Hemophilia is controlled by a recessive gene (h) located on the x chromosome, and diabetes mellitus is controlled by an autosomal recessive gene (b). The healthy spouses had a son with hemophilia and diabetes. Analyze the genotypes of all possible children.
The development of diabetes mellitus will be determined by the combination of two recessive genes – bb, because homozygosity is the condition under which a recessive trait acquires the ability to be realized in a phenotype. A person without diabetes will have at least one dominant gene (B) in their genotype. A child with diabetes can be born to healthy parents, provided both of them are heterozygous (Bb). Both parents will produce both types of sex cells: B and b.
Let’s designate the chromosome with which the gene responsible for the development of hemophilia is linked as “Xh”, then the sex chromosome responsible for the development of the normal blood coagulation system will look like “XH”.
A male child with hemophilia will have a set of XhY chromosomes, which is formed by the fusion of an Xh egg and a Y sperm.
A healthy woman, whose son is sick with hemophilia, necessarily carries a recessive gene – XH Xh, she produces eggs: XH and Xh.
Father with no signs of blood clotting disorder – XHY, his sperm: XH and Y.
Possible genotypic variants of the couple’s offspring:
healthy girls (XHXH BB; XHXh BB; XHXH Bb; XHXh Bb);
girls with diabetes mellitus (XHXH bb);
healthy boys (XHY BB; XHYBb);
boys with hemophilia (XhYBb);
boys with both hemophilia and diabetes mellitus (XhYbb).