Question

What is haemophilia? Write its sex-linked inheritance pattern.

Answer 1

Hint: Our DNA contains the genetic instructions of the most inherited traits. We inherit two sex chromosomes from our parents, either X or Y. Females inherit two X-chromosomes, each from the mother and the father. Whereas the males inherit one X-chromosome from mother and one Y-chromosome from father.

Complete answer:

• Haemophilia is a sex-linked recessive blood disorder.
• Clotting factors are absent in the blood of people affected with haemophilia and hence blood does not clot well.
• The abnormal gene liable for haemophilia is carried on the X- chromosome.
• Haemophilia is of two types - Haemophilia type A and Haemophilia type B.
• In a male, the presence of the abnormal gene responsible for haemophilia results in the mutation of either factor VIII or factor IX gene.
• The affected males cannot transmit the haemophilic gene to their sons as they will only inherit a Y-chromosome from their father.
• However, all the daughters of an affected father will receive the affected X-chromosome and are considered to be obligate carriers of haemophilia. They can pass these genes to their sons.
• Females have two X chromosomes, hence the presence of a gene responsible for haemophilia on one X chromosome does not cause haemophilia.
• The presence of the gene that codes for the synthesis of the factors compensate for the defective gene that doesn’t.

Note:
• These patients affected by Haemophilia B Leyden experience episodes of excessive bleeding during childhood but have fewer bleeding problems after puberty.
• Haemophilia type C is the rarest type of haemophilia. A deficiency of blood-clotting factor XI occurs and is characterized by bleeding episodes after dental extractions or similar events.
• Acquired haemophilia is a very rare type of non-inherited haemophilia caused by the immune system mistakenly attacking blood factors that control clotting.