Question

A woman with two genes, one for hemophilia and one for colour blindness on one of its X-chromosomes, marries a normal man. The progeny will be.
A) All sons are haemophilia and colour blind
B) 50% haemophilia and colour blind sons and 50% normal ones
C) All daughters haemophilia and colour blind
D) 50% haemophilic daughters and 50% colour blind daughters.

Answer 1

Hemophilia is a defect of blood that prevents blood from clotting. Color blindness is the failure of certain human beings to differentiate red from green color. Both these diseases are produced by a recessive gene which lies on the X-chromosomes.

Complete answer:
Haemophilia and Color blindness are X-linked recessive disorders. It means that both of these disorders are articulated only when both the X chromosomes in a human female carry the recessive alleles or when the recessive allele is present on the X chromosome of a male. Let us consider that h and c denote the recessive alleles for Haemophilia and Color blindness correspondingly. For a female who suffers from Haemophilia, she should be supposed to have a genotype of XhXh and for Colorblindness, she should have the genotype XcXc. While these disorders are X-linked, a male will undergo these when the recessive trait is present on the X- chromosome. So for a man to suffer from Haemophilia, he should have a genotype of XhY and for Color blindness, he should have a genotype of XcY. In the case of a woman to be a carrier, she should have one recessive allele. So a Carrier for Haemophilia may have genotype XXh and for Color blindness will have genotype XcX. For a woman to be a carrier of both Haemophilia and Color blindness, she should have a genotype of XhXc. A normal man exhibits to have genotype XY. The nature of progeny can be predicted by understanding the genotypes of the cross between the Carrier woman and a normal man that is XhXc × XY.
The traits of haemophilia and color blindness are expressed in males when present in a single copy (known as heterozygous) but in the case of females, they express only when present in homozygous condition.

So, the correct answer is 50% haemophilia and colour blind sons and 50% normal sons.

Note: It is well recognized that color-blindness and haemophilia are due to sex-linked genes. These genes appear to be evident in all males who carry them. In women the gene for haemophilia is most likely always recessive, the cases of haemophilia in heterozygous women are very uncertain.