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Biology LibreTexts

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 - and complete the online test for colorblindness.

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


    • XD = normal allele
    • Xd = colorblind allele

    Male genotypes

    • XDY    normal vision
    • XdY    colorblind

    Female Genotypes

    • XDXD     normal vision
    • XDXd    carries colorblind allele, normal vision
    • XdXd    colorblind
    1. Examine the colorblindness chart. Are you colorblind?
    2. What is your probable genotype?
    3. 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 XD and Xd in the Punnett square.