Read the following passage and answer the questions that follow : Haemophilia is a sex-linked recessive disease. Study the pedigree analysis given below showing the inheritance of the disease in a family and answer the questions that follow :
(a) Analyse the given pedigree chart to explain why Haemophilia is considered a sex-linked recessive disorder.
Show Hint
Only males are affected in the pedigree and the trait is carried by normal mothers, so it is X-linked recessive.
Step 1: Read the pedigree:
The affected (shaded) individuals are 5, 10 and 12. All three are males.
No female in the chart is affected.
Step 2: Trace the inheritance:
The parents of 5 (1 and 2) are normal. Son 5 got the disease allele from his normal mother 2, who is a carrier.
Daughter 4 is normal, but her son 10 is affected, so 4 is a carrier. Daughter 7 is normal, but her son 12 is affected, so 7 is a carrier.
The trait therefore passes from carrier mothers to sons, skipping a generation.
Step 3: Conclusion:
A recessive trait that shows mainly in males and passes through unaffected carrier mothers is X-linked recessive.
Males have one X chromosome, so a single recessive allele is enough to cause the disease.
Final Answer:
The disease appears only in males (5, 10, 12) and is passed through normal carrier mothers (2, 4, 7). So it is sex-linked (X-linked) and recessive.
