Question

Phenylketonuria is a genetic disorder of

• A. Trisomic condition
• B. Monosomic condition
• C. Autosomal dominant gene
• D. Autosomal recessive gene
• E. X-linked

Answer 1

Answer: (D)

Autosomal recessive gene

Presence or absence of one or more extra chromosome in genome is referred to as aneuploidy. Trisomy is presence of one chromosome in three copies (2n+1) while absence of one chromosome from homologous pair is called monosomy (2n-1). A gene which is present on a chromosome other than sex chromosome is called autosomal gene. A gene which needs to be present in single copy (heterozygous condition) to express its effect is called as dominant gene while the gene which can express itself when present in two copies (homozygous condition) is called recessive gene. Gene present on X-chromosome is called X-linked as it follows the X-chromosome inheritance. Phenylketonuria is an inborn error of metabolism characterized by inability to properly break down the amino acid called phenylalanine. It is caused by mutation in PAH gene, present on chromosome 12, responsible for an enzyme called phenylalanine hydroxylase, which breaks down phenylalanine into tyrosine not into alanine. The mutated gene can express itself in homozygous condition only; hence it is an autosomal recessive disorder.