Select the odd one out of the given Mendelian disorders.

Haemophilia

The correct answer is Option (4) → Haemophilia

Explanation:

All listed disorders are Mendelian (single-gene inherited), but Haemophilia differs in inheritance pattern:

1. Phenylketonuria (PKU):
• Autosomal recessive : Phenylketonuria is an inborn error of metabolism is inherited as an autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine.
2. Sickle-cell Anaemia:
• Autosomal recessive : This is an autosome-linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous).The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the beta globin chain of the haemoglobin molecule.
3. Thalassemia:
• Autosomal recessive : This is also an autosome-linked recessive blood disease transmitted from parents to the offspring when both the partners are unaffected carrier for the gene (or heterozygous).The defect could be due to either mutation or deletion which ultimately results in reduced rate of synthesis of one of the globin chains (α and β chains) that make up haemoglobin.
4. Haemophilia:
• X-linked recessive : This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non-stop bleeding.

So,In Haemophilia:

• It is the only X-linked disorder in the list, while others are autosomal recessive.
• Males are more severely affected due to hemizygosity (single X chromosome).

Answer: Haemophilia (the inheritance pattern makes it distinct).