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

(a) Thalassaemia and haemophilia are categorised as Mendelian disorders because these are determined by alternation or mutation in a single gene.
Symptoms of thalassaemia: The main symptoms of thalassaemia are anaemia, jaundice, hepatosplenomegaly, cardiac enlargement and skeletal deformities.
Symptoms of haemophilia: Haemophilia is also called bleeder’s disease in which a single cut leads to non-stop bleeding. It prevents clotting of blood. A seriously affected person may bleed to death after even a minor skin cut.
Inheritance pattern of haemophilia:
This is a sex-linked recessive disease which shows its transmission from an unaffected carrier female to some of the male progeny. It shows criss-cross inheritance. The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because the mother of such a female would have to be at least a carrier and the father should be haemophilic.

Inheritance pattern of thalassaemia:
Thalassaemia is an autosomal, recessively inherited blood disorder transmitted to the offspring when both parents are heterozygous. The defect arises because of either mutation or deletion which results in the reduced rate of synthesis of one of the globin chains of haemoglobin.