If John and Jane have another child, then what are the chances that they will have another child with hemophilia?
If Jane becomes pregnant, and pre-natal testing tells her that it is a boy, what are his chances of having hemophilia? If pre-natal testing tells her that it is a girl, then what are the odds of the baby girl being born with hemophilia?
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If John and Jane's son was born with hemophilia, the son inherited this gene from his mother Jane. It is a sex-linked trait which means that it is transmitted on the X chromosome. The son would inherit an X from his mother and a Y from his father. Therefore, Jane passed the trait to the son. The Y chromosome doesn't carry the hemophilia gene and plays no role in its transmission. Let us use the symbol X for normal and X- to represent hemophilia gene. If you cross John who has no hemophilia and thus his genes are XY with Jane, who is clearly a carrier and her genes are XX-(one of her X's has the normal gene and one is carrying the hemophilia gene), then you get the following results. For the girls, 50% are normal (XX) and 50% are carrier females(XX-) . For the boys, 50% are male hemophiliacs, or (X-Y) and 50% are normal males (XY). Since the hemophilia gene is recessive to normal, in females, they can be carriers without actually inheriting the disease, since females possess two X chromosomes. However, with males, since they only have one X chromosome, if they inherit from their mother the hemophilia gene, then, they get the disease and if they inherit the normal gene on the X chromosome, then, they are normal.
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