8.3: Sex Linkage

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About 2000 genes reside on the X chromosome. Female humans inherit 2 X chromosomes, one from the mother and one from the father and so, have 2 alleles for each X-linked gene. Males inherit an X chromosome from the mother and a Y from
the father and have 1 allele for each X-linked gene. The Y chromosome determines sex in humans and many other animals.

Colorblindness is the inability to perceive differences between some colors. The red-green colorblindness, or deuteranopia, gene is located on the X chromosome. Individuals with this type of colorblindness have difficulty discriminating reds, yellows, and greens. About 10 million US men are colorblind (~7% of the male population).

Go to the following website - http://colorvisiontesting.com/ishihara.html and complete the online test for colorblindness.

The use of the X to symbolize X-linked genes is useful in pedigree analysis.

Alleles

X^D = normal allele
X^d = colorblind allele

Male genotypes

X^DY normal vision
X^dY colorblind

Female Genotypes

X^DX^Dnormal vision
X^DX^d carries colorblind allele, normal vision
X^dX^d colorblind

Examine the colorblindness chart. Are you colorblind?
What is your probable genotype?
A carrier female has children with a male who has normal vision. What is the chance their son will be colorblind? Show the cross and explain your answer. Use the allele symbols X^D and X^d in the Punnett square.