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.