Q.Which of the following is not an example of aneuploidy ? (A) Turner's syndrome (B) Down's syndrome (C) Phenylketonuria (D) Klinefelter's syndrome

Concept: Aneuploidy vs. gene mutation

Aneuploidy refers to a change in chromosome number — having an abnormal count of chromosomes (not a multiple of the haploid set). This arises from nondisjunction during meiosis.

Analysis of each option:

• (A) Turner's syndrome: 45,X — monosomy (one sex chromosome missing). Aneuploidy.
• (B) Down's syndrome: Trisomy 21 — three copies of chromosome 21 instead of two. Aneuploidy.
• (D) Klinefelter's syndrome: 47,XXY — an extra X chromosome. Aneuploidy. …

The concept: What is aneuploidy?

Aneuploidy refers to a condition where an organism has a chromosome number that is not an exact multiple of the haploid number. In simpler terms, instead of having the normal diploid set (46 chromosomes in humans, arranged as 23 pairs), an aneuploid individual has one or more extra chromosomes or is missing one or more chromosomes.

The key is that aneuploidy involves whole chromosome gain or loss, not changes within a gene on a chromosome. Common types include:

• Monosomy: loss of one chromosome (2n - 1 = 45 chromosomes)
• Trisomy: gain of one chromosome (2n + 1 = 47 chromosomes)

This typically arises from nondisjunction during meiosis, when chromosomes fail to separate properly.

Examining each option

Let's identify which conditions involve chromosome number abnormalities and which do not.

1. Turner's syndrome (Option A)

   This is a monosomy of the sex chromosomes, denoted as 45,X. Affected individuals (phenotypically female) have only one X chromosome instead of the normal XX pair. Total chromosome count: 45.

   This is aneuploidy.

2. Down's syndrome (Option B)

   Also called Trisomy 21, this condition results from three copies of chromosome 21 instead of two. The karyotype is 47,XX,+21 or 47,XY,+21. Total chromosome count: 47.

   This is aneuploidy.

3. Phenylketonuria (Option C) …