Races of man / by Sonia Cole.

  • Sonia Mary Cole
Date:
1965
Catalogue details
    29/150 page 24
    Genetics and Evolution Geneticists believe that racial categories must be based on the genetic constitution or genotype, rather than on the outward appearance, or phenotype. The genotype is determined entirely by heredity and is fixed at conception5 its inherent poten tialities, however, may result in different phenotypes, which change throughout the life of an individual and vary according to the environment. Unfortunately, most of the outward characters which distinguish one race from another are controlled by a large number of genes—the units of heredity— and their mode of inheritance has not yet been worked out. A few characters, notably the blood groups, are determined by only a few genes—sometimes only by one—and it is for this reason that they have proved so valuable in tracing origins and relationships. Principles of Genetics The study of genetics originated with Mendel's discovery of the mechanism of inheritance, a work which remained unnoticed for 40 years until it was redis covered at the beginning of the present century. The unit of heredity, the gene, is a complex nucleo-protein. The genes are part of the chromosomes, which are rod-like structures in the nucleus of the cells, composed of nucleoproteins and deoxyribosenucleic acids, generally known as DNA. Each member of a gene pair is known as an allele, the two genes having been inherited one from each of the parents. Alleles may be composed of two dominant genes, expressed with capital letters (AA), or recessive genes, expressed with small letters (aa). In these two cases the individual would be homozy gous for the character concerned. The offspring of homozygous parents, however, might inherit the dominant gene A from one and the recessive gene a from another5 in this case, he would be heterozygous for that character, Aa. Man has 23 pairs of chromosomes and the number of possible combinations 26
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