Question 6
This problem is an example of the inheritance of a sex-linked trait involving a genetic locus on the X chromosome that is missing on the Y chromosome. The allele for red-green colorblindness at that locus is recessive to the normal color vision allele.
Because human females possess two X chromosomes, they may either be homozygous dominant (i.e., have two normal color vision alleles), homozygous recessive (i.e., have two red-green colorblind alleles), or heterozygous (i.e., have one normal color vision allele and one red-green colorblindness allele).
Human males, on the other hand, have only one X chromosome and the Y chromosome does not have a genetic locus for color vision. Therefore males can have only one color vision allele. Whichever allele they have determines their phenotype with respect to this character. Since human males can only inherit their X chromosomes from their mothers, the only way for a male to inherit red-green colorblindness is when the mother provides the allele in her gamete.
The genetics of this situation is explained further below.
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Alleles defined below:
X = normal color vision allele carried on the X chromosome
Xc = red-green colorblind allele carried on the X chromosome
Y = the lack of any allele influencing vision on the Y chromosome
Possible genotypes and their respective phenotypes:
XX = female with normal color vision (homozygous)
XXc = female with normal color vision, but is a carrier for the recessive red-green colorblind allele (heterozygous)
XY = male with normal color vision
XcY = male with red-green colorblindness
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In the problem presented, both the mother and father exhibit normal color vision, but some of the male children exhibited red-green colorblindness. Because the red-green colorblindness allele in these male children could only have been inherited from their mother, she must a carrier for this recessive allele. See the Punnett square diagram below to understand how the two parents could have produced their various offspring.
Based upon these results, we would expect 50% of their male children to exhibit red-green colorblindness, while none of the female children would be colorblind (although 50% of these female children would be carriers for the allele).