Q.Differentiate between the pattern of inheritance in humans of the blood diseases, haemophilia and thalassemia.
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Start your 14-day free trial to unlock the full solution →Both haemophilia and thalassemia are genetic blood disorders, but they differ fundamentally in inheritance pattern: haemophilia is X-linked recessive (passed from carrier mothers to sons), while thalassemia is autosomal recessive (requires both parents to carry the gene, affecting both sexes equally).
The Concept: Why Inheritance Patterns Differ
The key to understanding these two diseases lies in where the faulty gene lives — on which chromosome. Haemophilia is caused by mutations in genes on the X chromosome (specifically the F8 or F9 genes for clotting factors VIII and IX). Since males have only one X chromosome, a single recessive allele on that X will cause the disease. Females, with two X chromosomes, need both copies to be faulty to be affected — making them mostly carriers.
Thalassemia, on the other hand, involves genes on autosomes (non-sex chromosomes) — specifically chromosome 11 for beta-thalassemia and chromosome 16 for alpha-thalassemia. These code for the globin chains of haemoglobin. Because the genes are autosomal, the inheritance follows standard Mendelian recessive rules: both parents must contribute a faulty allele for a child to be affected, and males and females are equally at risk.
A common mistake is to think both are X-linked because they are "blood diseases." Haemophilia is indeed X-linked, but thalassemia is autosomal — never confuse the two in exams.
Step-by-Step Comparison
1. Chromosomal location of the defective gene
- Haemophilia: The gene is on the X chromosome (Xq28 for haemophilia A, Xq27 for haemophilia B). This makes it a sex-linked disorder.
- Thalassemia: The genes are on autosomes — chromosome 11 for beta-thalassemia, chromosome 16 for alpha-thalassemia. This makes it an autosomal disorder.
2. Mode of inheritance
- Haemophilia: X-linked recessive. A male inherits his single X from his mother. If that X carries the defective allele, he will have haemophilia because there is no second X to compensate. A female needs two defective X chromosomes (one from each parent) to be affected — extremely rare. Heterozygous females are carriers, usually asymptomatic.
- Thalassemia: Autosomal recessive. Both copies of the gene (one from each parent) must be defective for the disease to manifest. Heterozygous individuals have thalassemia minor (mild or no symptoms), while homozygous individuals have thalassemia major (severe anaemia requiring regular transfusions).
3. Who gets affected — sex distribution
- Haemophilia: Almost exclusively males are affected. Females are almost always carriers (unless the father is affected and the mother is a carrier — a very rare event).
- Thalassemia: Both males and females are affected equally, since the gene is on an autosome.
4. Carrier status and transmission
- Haemophilia: A carrier mother (heterozygous) has a 50% chance of passing the defective X to her son (who will be affected) and a 50% chance of passing it to her daughter (who will be a carrier). An affected father passes his defective X to all his daughters (who become carriers) but to none of his sons (since sons get his Y chromosome).
- Thalassemia: Both parents must be carriers (heterozygous) for a child to be at risk. Each pregnancy has a 25% chance of an affected child (homozygous), 50% chance of a carrier child, and 25% chance of a completely normal child. There is no sex bias.
For exam questions: If a pedigree shows only males affected and transmission through carrier females, think haemophilia. If both sexes are affected and both parents are often carriers, think thalassemia.
5. Molecular basis — what goes wrong …
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