Question

Case of haemophilia A reported in a family. In this family mother was carrier because she gave birth to an affected son ali. Ali passed on this recessive x linked trait in a zigzag fashion through his carrier daughter (III-1) to his grandson Ahmed (IV-1). Assign genotype to each individual. Can you explain how zain (IV-3) become haemophiliac?​

Answer 1

Haemophilia is a X linked trait which only affects men. Even if it ended up in a female genome, there wouldn't be any serious complications unless both of the chromosomes are affected.

Here,

A carrier mother (parental generation) passes the gene down to her son Ali (F1 Generation).

Assuming she married an unaffected male.

$\begin{array}{|c|c|c|}\cline{1-3} & \sf X^h & \sf X \\ \cline{1-3} \sf X & \sf XX^h & \sf XX \\ \cline{1-3} \sf Y & \sf X^hY & \sf XY \\ \cline{1-3} \end{array}$

• A carrier female and a normal female offsprings are born.
• An affected male and a normal male is formed.

• Genotype of Ali $\longrightarrow \sf X^hY$

Further, Ali's daughter is a carrier.

• Genotype of his daughter $\longrightarrow \sf X^hX$

Assuming she married an affected male.

$\begin{array}{|c|c|c|}\cline{1-3} & \sf X^h & \sf X \\ \cline{1-3} \sf X^h & \sf X^hX^h & \sf XX^h \\ \cline{1-3} \sf Y & \sf X{^h}Y & \sf X^hY \\ \cline{1-3} \end{array}$

Here, two male offsprings would inherit the haemophilic gene explaining how both the grandsons end up haemophilic.

• A female is unaffected by haemophilia is because she has two X chromosomes. Even when one of the chromosome fails to express the required genes, the other X chromosome ensures the expression of genes but here one of the female off springs has inherited two haemophilic genes, thus ending up haemophilic.

• Genotype of the grandsons $\longrightarrow \sf X^hY$