Haemophilia is a recessive X-linked trait. A man who does not have hemophilia marries a woman who is a carrier. what is the probablity, compared to all possibilities, they will have?

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Hemophilia is inherited as a sex-linked gene found on the X chromosome. Let us use the symbol X to represent a person who does not have hemophilia(normal) and X- to represent an X chromosome with the hemophilia gene. Since women have two X chromosomes, they can have the following genotypes: XX(normal) XX- (female carrier but still normal because the normal gene is dominant) and X-X- ( female with hemophilia). Males are hemizygous for the X chromosome because they are XY. Therefore, there are only two possible genotypes for males: XY(normal male) and X-Y (male with hemophilia). In the problem, the male is normal XY and the female is a carrier X X-.  Therefore, when they produce gametes by meiotic cell division, the male with make an X sperm and a Y sperm. The female will make an X egg and an X- egg. The genotypic results are as follows-

    XX (normal female)

X X- (carrier female)

    XY (normal male)

X-Y(male hemophiliac)


 The phenotypes are: 25% normal female, 25% normal male, 25% carrier female and 25 % male hemophiliac.                                                                     

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