Practice Problems, p. 158. 2. Color-blindness is an X-linked recessive disorder.

Question

Practice Problems, p. 158.
2.  Color-blindness is an X-linked recessive disorder.  Mike is color-blind.  His wife, Meg, is homozygous for the normal color vision allele.  Using Punnett squares, derive and compare the genotypic and phenotypic ratios for the offspring of this marriage.
a.  What is Mikes's genotype?

b.  What is Meg's genotype?

c.  What is the genotypic ratio of offspring?

d.  What is the phenotypic ratio of offspring?

e.  If they have eight children, how many of them would you expect to be color-blind?

Explanation / Answer

Colour blindness is an X-linked recessive disorder.

So Mikes's genotype will be XcY where Xc stands for X-chromosome that carries colour blindness gene.

Meg has homozygous normal colour vision alleles.

So the genotype of Meg is XX. Where X stands for normal chromosome.

So the genotypic ratio of offspring is 1:1 where 50% offspring has carrier gene and 50% have normal genes.

Phenotypic ratio is 1 i.e. NO offspring will be colour blind.

If they have 8 children then no child will be colour blind.

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