Variation in Chromosome Structure and Number Abbreviated

Variation in Chromosome Structure and Number Abbreviated -...

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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 . 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. 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.
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2 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. 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. A duplication creates extra genetic material (Figure 8.5). Duplications are usually the result of incorrect crossing over events. These are usually rare, spontaneous events during the evolution of species. The effects of a duplication on the phenotype are associated with the size of the duplication and number of genes that are duplicated. Usually, duplications are less detrimental than deletions.
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Variation in Chromosome Structure and Number Abbreviated -...

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