Question

How do chromosomal abnormalities due to change in the number of chromosomes arise? Give three examples of the occurrence of such abnormalities in humans.

Answer 1

Chromosomal-number abnormalities arise when whole chromosomes are lost or gained because of errors in cell division (most commonly nondisjunction during meiosis or mitosis), producing gametes or cells with extra or missing chromosomes (aneuploidy); complete extra sets (polyploidy) can arise from fertilization or cytokinesis failures.

Three human examples:

• Down syndrome: trisomy 21 (47, +21): one extra copy of chromosome 21, causing intellectual disability and characteristic physical features.
• Turner syndrome: monosomy X (45, X): females with a single X (45,X), short stature, ovarian failure and sterility.
• Klinefelter syndrome: 47, XXY: males with an extra X (XXY), small testes, reduced fertility and some feminized features.