Essentials of behaviour genetics / David A. Hay.

  • Hay, David A.
Date:
1985
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Essentials of behaviour genetics / David A. Hay. Source: Wellcome Collection.

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    require special educational or other assistance because of their inability to cope intellectually. The rest 'function within the limits of the community's tolerance in a welfare state (p. 24). Having indicated some of the current issues involved in defining retardation, we can consider the three ways in which genetics can contribute to it. Chromosome aberrations The chromosomes The chromosomes are the structures within the cells of the body containing all the genetic material. They can be seen quite easily in a sample of somatic cells, the name for any cells other than the gametes, the collective term for sperm and eggs. In practice, white blood cells are generally used for somatic chromosome preparations, but skin or other tissues are employed in certain cases. When the cells are suit¬ ably cultured, chemically treated and stained, the chromosomes become visible under the microscope as darkly staining structures. The normal human chromosome complement or karyotype consists of 46 chromosomes, with other species having different characteristic numbers, for example 42 in the rat and 78 in the dog. We refer to 46 as the diploid chromosome number in humans, to indicate that in females at least, these 46 can be sorted into 23 pairs on the basis of absolute size and relative length of the two arms around the central region, the centromere. The arrangement in males is slightly different, as can be seen by comparing the two parts of Fig. 2.1. Forty four (or 22 pairs] of a male's chromosomes are similar to those in females and are called autosomes. These have no direct influence on sex determination in contrast to the remaining two chromosomes, the sex chromosomes. In females these consist of two X chromosomes, which are homologous, matching in size and shape. In males there is only one X, the other chromosome being a very small, largely non¬ homologous Y chromosome. Thus the normal male is denoted formally as 46,XY and the normal female as 46,XX where the 46 represents the total chromosome complement and the XX or XY the sex chromosomes. Two extensions to this notation are to call the long arm of each chromosome q, and the short arm p (for 'petite') and to identify numerically the specific region of each arm (see Yunis 1977 for details). For example, a female with Cri du chat Syndrome (Fig. 2.8) where part of the short arm of chromosome 5 has been deleted is missing is 46,XX, del(5) (p 13). Our information on human chromosome structure is surprisingly recent. It was only in 1956 that improved techniques revealed that humans had 46 chromosomes rather than the 48 as previously believed. Major advances occurred in the 1970s with the widespread adoption of several chemical techniques that would differentially stain portions of the chromosome arms. Since specific regions of each 32 CHAPTER 2