Abnormal chromosomal number which is exact multiple of the 23 -
Ans. is 'a' i.e., Euploidy Chromosomal disorders (cvtogenetic disorders) o Chromosomal mutation may cause ? 1.Abnormal number of chromosomes 2.Alteration in the structure of chromosomes. 1. Abnormal number of chromosomes o There are 23 pairs of chromosomes (46, XX in females and 46, XY in males). o An exact multiple of the 23 (haploid number) is called euploidy. o When the number of chromosomes are not an exact multiple of 23, it is referred as aneuploidy. eg - Trisomy or monosomy. Pathogenesis of aneuploidy o In aneuploidy error either in meiosis or in mitosis. (a) Error in meiosis o It is the most common cause of aneuploidy. o The usual causes are nondisjunction or anaphase lag. o In nondisjunction, chromosomal pair fails to separate in first meiotic division during gametogenesis, so one gamete has an extra chromosome (n+1) and the other has one less chromosome (n-1). Feilization of such gametes with normal gamete results in two types of zygotes - trisomy (2n+1) or monosomy (2n-1). o In anaphase lag, one chromosome lags behind and is left out of the cell nucleus. The result is one gamete with normal chromosomes (n) and other gamete with one less chromosome (n-1). Feilization in two types of zygote -4 normal (2n) or monosomy (2n-1). (b) Error in Mitosis o Error in mitosis in early development give rise to two or more different types of populations in the same individuals, a condition referred as mosaicism. o Mosaicism affecting the sex chromosome is common. o In the division of an feilized ovum, an error may lead to one of the daughter cell receiving three sex chromosomes, where as the other recieves only one, yielding, a 45, X/47, XXX mosaic. That means the same individual will have both types of cells cells having 45, X and cells having 47, XXX. Then what is the difference between Monosomy / trisomy and mosachism ? Trisomy/monosomy are due to defect in meiosis and before feilization, so that after feilization all cells have same number of chromosome either monosomy or trisomy. In mosaicism, defect is in mitosis and after feilization so that same individual has cells of two different population eg. 45, X and 47, XXX. 2. Abnormal structure of chromosome o Chromosomal structural changes usually result from chromosome breakage followed by loss or rearrangement. o Beside these chromosomal structural abnormalities some autosomal recessive genetic disorders are associated with such a high level of chromosomal instability that they are known as chromosomal breakage syndrome --> Ataxia - telangiectasis, bloom syndrome, and fanconi anemia. There is significantly increased risk of cancer in these syndromes.
Euploidy
Aneuploidy
Mosaicism
Trisomy