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?
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.