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Variations in chromosome number involving individual chromosomes are included in aneuploidy. Aneuploid plants and animals have incomplete genomes. Individual chromosome may either be less than the diploid number, or more than the diploid number (polysomics).

1. Monosomics (2n – 1) are organisms having one chromosome less than the diploid complement, (ABC, AB)

2. Double monosomics (2n – 1 – 1) have two chromosomes less, that are different memebers of the genome, (ABC, A)

3. Nullisomics (2n – 2) have two homologous chromosomes less than the diploid complement, (AB, AB)

4. Polysomics. In polysomics there is duplication of one or more chromosomes of a set.

(a) Trisomics (2n + 1) have two complete genomes plus an extra chromosome, i.e. one chromosome is present in triplicate (ABC, ABC, A).
(b) Double trisomics (2n + 1 + 1) have two extra chromosomes, that are different members of the genome. (ABC, ABC, AB).
(c) Tetrasomic (2n + 2) have one chromosome in quadruplicate, while the others are in duplicate, (ABC, ABC, AA).
(d) Pentasomic (2n + 3) have one chromosome in pentaplicate, while the others are in duplicate, (ABC, ABC, AAA).

Aneuploidy may arise by two methods. (1) There may be a union of gameters of which one contains an unbalanced number of chromosomes. (2) It may also arise by the perpetuation of a cell in which, through a failure of anaphase of mitosis, one or more chromosomes may be added or lost.
True aneuploids should be distinguished from false ones. In true aneuploids there are complete genomes, plus a fraction of genome.

Monosomic and trisomic in Drosophila. In Drosophila melanogaster there are four pairs of chromosomes or heterosomes or hetrosomes (X and X). Males have three pairs of autosomes, and X and Y sex chromosomes.

Meiosis in Drosophila

Sometimes due to non-disjunction during meiosis, two types of eggs are produced, one having two X chromosomes and the other without any sex chromosomes.

Trisomics in Datura. Belling and Blakeslee observed mutants in the Jimson weed Datura stramonium. Trisomy of chromosomes A can be represented as 2n + A, and is characterized by rolled leaves.

Aneuploidy in man. In man there are 46 chromosomes of which 22 pairs are autosomes and one pair the sex chromosomes. The sex chromosomes are two types X and Y. Females have two X chromosome, while males have one X and one Y chromosomes.
trisomy in datura

aneuploidy of sex chromosome

Aneuploidy of sex chromosomes. Both monosomics and polysomics are found with reference to the sex chromosomes. In monosomics there is only one X chromosome present in each cell, and the individuals show the Turner syndrome.

Kelinfelter syndrome individual are trisomics with XXY sex chromosomes. A total of 47 chromosomes are present in each cell. The Klinefelter syndrome individual has external genitalia of the male, but has female, like breast development, small testes, and sparse body hair.

Aneuploidy of autosomes. Trisomy of chromosome 21 results in Down’s syndrome or Mongolian idiocy. The individuals are characterized by mental deficiency, short stature, round face, flaccid muscles, small, ears, protruding tongue and epicanthal folds over the eyes.

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