Non-dysjunction

• when homologous chromosome cells fail to seperate in meiosis

• can occur in Anaphase I (where homologous chromosomes fail to seperate), which could lead to only two cells with genetic material (chromsosomes) and two empty cells

• can also occur in Anaphase II (where at least one of the chromosomes fail to seperate) which leads to one cell with a full chromosome, two with connected chromatids, and one empty cell

Trisomy-21 (Down syndrome)

→ Non dysjunction in pair 21 due to an extra chromosome

→ usually occurs with a normal gamete (23 chromosomes)(haploid) and an abnormal gamete (24 chromosomes) (haploid)

→ the abnormal chromosome is more likely to come from the female, and often occurs with older age in women (is incurable)

→ is possible to test early while it is as a foetus via amniocentesis (process of removing amniotic fluid that the karyotype of the foetal cells in the amniotic fluid can be analysed)

→ mutation of Trisomy-21 can lead to down-syndrome:

• upwardly slanted eyes

• small, flat bridged nose

• small mouth

• degree of retardation

• decreased muscle tone

• hearing loss

• heart defects