenced manner in which the autosomal horn allele expresses itself in sheep.

This explanation and genetic insight can assist students, educators, and breeders in understanding the complex inheritance patterns of sexually dimorphic traits such as horns in sheep.