Trisomy occurs due to non-disjunction, the failure of one or more chromatids to separate in the second meiotic division. Eggs and sperm formed when non-disjunction occurs either have one chromosome too many, or one chromosome missing.

Down syndrome (trisomy 21)

A chromomsomal mutation that occurs frequently is Down syndrome, or trisomy 21, in which the affected child has 3 of chromosome 21 as opposed to 2.

Persons affected by Down syndrome have a characteristic facial expression, intellectual disability and weak muscles; they may also suffer from birth defects like heart defects or digestive abnormalities.

Many symptoms of Down syndrome when part of an extra copy of chromosome 21 is attached to one of the other chromosomes, called partial trisomy.

Patau syndrome

Patau syndrome occurs when an extra chromosome 13 produces offspring impacted by intellectual diability, microcephaly, an extra finger on each hand, a cleft palate/cleft lip, and ocular/ear malformation.

The extra chromosome 13 may be passed paternally or maternally, and the features of trisomy 13 result from having this extra chromosome in each of the body's cells.

Trisomy 13 occurs in one of every 5000 births, yet 80% of offspring affected die within a month of birth.

Sex chromosome trisomy

In males, non-disjunction may occur during either the first or second meiotic division, producing individuals with an extra X chromome (XXY), or an extra Y chromosome (XYY). Individuals with trisomy XXY are normal as boys, but develop Klinefelter syndrome as adults; they have testes of reduced size that do not produce sperm, their breasts are enlarged and body hair is sparse. Klinefelter syndrome may cause intellectual disability on occasion.