Write in brief on numerical alterations of chromosomes.
Answers
Chromosomal Abnormalities
Normally, humans have 23 pairs of chromosomes - making 46 in total. This includes one pair of chromosomes which are the sex chromosomes. The ova and the sperm each carry 23 chromosomes.
Numerical Aberrations
Structural Aberrations
Chromosomal abnormalities occur when there is a defect in a chromosome, or in the arrangement of the genetic material on the chromosome. Very often, chromosome abnormalities give rise to specific physical symptoms, however, the severity of these can vary from individual to individual.
Abnormalities can be in the form of additional material which may be attached to a chromosome, or where part or a whole chromosome is missing, or even in defective formation of a chromosome. Any increases or decreases in chromosomal material interfere with normal development and function.
There are two main types of chromosomal abnormality which can occur during meiosis and fertilization: numerical aberrations and structural aberrations.
Numerical Aberrations
These are usually caused by a failure of chromosome division, which results in cells with an extra chromosome or a deficiency in chromosomes.
Gametes with these anomalies can result in conditions such as Down syndrome (who have 47 chromosomes instead of 46), or Turner syndrome (45 chromosomes). Common types of numerical aberrations are: triploidy, trisomy, monosomy and mosaicism.
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