Aneuploidy, What Causes Aneuploidy and Types of Aneuploidy

What is Aneuploidy – What Does Aneuploidy Mean and Define Down Syndrome

Aneuploidy can be either due to loss of one or more chromosomes or due to addition of one or more chromosomes. It leads to variation in number of chromosome and do not involve the whole set of the chromosome. The nuclei of the aneuploids contain chromosomes whose number is not true multiple of the basic number (n).

What Causes Aneuploidy: The aneuploidy causes due to a result of primary or secondary non disjunction. The loss of one chromosome produces a Monosomic (2n – 1) the condition is termed as Monosomy; the gain of one chromosome produces a Trisomic (2n + 1) and the condition is known as Trisomy; the addition of two or more chromosomes is known as Tetrasomy and Pentasomy, respectively, and the individuals are known as Tertrasomics and Pentasomics. In some cases, a pair of homologous chromosomes is lost (2n – 2), such individuals are termed as Nullisomics and the condition is called Nullisomy.

Types of Aneuploidy

Monosomy (2n – 1)

The monosomics lack a complete chromosome and this create genetic imbalance as the expression of only one allele at each locus of the chromosomes is inadequate. The expression of a genetic information during early development is very delicately regulated because a sensitive equilibrium of gene product is required to ensure normal development. Due to loss of one chromosome in a monosomic, this equilibrium is not achieved, therefore, monosomy is not tolerated in diploids. But this requirement does not appear to be important in plants.

In animals monosomy for one of the sex chromosome is fairly common, e.g., XO individuals in Drosophila have a normal appearance but are sterile males, but monosomy for one of the autosome is not tolerated in animals, e.g., in Drosophila, loss of chromosome 4, results in slow development of the individual, a reduced body size and an impaired viability, while the loss of autosome 2 or 3 is lethal, because such flies never recover.

In plants, monosomy has been observed in Maize, Tobacco, the evening primrose Oenothera, and the weed Datura. Such monosomic plants are usually less viable than their diploid derivatives. The monosomics can easily be produced in ployploids. A polyploid has several chromosomes of same type, and therefore, the loss can be easily tolerated.

The number of possible monosomics in an organism will be equal to haploid chromosome number. In common Wheat, since 21 pairs of chromosomes are present, 21 possible monosomics are known. Monosomics were also isolated in Cotton (2n = 52) and in Tobacco (2n = 48). In Tomato, which is a diploid (2n = 24), rarely monosomics could be produced. Similarly monosomics have been produced in diploid Maize.

Double Monosomics (2n – 1 – 1), i.e., loss of two chromosomes, are also found, but these differ from nullisomics in the respect that the chromosomes lost are non-homologous. Also Triple Monosomics (2n – 1 – 1 – 1) could be produced in polyploids like Wheat.

Trisomy (2n + 1)

The addition of an extra chromosome produce somewhat more viable individuals in animals and plants, than does the loss of a chromosome. As in monosomy, the sex chromosome variations of trisomic type has a less dramatic effect on the phenotype than autosomal variations. Drosophilla females with three X chromosomes and a normal complement of autosomes (3X : 2A) may be fertile but less viable than normal 2X : 2A females.

In plants, the trisomic individuals are viable, but their phenotype may be altered, e.g., the diploid number of jimson weed Datura is 24. Twelve different Primary Trisomics, where extra chromosome is identical to its homologous, are recognized. Each trisomy alters the phenotype of the capsule and a unique phenotype is produced. Secondary Trisomics, where the chromosome is an extra isochromosome (both arms genetically similar), and Tertiary Trisomics are also possible. The extra chromosome in a tertiary trisomic is the product of a translocation.

The trisomy may originate spontaneously due to production of n – 1 type of gametes due to non-disjunction of a bivalent. The trisomics are more often produced artificially by selfing triploids or by crossing triploid females with diploid males (3X x 2X). In plants and animals the trisomy may be detected during cytological observation of meiosis. Since three copies of a chromosome are present, pairing configurations are different. In some cases, only two of these three homologues may synapse. At various regions different members of the trio maybe paired and are known as Trivalents. In some cases, one bivalent and one univalent (unpaired chromosome) may be present. The trivalent is usually arranged on the spindle so that during anaphase one member moves to one pole, and two go to the opposite pole.

Trisomics are used to locate genes on specific chromosomes. If a particular gene is located on the chromosome involved in trisomy, segregation in the progeny of this trisomic will not follow Mendelian pattern, but the ratio will deviate from normal 3 : 1 and 1 : 1 test cross ratios.

Down Syndrome: In human beings, trisomy for certain chromosomes causes definite morphological abnormalities. Mongolism (Down’s syndrome) is one of them. The monogolism is common in children and is characterized by mental retardation, a short body, swollen tongue and eyelid folds resembling Mongols. Down’s syndrome is due to trisomy of chromosome number 21. The trisomics among the humans are the only trisomics with a higher survival rate.

Tetrasomy (2n + 2)

Tetrasomics are a particular chromosome represented in four doses. Therefore, general chromosome formula for tetrasomics is 2n + 2. In Wheat, 21 possible tetrasomics are available.

E. R. Sears was able to synthesize a complete set of Compensating Nullisomic Tetrasomic (2n + 2 + 2), where addition of a pair of homologous chromosomes would compensate for the loss of another pair of homologues. Such non-homologous chromosomes which are able to compensate for each other are called Homologous Chromosomes.

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