8.3: Sex Linkage
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 D Y normal vision
- X d Y colorblind
Female Genotypes
- X D X D normal vision
- X D X d carries colorblind allele, normal vision
- X d X 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.