Question

If these is a history of haemophilia in the family, the chances of male members becoming haemophilic are more than that of the female.

(a) Why is it so ? 

(b) Write the symptoms of the disease. 

Answer 1

(a) Defective gene is on X chromosome, in case the carrier female (mother) passes X^h to the son he suffers, if she passes X^h to the daughter, she has the other X (from father) to make it heterozygous so the daughters escape as carriers.

(b) The blood does not clot in the affected person after an injury or a small cut.

Detailed Answer :

(a) Haemophilia is a sex linked recessive trait. The gene for this trait is located an X chromosome. There are greater chances of males getting haemophilic because the males have only one X chromosome and haemophilic gene located on this chromosome (X^hY) expresses it  self. Human males are hemizygous. Therefore X^hY would by haemophilic where X^hX would not be haemophilic she will be a carrier. It carrier mother X^hX passes X^h to son, he would be haemophilic but if she passes it. to daughter, and she has the other normal X from father, she escapes as carrier.

(b) In haemophilic persons, the blood does not clot after injury due to the absence of blood clotting factor.