Genetic structure and function / P.F. Smith Keary.

  • Smith-Keary, P. F.
Date:
1975
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic structure and function / P.F. Smith Keary. Source: Wellcome Collection.

    104/392 (page 83)
    THE GENETICS OF HIGHER ORGANISMS 85 20 phenotype phenotype (a) (b) height in metres (men) (c) Figure 5.11 Continuous variation. (a) The segregation of a single pair of alleles will, when there is no dominance, produce three phenotypes in a 1 ;2г1 ratio. (b) When there are six pairs of alleles segregating and all affect the same character there are seven phenotypes. As the number of pairs of alleles increases the distinctions between the phenotypic classes diminish until eventually the small remaining differences are smoothed out by the environment, producing a continuous distribution. (c) Height in man is determined by very many pairs of alleles each with a very small but cumulative effect so that the distribution of height in a population follows a normal, or gaussian, curve. polygenes cannot be studied by Mendelian methods, they do behave according to the neo-Mendehan principles of heredity. These genes are studied in populations by the use of statistical techniques outside the scope of this book. Sex Linkage The genes we have considered so far have shown no tendency to segregate with a particular sex but other genes, because they are carried on one of the chromosomes assisting in the determination of sex, do not show this independence; such genes and the characters they determine are said to be sex linked.