59. In an organism, allele for red eye colour is dominant over the allele for white eye colour. A cross is made between a white eyed male and a red eyed female. In the progeny all males are red eyed while the females are white eyed. The reciprocal cross leads to all red eyed progeny, Based on the above information which one of the following conclusions is correct?
(1) This is a sex-limited trait, and the male is the homomorphic sex
(2) This is a sex-linked trait, with male being the homomorphic sex
(3) This is a sex-linked trait, with female being the homomorphic sex
(4) This is a case of autosomal inheritance, with incomplete penetrance

Introduction

In genetic studies, the inheritance pattern of traits like eye color often reveals whether a trait is autosomal, sex-linked, sex-limited, or influenced by penetrance. A classic problem involves a cross between white-eyed males and red-eyed females, resulting in all red-eyed males and white-eyed females in the progeny, while the reciprocal cross produces all red-eyed offspring. Understanding this pattern is essential for competitive exams and genetics courses.

Detailed Solution and Option Explanation

Sex-Linked Inheritance Pattern

• In this scenario, the allele for red eye color is dominant over white and the trait shows a specific pattern based on the sex of the parent and offspring.

• When a white-eyed male (genotype XᵂY) is crossed with a red-eyed female (likely homozygous XᴿXᴿ), all male progeny inherit the Xᴿ from the mother and Y from the father, making them red-eyed (XᴿY), and all female progeny inherit Xᴿ from the mother and Xᵂ from the father, resulting in white-eyed females (XᴿXᵂ) because in Drosophila, red is dominant.​

• The reciprocal cross (red-eyed male and white-eyed female) produces all red-eyed offspring because males get Xˢ from the mother and Y from the father, and females get Xˢ from both parents.​

• This confirms X-linked inheritance, with females (XX) being the homomorphic sex (same shape chromosomes) and males (XY) heteromorphic (different shape chromosomes).​

Explanation of Each Option

• Option 1: Sex-limited trait, male homomorphic sex
  Sex-limited traits are expressed only in one sex due to hormonal or anatomical restrictions, not because of chromosome-linked inheritance. This trait is seen in both males and females, so it’s not sex-limited.​​

• Option 2: Sex-linked trait, male homomorphic sex
  A sex-linked trait means the gene is located on a sex chromosome. Typically, in species like humans and Drosophila, males (XY) are the heteromorphic sex, not homomorphic.​

• Option 3: Sex-linked trait, female homomorphic sex
  Correct answer. Here, the trait is X-linked, and females (XX) have two identical sex chromosomes (homomorphic), while males (XY) have heteromorphic sex chromosomes. The cross results fit classic sex-linked inheritance.

• Option 4: Autosomal inheritance, incomplete penetrance
  Autosomal inheritance would yield both red and white eyes in both sexes if dominant/recessive alleles and no sex linkage, which does not fit the cross pattern observed. Incomplete penetrance means not all individuals with the allele express the trait, but in this case, the phenotype matches Mendelian expectations for sex-linked inheritance.​

Summary Table: Options vs. Observations

This detailed genetics explanation helps clarify why option 3 is correct and addresses each alternative based on scientific reasoning and inheritance models.​