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.

    395/424 (page 375)
    heterozygous (heterozygote)—if the two members of a gene pair are different, the person is said to be heterozygous for that pair of genes, holandric—pertaining to genes carried by the Y chromosome, or to character¬ istics inherited only through the paternal line (holandric inheritance), homozygous (homozygote)—if the two members of a gene pair are the same, the person is said to be homozygous for that pair of genes, idiogram—a diagrammatic representation of a karyotype, which may be based on many observations and measurements of chromosomes. The terms idiogram and karyotype (q.v.) are sometimes used synonymously; however, the word idiogram is best reserved for the sense just given, karyotype—the character of the nucleus of a single cell, as defined by the size, shape, and number of chromosomes it contains as seen at mitosis; i.e., it is the chromosome picture. Usually, the chromosomes are drawn or photographed and presented in a schematized array, locus—see Gene. mutation—a sudden, heritable change in the structure of the genetic material. The most .important mutations are those occurring in the gametes or their precursors, since these can produce inherited changes in the char¬ acteristics of the organisms developing from them. These changes are the principal raw material upon which nature works to bring about evolu¬ tion. The majority of mutations are gene-mutations—changes in indi¬ vidual genes—but some are gross structural alterations of chromosomes (e.g., inversions and translocations) or changes in the number of chromo¬ somes per nucleus, as in polyploidy, for example. Mutation is normally a very infrequent event, though the mutation rate can be increased by irradiation and other treatments, nondisjunction—a phenomenon which occurs when a given pair of homologous chromosomes fail to pair properly or to orient themselves in regular fashion at metaphase, so that both members go to the same pole. As a consequence, half of the daughter cells will have one chromosome too many, and the other half will be one chromosome short. Union of the first type of abnormal product with a normal gamete of the opposite sex produces an individual who is trisomie with respect to a particular chro¬ mosome. Union of the second type of abnormal product with a normal gamete of the opposite sex produces an individual who is monosomic with respect to a particular chromosome, penetrance—the proportion of individuals who bear a particular dominant gene, or who are homozygous for a recessive gene, and who show the effect of that gene. The value is affected by environment or genotype. The penetrance of a gene is expressed as a percentage of individuals with appropriate genotype who fail to show the phenotype. When a dominant gene sometimes fails to express itself in the heterozygote, or a recessive gene in the homozygote, this is referred to as reduced penetrance. It will lead to occasional skipping of a generation in otherwise dominant pedigrees. phenotype—the outward appearance of the individual, with respect to a gene- controlled trait. Thus different genotypes may have the same phenotype —e.g., a non-albino phenotype may have a CC or Cc genotype, since the albino gene, c, is recessive. It is the sum of the characters manifested by an organism, as contrasted with the set of genes possessed by it (geno¬ type) . Genes interact with the environment to produce the characteristics for which they are responsible. It is therefore possible for organisms of similar genotype to have different phenotypes, owing to environmentally produced variation. Likewise, organisms of similar phenotype may differ genotypically. polygenic inheritance—inheritance influenced by several different genes. It results in normal differences between individuals, such as in stature. Such differences display continuous variation and are influenced by the joint action of several or many genes at different loci, each of which indi- 375