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9 February, 07:06

In humans, hemophilia is caused by a recessive allele on the X chromosome. Suppose a man with hemophilia has children with a healthy woman whose mother had hemophilia. What is the probability that their second child will have hemophilia? (Enter the probability as a percent. Enter the number only without the percent sign. For example, enter 100% as 100 and enter 12.5% as 12.5)

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  1. 9 February, 07:45
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    The answer is 50

    Explanation:

    Hemophilia is a X-linked disease. That means the recesive allele which causes the disease is not in Y chromosome, but only in X chromosome. If we denote:

    X⁺ : dominant allele

    X⁻: recesive allele

    Then, in a woman there are three posibilities:

    X⁺X⁻: is a carrier woman (without hemophilia)

    X⁺X⁺: healthy woman

    X⁻X⁻: woman with hemophilia

    But in a man there are two posibilities:

    X⁺Y: healthy man

    X⁻Y: man with hemophilia

    A man with hemophilia is X⁻Y, and a healthy woman whose mother had hemophilia is a carrier woman (X⁺X⁻). If they have a child, there are 4 posibilities, and each one has 1/4 of occurrence probability:

    X⁻Y x X⁺X⁻ = X⁻X⁺, X⁻X⁻, X⁺Y, YX⁻

    Posibilities with hemophilia are X⁻X⁻ and YX⁻ (1/4 + 1/4 = 1/2 = 0.5 = 50%)

    In each filial, there is a 50 percent of probability of having a child (man or woman) with hemophilia.
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