Aneuploidy:
In Aneuploidy, there is either reduction or partial increase of the chromosome number. Reduction is said to be hypoploidy, while increase is said to be hyperploidy. Hypoploidy is due to loss of one or two chromosomes. If a single chromosome is lost from a diploid complement it is said to be Monosomy (individuals are called monosomies) and is symbolically designated as 2n-1.
If two chromosomes are lost (i.e., both the homologues in a pair) it is said to be Nullisomy (2n-2). Similarly the addition of a third chromosome to one of the bivalents is called Trisomy (2n+l) and addition of 2 chromosomes to a bivalent is said to be tetrasomy (2nucleic acids+2).
Monosomy:
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In monosornic diploid individuals, (in the genome) one of the pairs of homologues would undergo a loss of one chromosome and it becomes a univalent while all others are bivalents. The diploids cannot generally withstand such a loss, while polyploids can withstand it. This is due to the fact that polypliods (see later in the same chapter) have more than two homologues for every pair.
Theoretically the number of monosomies possible in an individual equals its haploid number. In common wheat where there are 21 pairs of chromosomes; there are 21 possibilities for monosomies.
E.R. Sears has artificially induced these 21 monosomies in a wheat variety called Chinese spring. Monosomies have been induced in cotton (Endrizzi et al) and tobacco (Clausen and Cameron).
While diploids with monosomies do not survive, there are instances as in tomato, where monosomies could be produced. Here the chromosome number is 2n=24. Double monosomies (2n-1 -1) have two homologues lost from two different bivalents.
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During meiotic prophase together with the normal bivalents there will be two separate univalents indicating that both these have lost their partners. Similarly in triple monosomy one homologue each is lost from three different bivalents and three univalents appear at the meiotic prophase.
Aneuploidy has been noticed in human beings also. A classical instance is the Turner’s syndrome named after the discoverer H.H. Turner (1938). It occurs in .23 infants for every 1000 births.
Nullisomy: In this, an entire pair of homologous chromosomes will be missing (2n-2). This should not be mistaken with double monosomy where 2 chromosomes are lost from two different pairs. In nullisomics, the metaphase plate at meiosis I will apparently show no abnormality and only a comparison with the normal can detect the loss of one whole pair of chromosomes. Nullisomics have been identified in wheat and other plants.