Chapter 4, Problem 33P

In 1919, Calvin Bridges began studying an X-linked recessive mutation causing eosin-colored eyes in Drosophila. Within an otherwise true-breeding culture of eosin-eyed flies, he noticed rare variants that had much lighter cream-colored eyes. By intercrossing these variants, he was able to make a true-breeding cream-eyed stock. Bridges now crossed males from this cream-eyed stock with true-breeding wild-type females. All the F progeny had red (wild-type) eyes. When F flies were intercrossed, the F progeny were 104 females with red eyes, 52 males with red eyes, 44 males with eosin eyes, and 14 males with cream eyes. Assume that these numbers represent an 8:4:3:1 ratio.

1. Formulate a hypothesis to a. explain the F₁ and F₂ results, assigning phenotypes to all possible genotypes?

2. What do you predict in the F₁ and F₂ generations if the parental cross is between true-breeding eosin eyed males
   and true-breeding cream eyed females?

3. What do you predict in the F₁ and F₂ generations if the parental cross is between true-breeding eosin eyed females
   and true-breeding cream eyed males?

Summary Introduction

a.

To determine:

The hypothesis derived from the results obtained from crossing true breeding cream-colored male and true-breeding wild female.

Introduction:

Fertilization is the process by which the male gamete, sperm, fertilize with the female gamete, ovary. The process brings about the fusion of two haploid pronuclei into a diploid zygote.

Explanation of Solution

The hypothesis that can be derived from the results is that mutation in the wild type allele results in the production of eosin eyed progeny. The presence of criss-cross inheritance in the wild and the mutated variant results in the production of cream eyed progeny. This hypothesis supports the presence of criss-cross inheritance.

Summary Introduction

b.

To determine:

The predicted progeny in F₁ and F₂ generation if the cross is made between true-breeding eosin eyed males and true-breeding cream-colored females.

Introduction:

The dominant allele masks the expression of the recessive allele. Therefore, the dominant allele is expressed in homozygous and heterozygous genotype while the recessive phenotype is only expressed under homozygous condition.

Explanation of Solution

The progeny from the cross between true-breeding eosin eyed males, and true-breeding cream-colored females are as follows:

Gametes	Xe	Y
Xc	Xe Xc	Xc Y
Xc	Xe Xc	Xc Y

The F₂ progeny is derived as follows:

Gametes	Xc	Y
Xe	Xe Xc	Xe Y
Xc	Xc Xc	Xc Y

The progeny obtained will be cream eyed males (X^c Y), eosin eyed males (X^e Y), cream-colored females (X^c X^c) and variant cream colored female (X^e X^c).

Summary Introduction

b.

To determine:

The predicted progeny in F₁ and F₂ generation if the cross is made between true-breeding eosin eyed females and true-breeding cream colored males.

Introduction:

The Punnett square is used to depict the genetic cross between two parents and all the possibilities of the resulting progeny that can help in identifying the genotype and phenotype of the hybrids produced.

Explanation of Solution

The progeny from the cross between true-breeding eosin eyed females, and true-breeding cream colored males are as follows:

Gametes	Xc	Y
Xe	Xc Xe	Xe Y
Xe	Xc Xe	Xe Y

The F2 progeny will be derived as follows:

Gametes	Xe	Y
Xe	Xe Xe	Xe Y
Xc	Xe Xc	Xc Y

The progeny obtained will have male genotype X^c Y, and X^e Y and females will have genotype X^e X^e and X^e X^c