Question

Which of the following is not a Mendelian Disorder?

• A. Haemophilia
• B. Sickle-cell anaemia
• C. Down's Syndrome
• D. Phenylketonuria

Hint:

Mendelian disorders involve single gene mutations, while Down’s syndrome is a chromosomal disorder.

Answer 1

The Correct Option is C

Concept: Mendelian disorders arise due to mutations in a single gene and follow Mendel’s laws of inheritance. They may be autosomal dominant, autosomal recessive, or sex-linked disorders.

Step 1: Analyze haemophilia.
Haemophilia is a sex-linked recessive disorder caused by mutation in genes responsible for blood clotting factors. It follows Mendelian inheritance, so it is a Mendelian disorder.

Step 2: Analyze sickle-cell anaemia.
This disorder is caused by a point mutation in the beta-globin gene, leading to abnormal hemoglobin structure. It is autosomal recessive and therefore Mendelian.

Step 3: Analyze phenylketonuria.
Phenylketonuria is caused by mutation in a gene encoding phenylalanine hydroxylase enzyme. It follows autosomal recessive inheritance and is Mendelian.

Step 4: Analyze Down’s syndrome.
Down’s syndrome is caused by trisomy of chromosome 21, which is a chromosomal abnormality due to nondisjunction. It is not caused by a single gene mutation, hence not Mendelian. Final Conclusion: Down’s syndrome is not a Mendelian disorder.