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Color-blindness is an X-linked recessive disorder. Mike is color-blind. His wife, Meg, is homozygous for 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?

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namordu

Answer:

a) XᵇY; b) XᴮXᴮ; c) 1 XᴮXᵇ : 1 XᴮY; d) 1 normal vision children; e) no children will be color-blind.

Explanation:

Color-blindness is an X-linked recessive disorder. Women have two X chromosomes, while men have one X and one Y chromosome.

The possible genotypes and phenotypes for this trait are:

  • XᴮXᴮ or XᴮXᵇ: woman with normal color vision
  • XᵇXᵇ: color blind woman
  • XᴮY: man with normal color vision
  • XᵇY: color blind man

Mike is color blind, so his genotype is XᵇY. Meg is homozygous for normal color vision, therefore her genotype is XᴮXᴮ.

Mike can produce Xᵇ and Y gametes. Meg can produce only Xᴮ gametes.

Looking at the Punnett square, their offspring would be:

  • 1/2 XᴮXᵇ women with normal vision
  • 1/2 XᴮY men with normal vision

All their children will have normal vision (the daughters will be carriers of the color blind allele).

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