53. A cross between a red eyed male fly and white eyed female fly produces red eyed female and white eyed male progenies. While reciprocal cross produces all offsprings with red eyes. The trait for eye color is
(1) Sex linked traits
(2) Sex influenced trait
(3) Sex linked homogametic male
(4) Sex linked heterogametic male
Introduction
Eye color inheritance in Drosophila (fruit flies) is a classic example of sex linked traits, where the gene controlling eye color is located on the X chromosome. This results in distinctive patterns of inheritance between males and females due to their differing sex chromosomes. Understanding this trait helps clarify broader concepts in genetics related to sex chromosomes.
Explanation of the given cross and options
In the problem, crossing a red-eyed male (XY, with dominant allele) to a white-eyed female (XX, recessive homozygous) produces red-eyed females and white-eyed males. The reciprocal cross (white-eyed male × red-eyed female) produces all red-eyed offspring. This pattern suggests the trait is controlled by a gene on the X chromosome showing typical sex linked inheritance.
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(1) Sex linked traits: Correct option. The eye color gene is on the X chromosome. Males (XY) have only one X, so the allele they carry is fully expressed (hemizygous). Females (XX) have two X chromosomes, so they can be heterozygous carriers if they carry one dominant and one recessive allele. This explains why in the first cross white-eyed females (homozygous recessive) produce white-eyed males (inheriting their only X from mother) and red-eyed females (inheriting dominant X from father).
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(2) Sex influenced trait: Incorrect. Sex influenced traits are autosomal but expressed differently in males and females due to hormonal influences. Eye color in Drosophila is not influenced by hormonal differences but by chromosome-linked inheritance.
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(3) & (4) Sex linked homogametic male and Sex linked heterogametic male: Incorrect. In Drosophila, females are homogametic (XX) and males heterogametic (XY). The terms contradict the known sex chromosome system in flies.
How sex linked inheritance explains the cross outcomes
The red eye allele (X^W) is dominant over the white eye allele (X^w).
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In the first cross: red-eyed male (X^W Y) × white-eyed female (X^w X^w)
Offspring:-
Females inherit X^W from father and X^w from mother = red-eyed (heterozygous)
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Males inherit Y from father and X^w from mother = white-eyed (hemizygous recessive)
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Reciprocal cross: white-eyed male (X^w Y) × red-eyed female (X^W X^W)
Offspring: all inherit at least one X^W allele, so all have red eyes.
Summary table of options
| Option | Explanation | Correct/Incorrect |
|---|---|---|
| Sex linked traits | Trait on X chromosome causing inheritance pattern described | Correct |
| Sex influenced trait | Autosomal trait influenced by sex hormones | Incorrect |
| Sex linked homogametic male | Incorrect; males are heterogametic XY in flies | Incorrect |
| Sex linked heterogametic male | Incorrect; males are heterogametic, females homogametic | Incorrect |
This classical example confirms that eye color in Drosophila is a sex linked trait, with inheritance patterns consistent with genes located on the X chromosome, as demonstrated by Thomas Hunt Morgan’s experiments. It highlights the difference in allele expression between hemizygous males and homozygous or heterozygous females.
This article explains how the fly eye color inheritance pattern matches “sex linked traits,” clarifies the terminology of sex chromosomes in Drosophila, and disproves other options. These insights are crucial for understanding sex linked genetics in fruit flies and serve as an important foundation for genetics education.
