Question

(a) Why are thalassemia and haemophilia categorized as Mendelian disorders? Write the symptoms of these diseases. Explain their pattern of inheritance in humans.
(b) Write the genotypes of the normal parents producing a haemophilic son.

Answer 1

Mendelian disorders include genetic disorders caused by alterations or mutations in a single gene as in haemophilia and thalassemia.

a) Haemophilia: It is a type of genetic disorder in which blood clotting ability of the body is impaired due to the defect in one of the blood clotting factors. It is an X- linked recessive disorder and therefore is more common in males than in females. Males can inherit this disease from heterozygous carrier mother and such males become infertile. Females are rarely haemophilic as both the X- chromosomes need to be in recessive form. This disease leads to spontaneous bleeding on injury.

b) Thalassemia: It is an inherited autosomal recessive disorder of blood. In this disease abnormal haemoglobin is synthesized and this decreases the oxygen carrying capacity of blood. Thus destruction of blood cells takes place causing anaemia. The individuals whose one of the parents is a carrier also becomes a carrier of the disease. If both the parents are carriers of the disease, the individual has 25% chance of inheriting the disease.

Thalassemia is life seizing disorder which does not allow the individual to live more than early stages of life. Different types of thalassemia exhibit different symptoms and severity.

(b) The genotype of the normal parents having a haemophilic son are:
Mother: X${}^c$X (normal) ; Father: XY (normal)
Son: X${}^c$Y (haemophilic)