Variation in Chromosome Structure and Number

Variation in Chromosome Structure and Number - Variation in...

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1 Variation in Chromosome Structure and Number Introduction Genetic variation refers to genetic differences among members of the same species or those between different species. Allelic variation refers to variation within a specific gene. Variation in Chromosome Structure Natural variation exists in chromosome structure. The chromosomes for a given species vary in both size and shape (Figure 8.1a). Chromosomes are named according to the location of their centromere (Figure 8.1b). Based on the location of the centromere, the chromosome is called metacentric, submetacentric, acrocentric, or telocentric. All chromosomes have a long and short arm. The long arm of the chromosome is called q . The short arm of the chromosome is called p . A karyotype is a micrograph that arranges the chromosomes with the short arm at the top, and then in descending order by size (Figure 8.1c). Cytogeneticists, scientists who study chromosomes microscopically, may use stains to further identify chromosomes. The use of Giemsa stain produces a G banding pattern (Figure 8.1d), which is used as a standard identification pattern for chromosomes. Banding patterns may also be used to identify changes in chromosome structure. Changes in chromosome structure include deletions, duplications, inversions, and translocations. Changes in chromosome structure may either change the amount of genetic material within the chromosome (increase or decrease) or rearrange the genetic material within a chromosome or between two chromosomes. Examples of changes to chromosome structure include (Figure 8.2): Deficiencies and deletions. This decreases the genetic content of the chromosome. Duplications. This increases the genetic content of the chromosome. Inversions. This changes the arrangement of the chromosome.
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2 Translocations. These may be either simple translocations (a single piece of a chromosome is attached to another chromosome) or reciprocal translocations (two different chromosomes exchange pieces. These typically change both the arrangement of the chromosome and the genetic content. The loss of genetic material in a deletion tends to be detrimental to an organism. Chromosomal deletions are the result of breaks in a chromosome or aberrant recombination events. If there is a single break, the piece without the centromere will be lost and the result is a terminal deletion (Figure 8.3a). If the chromosome breaks at two locations, the central fragment is lost, and the outer pieces rejoin, the result is an interstitial deletion (Figure 8.3b). The effect of a deficiency depends upon the size of the deletion and whether it includes genes or portions of genes. Cri-du-chat (pronounced cree du shaw) syndrome in humans is caused by a deletion in the short arm of chromosome 5 (Figure 8.4), and Prader-Willi and Angelman syndromes are due to a deletion in chromosome 15. Duplications tend to be less harmful than deletions.
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Variation in Chromosome Structure and Number - Variation in...

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