Question

Define aneuploidy. Where (which parent), and when (which stage of meiosis) does the aneuploidy resulting in:

(a) Klinefelter syndrome

(b) Down syndrome occur?

Explain your answers

Answer 1

Aneuploidy: A condition in which a person has one or a few chromosomes above or below the normal chromosome number. for example a human cell having 45 or 47 chromosomes instead of the usual 46.Aneuploidy can be caused by a failure of homologous chromosomes to separate properly at meiosis or mitosis, a phenomenon called nondisjunction.

a.

Klinefelter syndrome can be caused by:

One extra copy of the X chromosome in each cell (XXY), the most common cause.Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.

The extra chromosome is retained because of a nondisjunction event during paternal meiosis I, maternal meiosis I, or maternal meiosis II (gametogenesis)

Other way for retaining the extra chromosome is through a nondisjunction event during meiosis II in the egg.

Klinefelter syndrome occurs randomly. The extra X chromosome comes from the father and mother nearly equally. An older mother may have a slightly increased risk .

Down Syndrome

Down syndrome (DS), also called Trisomy 21, is a condition in which a person is born with an extra chromosome.

Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction or mosaicism.

Trisomy 21 is caused by a failure of the 21st chromosome to separate during egg or sperm development (nondisjunction). As a result, a sperm or egg cell is produced with an extra copy of chromosome 21; this cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21.