Q.Although Haemophilia and sickle cell anemia are two blood related Mendelian disorders, yet, they differ in their pattern of inheritance. State any two differences.
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Start your 14-day free trial to unlock the full solution →Haemophilia is X-linked recessive (affects mainly males, carried by females), while sickle cell anemia is autosomal recessive (affects both sexes equally when homozygous). They also differ in whether heterozygotes express the trait.
Why inheritance patterns matter
Both disorders follow Mendelian genetics, but the chromosome carrying the mutant allele determines who gets affected and how the trait passes through families. Haemophilia genes sit on the X chromosome, making sex a critical factor. Sickle cell anemia genes occupy an autosome (non-sex chromosome), so sex plays no role in inheritance probability.
Understanding these patterns helps predict risk in offspring and explains why certain disorders appear more frequently in one sex or show different severity in carriers.
Key differences in inheritance pattern
1. Chromosomal location and sex-linkage
Haemophilia is X-linked recessive. The defective gene (for clotting factor VIII in Haemophilia A, or factor IX in Haemophilia B) resides on the X chromosome. Males (XY) need only one mutant allele to express the disease because they have just one X chromosome. Females (XX) require two mutant alleles, making affected females extremely rare. Typically, females are carriers (X^H X^h) who transmit the disorder to sons without expressing it themselves.
Sickle cell anemia is autosomal recessive. The mutant gene (β-globin, HbS) sits on chromosome 11, an autosome. Both males and females have two copies of this chromosome, so both sexes are equally likely to be affected when homozygous recessive (HbS/HbS). There is no sex bias in disease expression.
2. Expression in heterozygotes
In haemophilia, female carriers (X^H X^h) are usually asymptomatic or show very mild symptoms due to random X-inactivation (lyonization). The normal allele on one X chromosome typically provides enough clotting factor. Males cannot be carriers—they are either affected or normal.
In sickle cell anemia, heterozygotes (HbA/HbS) have sickle cell trait. They produce both normal and abnormal hemoglobin (roughly 60% HbA, 40% HbS). While generally healthy under normal conditions, they may experience sickling under extreme oxygen deprivation. This partial expression in heterozygotes is absent in haemophilia carriers. …
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