This module looks at variation in chromosome number, both of entire sets of chromosomes and of individual chromosomes, and how that can affect inheritance and phenotype. It will also examine changes and abnormalities in chromosome structure and how they can affect phenotype.
Objectives
Ploidy
Ploidy refers to the number of chromosome sets found in a cell. We have encountered variations of this word before: haploid, which describes cells with one set of chromosomes, and diploid, which describes cells with two sets of chromosomes. These two situations are by far the most common, but they are not the only only possibilities. For example, polyploidy, the presence of more than two sets of chromosomes, is fairly common in plants (particularly in grasses). Polyploidy is much rarer among animals. Polyploidy includes triploidy (three sets of chromosomes), tetraploidy (four sets), on up to octaploidy (eight) and beyond.
Cells of polyploid organisms tend be large, as do the organisms themselves. This has agricultural significance, because many food plants, such as wheat, potatoes, and bananas are polyploid species. Unfortunately, many polyploid species (especially triploid species) are sterile, presumably due to irregular segregation of chromosomes in meiosis. Luckily for these plants (and for us), they can be propagated asexually (so meiosis isn't required).
Some polyploid species (those with even numbers of chromosome sets), on the other hand, are able to produce viable offspring. These species, upon closer examination, have two distinct sets of chromosomes (and two of each distinct set), indicating that they arose from chromosome duplication in a plant produced by a cross (called a hybridization) between two distinct but closely related species.
An example of a fertile polyploid is Triticum aestivum, a species of wheat used to make bread flour. This plant is hexaploid, with three different sets of seven chromosomes, each of which is duplicated. Therefore, somatic cells have 42 chromosomes and gametes have 21. This modern plant appears to have arisen through two separate hybridization events. First, two related diploid species combined to produce a tetraploid species, which subsequently crossed with yet another diploid species to produce the hexaploid plant. Candidate cereal plants (diploids) have been identified in the Middle East that might represent the ancestral plants.
Aneuploidy
Aneuploidy (translation: "not good ploidy") refers to chromosome sets that have an abnormal number of chromosomes. (The opposite term, euploidy - "good ploidy", refers to chromosome sets with the correct number of chromosomes.) Aneuploidy usually results from nondisjunction during meiosis. (For a quick review, check out the animation of nondisjunction during meiosis.) Nondisjunction results in an extra copy of a chromosome in some gametes, and a lack of that chromosome in other gametes. If any of these gametes participate in fertilization, the resulting individual can lack a chromosome, or have an extra one. If a chromosome is lacking, then the individual is suffering from monosomy. If an extra chromosome is present, then there is trisomy for that chromosome.
The best-known example of trisomy in humans is Down syndrome, which results from trisomy of chromosome 21 (this is referred to as 'trisomy 21' for short). Down syndrome is characterized by short stature, broad skull, wide nostrils, large, furrowed tongue, and impaired mental abilities.
An example of monosomy has been considered elsewhere: Turner syndrome, which results from monosomy X. Apart from the sex chromosomes, complete monosomy is rarely seen in animals. Apparently, monosomy is typically lethal during embryonic development. However, partial monosomies are occasionally observed. Partial monosomy is loss of part of a chromosome. This is due to some accident of replication or other trauma to a chromosome, rather than as a result of nondisjunction. One example of partial monosomy is found in cri-du-chat syndrome, which result fromloss of part of the short arm of chromosome 5. Individuals with this syndrome suffer severe mental and physical impairment, and have a distinctive cry like that of a cat (which gives the syndrome its name).
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