Sex-linked Crosses – Complete punnett squares to answer the question below. 1. Red-green colorblindness is a recessive allele carried on the X chromosome. A colorblind male mates with a non-colorblind heterozygous female. What are the expected ratios for genotype and phenotype of the offspring? Genotypic Ratios Phenotypic Ratios
The Correct Answer and Explanation is:
To solve this, let’s break it down step by step and use a Punnett square to find the genotypic and phenotypic ratios.
Given:
- Red-green colorblindness is recessive and located on the X chromosome.
- A colorblind male has the genotype XᶜY, where Xᶜ is the X chromosome with the colorblind allele and Y is the Y chromosome.
- A heterozygous female has the genotype XᶜX, where Xᶜ carries the colorblind allele and X carries the normal vision allele.
Cross:
- Male: XᶜY
- Female: XᶜX
Punnett Square:
The offspring will inherit one X chromosome from the mother and either an X or a Y chromosome from the father.
| Mother\Father | Xᶜ | Y |
|---|---|---|
| Xᶜ | XᶜXᶜ | XᶜY |
| X | XᶜX | XY |
Genotypic Ratios:
From the Punnett square, we get the following genotypes for the offspring:
- XᶜXᶜ (Colorblind female): 1
- XᶜX (Normal vision female but a carrier): 1
- XᶜY (Colorblind male): 1
- XY (Normal vision male): 1
Genotypic Ratio: 1 XᶜXᶜ : 1 XᶜX : 1 XᶜY : 1 XY
Phenotypic Ratios:
- Colorblind females (XᶜXᶜ) and colorblind males (XᶜY) will exhibit colorblindness.
- Normal vision females (XᶜX) and normal vision males (XY) will not be colorblind.
So, the phenotypic ratio will be:
- Colorblind offspring: 2 (1 XᶜXᶜ female + 1 XᶜY male)
- Normal vision offspring: 2 (1 XᶜX female + 1 XY male)
Phenotypic Ratio: 2 colorblind : 2 normal vision
Conclusion:
- Genotypic Ratio: 1 XᶜXᶜ : 1 XᶜX : 1 XᶜY : 1 XY
- Phenotypic Ratio: 2 colorblind : 2 normal vision
This cross demonstrates how X-linked recessive traits are inherited differently in males and females, as males only have one X chromosome and thus will express any X-linked recessive trait.
