Question

What is meant by non-disjunction and when in meiosis does it occur? What is the result of non-disjunction in humans? Can non-disjunction ever be beneficial to an organism? Explain briefly.

Answer 1

Answer:

Fail to separate paternal and maternal chromosomes (homologous chromosomes) after synapsis and crossing over during prophase I of meiosis is called non disjunction of chromosomes.

Due to non-disjuction, chromosomal aberrations are developed. They may be abnormal chromosome numbers (aneuploidy) in the daughter cells. Some abnormalities are developed by non-disjunction are as follows:

Down’s syndrome:

Duplications are usually lethal. Duplication of chromosome 21 in humans results in Trisomy-21 and Down syndrome. In these individuals an additional copy of chromosome 21 is present. This disorder was first reported in the year 1866 by Langdon Down. Down syndrome features are mental retardation, short statured with small round head, furrowed tongue.

Turner’s syndrome:

Deletion of a part of the short arm of one X- chromosome produces Turners syndrome. In human beings presence of Y chromosome determines maleness and its absence determines femaleness. So, in human beings males are represented as XY and females as XX. A female with 44 autosomes and only one X chromosome exhibits Turners syndrome. Such females are short and sterile besides other features including lack of other sexual characters.

Klinefelter syndrome:

This genetic disorder is also caused due to the presence of an additional copy of X chromosome. An extra X chromosome in addition to the usual male (XY) chromosome complement (47, XXY) has been associated with the abnormal male syndrome known as Klinefelter syndrome. This results in abnormal males. Such individual has overall masculine development, however feminine development is also expressed.

Always, non-disjunction is not at all beneficial as it results abnormal chromosome number in the daughter cells.