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In this article we will discuss about the aneuploidy in drosophila and humans.
Aneuploidy in Drosophila:
The strain of attached X-females further explains the consequences of aneuploidy. Besides sex chromosomes, autosomes in Drosophila can also have nondisjunction. The tiny dot-like chromosome 4 if present in the trisomic state produces viable but phenotypically deformed offspring.
But if one of the very large chromosomes 1 or 2 is present in the trisomic state, it results in lethality. Since there are many genes present on these two chromosomes, a gain or loss of one chromosome upsets the genetic balance to an extent enough to cause the lethal condition.
Aneuploidy in Humans:
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Non-disjunction can occur in both sex chromosomes and autosomes leading to a number of disorders in human beings, each phenotypically distinct from the other. Chromosome studies on foetuses obtained from spontaneous abortions have shown aneuploidy in about 20% of cases.
It is perhaps a good thing that abnormal embryos die early before birth. But this is not always so. Unfortunately, some of the aneuploid zygotes survive to produce abnormal individuals which are born to suffer throughout their lives.
The most well-known amongst the mentally retarded conditions is that of Mongolism or Down’s syndrome resulting from trisomy of a G group chromosome (No. 21). Individuals affected by this syndrome are characterised by a short stature, round face, short fingers and mental retardation of varying degrees.
Their average life span ranges between 10 and 20 years, and IQ between 25 and 49. Their sexual development is incomplete/impaired and most of them, especially males are frequently sterile. Some are fertile and have produced children.
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Besides Down’s syndrome, mental retardation can also result from trisomy of chromosome 18 (Edward’s syndrome) and chromosome 13 (Patau’s syndrome). Nondisjunction of the sex chromosome leads to XO (Turner’s syndrome) and XXY (Klinefelter’s syndrome) conditions, all of which are described in detail elsewhere.
Sometimes non-disjunction occurs during mitotic divisions in the zygote. This leads to the formation of two or more cell lines with different chromosome numbers. Such an event results in a mosaic individual with phenotypic abnormalities related to the chromosomal anomalies in the cell lines. Thus Turner’s females may be chromosomally XO/XX; Klinefelter’s XX/XXY or XXX/XXY/XXXY; Down’s syndrome 2n/2n + 1, and so on.
