Two people without hemophilia marry and have a hemophilic son. Their daughter is normal. However, when she marries she has a hemophilic son. How could this occur? Show all the genotypes: the original couple, their son and daughter and the daughter’s husband and their son. You can show this in a pedigree form if you wish.

The mother from the original couple must be a carrier for the Hemophilia gene. This is because:

-If the mother is a carrier of the hemophilia gene, and the father does not have hemophilia there's a 50% chance of all their daughters being carriers of the disease, and a 50% chance of all their sons having the disease.

-If the father has hemophilia, and the mother does not carry the hemophilia gene all their daughters will be carriers of the disease, but none of their sons will have the disease.

-If the father does not have hemophilia, and the mother does not carry the hemophilia gene none of their children (daughters or sons) will have hemophilia or carry the gene.

Therefore in this instance, the mother from the original couple must be a carrier while the father is completely unaffected.

the mother from the original couple must be a carrier while the father is completely unaffected. The daughter is a carrier while her husband is completely unaffected.

Explanation:

Hemophilia is a recessive condition where a person's blood soes not clot properly, it is a  sex-linked genetic disorder.

Hemophilia is caused by a mutation in either of two genes, both of which are located on the X chromosome.

The father has one X and one Y chromosome (XY), while the mother has two X chromosomes (XX).

Because hemophilia is a recessive disorder, a daughter must receive two disease alleles (one on each X chromosome) in order to inherit Hemophilia. Thus, she must get a disease allele from both her mother and her father.

Since hemophilia is an X-linked disorder, males are hemizygous for the hemophilia-related gene (have only one allele and display the phenotype associated with that allele). In order to have an hemophilia allele that can be passed on to an offspring, a male must himself be an hemophiliac.

Pairs of parents in which the male is not hemophiliac cannot produce a hemophiliac daughter.

Pairs of parents in which the female is neither hemophiliac nor a carrier cannot produce a hemophiliac daughter.

However, a pair of parents with a carrier mother and an unaffected father can have sons that are affected and unaffected daughters that are carriers.

Therefore in this instance, for the original couple to have an unaffected daughter the mother must be heterozygous for the Hemophilia allele which she then passed to both the son and daughter.

So, the son has Hemophilia  because he got a Y from his father instead of an X, so his only copy of the blood clotting gene comes from the mother.

The daughter is unaffected but went on to have a hemophilic son because she got an unaffected X chromosome from her father and an affected one from her carrier mother, making her a carrier too.

The daughter's husband must have an unaffected X chromosome in order to have a hemophilic son with the carrier daughter from the original couple.

All the genotypes are shown in the file attached to this answer.

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