Most members of diploid species normally contain precisely two haploid chromosome sets, many known cases vary from this pattern. Modifications that affect changes in the number of chromosomes as opposed to those that affect the structure of chromosomes will be considered in this article.
In this article also, phenotypic variation that results from changes that are more substantial than alterations of individual genes are considered. Such alterations cause modifications at the level of the chromosome.
Aneuploidy and Euploidy
In aneuploidy, an organism has gained or lost one or m ore chromosomes but not a complete set. The absence of a single chromosome from an otherwise diploid genome is called monosomy.
The gain of one extra chromosome results in trisomy. Such changes are contrasted with the condition of euploidy, where all chromosomes belong to complete haploid sets. If more than two sets are present, the term polypoloidy applies. Organisms with three sets are specifically triploid; those with four sets are tetraploid.
Origin of Variations in Chromosome Number
As cases that result from the gain or loss of chromosomes, it is useful to examine how such aberrations operate. Such chromosomal variation originates as a random error during the production of gametes, a phenomenon referred to as nondisjunction, whereby paired homologs fail to disjoin during segregation.
This process disrupts the normal distribution of chromosomes into gametes. The results of nondisjunction during meiosis I and meiosis II for a single chromosome of a diploid organism, abnormal games can form containing either two members of the affected chromosome or none at all.
Fertilizing these with a normal haploid gamete produces a zygote with either three members (trisomy) or only one member (monosomy) of this chromosome. Nondisjunction leads to a variety of aneuploid conditions in plants and human and other organisms.
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Concept of the Basic Chromosome Number
The number of chromosomes in a basic set is called the monoploid number (x). The haploid number (n) refers to the number of chromosomes in gametes. In most plants and animals haploid number and monoploid number are the same so n = x or 2n = 2x. However, in some plants like the hexaploid wheat 2n = 6x = 42. Hence n = 21, but x = 7.
Genome Formula
This is a way of representing the chromosome complement of an organisms using upper case letters. Thus wheat (6x) is represented as AABBCC-a hybrid of 3 different species. Unlike another hexaploid which can be AAAAAA, a hexaploid of the same genome.
In conclusion, mutation can occur at the chromosomal level in the form of loss or gain of chromosome. Such mutation affects the chromosome complement of the organism.
There are two broad categories of numerical chromosomal aberration in plants and other organisms: Aneuploidy and Polyploidy. Aneuploidy is when the change does not involve the whole set, if the change affects the whole set, it is known as euploidy.
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