Handbook of congenital malformations / edited by Alan Rubin.

Date:
1967
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Handbook of congenital malformations / edited by Alan Rubin. Source: Wellcome Collection.

    396/424 (page 376)
    vidually exerts only a very slight effect on the character in question. These multiple genes are usually referred to as polygenes. polyploidy—see Mutation. recessive—a gene which does not express itself in the heterozygous condition. It has an effect only when present in double dose, i.e., in the homo¬ zygote. recurrence (risk of)—see under Empiric risk. sex chromatin (Barr body)—a body about 1 /z in diameter and composed of the densely staining chromatin material found on the inner surface of the nuclear membrane in the interphase nucleus in a high proportion of female cells. Sex chromatin bodies are absent in the nuclei of male cells. An analogous sex difference is found in leukocytes, the nuclei of female polymorphonuclear cells having a pedunculate nodule known as a drum¬ stick, which is absent in male leukocytes. Barr bodies and drumsticks are important in the study of intersexes and in the interpretation of some karyotypes. It has been found empirically that in individuals having mul¬ tiple X chromosomes, the maximum number of Barr bodies is always one less than the niimber of X chromosomes. For example, in XXXX and XXXXY individuals, some nuclei contain three Barr bodies, sex-influenced—see Sex-limited, sex-limited—a trait appearing in only one sex. sex-limited genes or characters—those genes that are expressed phenotypically in one sex only, as opposed to a sex-controlled or sex-influenced char¬ acter, which refers to genotypes expressed in both sexes (albeit, in a different manner in each). For example, genes concerned with milk production are present in both male and female, but only express them¬ selves in the female. These genes are known as sex-limited. On the other hand, baldness is an example of a sex-influenced character, because it is recessive in females and dominant in males, sex-linked genes or characters—those genes which have a special distribution with regard to sex. The male has only one X chromosome. Thus, for X-borne genes in the male, only single, unpaired genes are represented for each locus. In regard to genes on the X chromosome, the question of dominance or recessiveness does not arise, since recessive genes cannot be masked by dominant alleles. If a male carries an abnormal sex-linked gene, he is abnormal with respect to the trait that it determines. If he carries the normal allele, then he is normal. The terms homozygous and heterozygous cannot be applied here. The male carrying the single, unpaired gene is usually called hemizygous. Females have two X chromosomes, so that there are paired loci and paired genes. Consequently, in females, sex-linked genes may be dominant or recessive or intermediate, just as in the case of genes borne by the autosomes. The male receives his X chromosomes, and therefore, all his sex- linked genes, from his mother, and he hands them on to his daughters, who get their second X chromosome from their mothers. Most of the sex-linked genes in man are recessive in the female. Disease showing sex- linked recessive inheritance, therefore, will usually be transmitted by unaffected (heterozygous) females and appear in about half their sons. The classic example of the phenomenon produced by sex-linkage is hemophilia. trisomy—a condition in which one particular chromosome in an otherwise diploid cell is present three times instead of twice. In Down's syn¬ drome or mongoloid idiocy, chromosome 21 is represented in triplicate in every cell. This situation is believed to come about through the acci¬ dent of nondisjunction, either in one of the two meiotic divisions of gametogenesis in one of the parents, or in the early mitotic divisions of the zygote, zygote—see Gamete. 376