Meiosis normally occurs in order and ensures that every phase of it finishes in appropriate way. However, sometimes the results are unexpected at any point and abnormalities are created. One of the common abnormality is non-disjunction of chromosomes; during which chromosomes fail to segregate during anaphase and telophase and don’t complete with equal distribution among daughter nuclei.
As a result, there is either increase or decrease in the number of chromosomes which causes serious physical, mental and social disorders. This non-disjunction happens in sex chromosome or in autosomes. Some of these disorders are discussed below:
Down’s syndrome:
It results due to autosomal non-disjunction in men. In this disorder, 21st pair of chromosome fails to segregate which results in gamete having 24 chromosome.
When this gamete fertilizes the normal gamete, the individual which is produced has 47 (2n +1) chromosomes. The non-disjunction appears to happen in ova and depends upon the age of mother. The chance of teenage mother having Down’s syndrome child is one in many thousands. At the age of 45, the risk is three times greater.
The affected individuals of Down’s syndrome have broad, flat face, tilted eyes with the skin folded in inner corner, mental retardation and defective development of central nervous system. The non-disjunction may also occur in other than 21 chromosome resulting in abortion or early death.
Klinefelter’s syndrome:
In this disorder, the individuals have extra sex chromosome i.e. 47 chromosome (44 autosome + XXY). These individuals are phenotypically male but they often have enlarged breasts, tendency to obesity, tallness, small tests having no sperms and have underdeveloped secondary sex characteristics.
In Klinefelter’s syndrome, males having 48 (44 autosomes + XXXY), having 49 chromosomes (44 autosomes + XXXXY) and having 47 chromosomes (44 autosomes + XYY) are also observed.
Turner’s syndrome:
In this disorder, the affected individuals have one missing X chromosome and having only 45 chromosomes (44 autosomes + X). These individuals don’t survive during pregnancy and are aborted. The individuals who survive have female appearance with short build, webbed neck, having no ovaries and germ cells.
Reference: Wikipedia