Homo- and Heterozygotes

HOMO- AND HETEROZYGOTES, terms introduced into genetics by Bateson to denote the constitution of organisms with respect to some hereditary factor (gene). If a certain gene is received from both parents, the organism will be homozygous for this gene. For example, if a child received the gene for brown eye color from both father and mother, they are homozygous for brown eyes. If we designate this gene by the letter A, the formula of the organism will be AA. If, however, the gene is received from only one parent, the individual is heterozygous. For example, if one parent has brown eyes and the other has blue, the offspring will be heterozygous for eye color. Designating the dominant gene for brown color as A and blue as a, we have the formula Aa for the offspring. An individual may be homozygous for a dominant gene (AA) as well as for a recessive gene (aa). An organism may be homozygous for some genes and heterozygous for others. For example, both parents may have blue eyes, but one of them has curly hair and the other straight. The formula of the offspring will be AaBB. A heterozygote for two genes is called a diheterozygote. In appearance, homo- and heterozygotes are either clearly distinguishable—the case of incomplete dominance (curly-haired are homozygous for the dominant gene, wavy-haired are heterozygous, and straight-haired are homozygous for the recessive gene, or black, blue, and Andalusian fowls)—or distinguishable by microscopic and other investigations (peas heterozygous for the wrinkled seed trait are distinguishable by not quite round grains), or completely indistinguishable in the case of complete dominance. Similar phenomena have been noted in humans as well: thus, for example, there is reason to believe that a mild degree of recessive myopia may also manifest itself in a heterozygote; the same applies to Friedreich's ataxia, etc. The phenomenon of complete dominance makes possible the spread in a hidden form of lethal or harmful recessive genes, because if two individuals, outwardly healthy but containing such a gene in the heterozygous state, marry, 25% of unviable or sick children will appear among the offspring (for example, ichthyosis congenita). From the marriage of two individuals homozygous for any trait, all offspring will also possess this trait: thus, for example, from the marriage of two genotypically deaf-mute individuals (a recessive trait, therefore the patient has the structure aa) all children will be deaf-mute; from the marriage of a recessive homozygote and a heterozygote, half of the offspring inherit the dominant trait. A physician most frequently has to deal with marriages of heterozygote-heterozygote (in the case of a recessive disease factor) and homozygote-heterozygote (in the case of a dominant disease factor). The sex having two identical sex chromosomes (female in mammals, male in birds, etc.) is called the homogametic (homozygous) sex. The sex having different sex chromosomes (X and Y, or only one X) is called the heterogametic (heterozygous) sex. The term hemizygous [introduced into genetics by Lippincott] is more convenient, since a heterozygote must have the structure Aa, and individuals with a single X chromosome cannot be Aa, but have the structure A or a. Examples of hemizygous patients are men suffering from hemophilia, daltonism, and certain other diseases whose genes are localized in the X chromosome.