THE PHYSICIAN interested in the etiology of a disease should always try to ascertain as many etiologic factors as possible, because the causal pathogenic web can often be disturbed from different angles. Although hereditary factors will be the main topic of this round table, we shall stress that they never act in a vacuum. The genes direct the development of the embryo and fetus, but the development depends also upon an environment limited by the mother's body and surroundings.
Certain terms are fundamental to an understanding of heredity:
Chromosomes. The nuclear carriers of the hereditary factors, the genes. Each nucleated somatic cell of a person's body has 24 pairs of chromosomes, each pair carrying hundreds of genes. One member of each pair is derived from the person's mother and one from the father. In the female there are 23 pairs of autosomes (non-sex chromosomes) and 1 pair of like sex chromosomes (X-chromosomes). In the male there are 23 pairs of autosomes and 1 pair of unlike sex chromosomes (one X and one Y-chromosome).
Gene. The particulate biochemical factor responsible for a particular hereditary characteristic. As the genes are carried on the chromosomes, they also occur in pairs. Each pair occupies a particular locus on the chromosomes. Genes located at the same locus are termed alleles. A child gets 1 member of each gene pair from each parent. Sometimes by the rare event of mutation, a gene becomes changed into one that may function abnormally—a "pathologic" gene.
Homozygote. An individual in whom the gene pair in question consists of 2 like genes.
Heterozygote. An individual in whom the gene pair in question consists of unlike genes.
Depending upon the type (dominant or recessive) and location (autosome or sex chromosome) of the gene, several types of inheritance patterns are possible.
- Copyright © 1956 by the American Academy of Pediatrics