Normally, humans have 46 chromosomes, with 44 being the autosomal chromosomes and the 2 being the sex chromosomes. The probability of nondisjunction is high in humans, and sometimes can be really destructive to the zygote as the probability of miscarriage is also very high during the early trimester of pregnancy.
Tabulated below are the disorders that are manifested by individuals who bear chromosomes that underwent nondisjunction.
| TYPE OF DISORDER | ABNORMALITY | NUMBER OF CHROMOSOMES | CHARACTERISTICS |
|---|
| Down Syndrome | Trisomy 21 (presence of a third copy of chromosome 21) | 47 | Individuals with this syndrome are often called as “mongoloids“[5] due to their physical attributes similar to the Mongolian descent: flat face, slanting eyes, short neck, below average intelligence, etc. |
| Edwards Syndrome | Trisomy 18 (presence of a third copy of chromosome 18) | 47 | Individuals with Edwards syndrome[6] are characterized by their small and abnormally shaped head, small jaw, clenched fists and fingers that tend to overlap. |
| Patau Syndrome | Trisomy 13 presence of a third copy of chromosome 13) | 47 | Among the three autosomal trisomies, this is the rarest yet the most severe[7]. Individuals are characterized by having polydactyly (having extra fingers or toes), holoprosencephaly (failure to create double lobes of the cerebral hemispheres), facial clefting, heart ailments, etc. |
| Turner Syndrome | Absence of one X chromosome | 45 | Only affecting females, Turner syndrome[8] is characterized by the offspring having short stance, inability to develop during puberty, infertility, heart ailments, and other physical, social, and mental disabilities. |
| Metafemale | Presence of extra X chromosome (XXX) | 47 | This syndrome is also referred to as the Triple X syndrome[9] as affected individuals (known as metafemales) are characterized by the presence of an extra X chromosome. Most of the time, metafemales do not exhibit unusual physical attributes but they can be described as having behavioral problems, wide-set eyes, and ovarian failure. |
| Klinefelter Syndrome | Presence of extra X chromosome (XXY) | 47 | Klinefelter Syndrome[10] only occurs in males (due to the presence of the X chromosome) and it specifically affects their physical and intellectual development. In most cases, affected individuals have small reproductive organs and produce less testosterone than normal. This can then lead to delayed puberty and expression of some female physical attributes like the enlargement of breasts. |
| Jacob’s Syndrome | Presence of extra copy of the Y chromosome (XYY) | 47 | Individuals with the Jacob’s syndrome[11] appear to be physically normal in general. Because of their excess Y chromosome, they are taller than average, weaker muscle tone, and may have some learning and speech problems. |
Due to the presence of numerous severe life-threatening medical problems, many individuals with trisomies or nondisjunction in their autosomal cells die either before birth or within their first month.
On the other hand, the last four syndromes may be fatal[12] but sometimes, offspring with these kinds of genotype may appear just fine.
- According to studies, nondisjunction tends to occur more commonly in the cells of older individuals. This is precisely because the cell tends to lose its complete control over some processes as it grows older.
- This fact is observed by the fact that women who get pregnant at an older age are more likely to conceive children who are affected by the aforementioned disorders.
Check out this video that explains how offspring can have the wrong number of chromosomes. You will learn how individuals are born with Down, Turner’s, and Klinefelter’s syndromes.
