Terminology - SU 4

Complete dominance

Type of dominance in which the same phenotype is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote.

Incomplete dominance

Type of dominance in which the phenotype of the heterozygote falls in between the phenotypes of the two homozygotes.

Codominance

Type of dominance in which the phenotype of the heterozygote falls in between the phenotypes of the two homozygotes.

Incomplete penetrance

The genotype does not always produce the expected phenotype.

Penetrance

Percentage of individual organisms having a particular genotype that express the expected phenotype.

Expressivity

Degree to which a trait is expressed.

Lethal allele

Allele that causes death at an early stage in development—often before birth—so that some genotypes do not appear among the progeny. A recessive lethal allele kills individuals that are homozygous for the allele; a dominant lethal allele kills both heterozygotes and homozygotes.

Multiple alleles

Presence of more than two alleles within a group of organisms; however, each individual member of the group has only two of the possible alleles.

Compound heterozygote

An individual who carries two different alleles at a locus that result in a recessive phenotype.

Gene interaction

Interaction between genes at different loci that affect the same characteristic.

Epistasis

Type of gene interaction in which a gene at one locus masks or suppresses the effects of a gene at a different locus.

Epistatic gene

Gene that masks or suppresses the effect of a gene at a different locus.

Hypostatic gene

Gene that is masked or suppressed by the action of a gene at a different locus.

Complementation test

Test designed to determine whether two different mutations are at the same locus (are allelic) or at different loci (are nonallelic). Two individuals that are homozygous for two independently derived mutations are crossed, producing F₁ progeny that are heterozygous for the mutations. If the mutations are at the same locus, the F₁ will have a mutant phenotype. If the mutations are at different loci, the F₁ will have a wild-type phenotype.

Complementation

Manifestation of two different mutations in the heterozygous condition as the wild-type phenotype; indicates that the mutations are at different loci.

Sex-influenced characteristic

Characteristic encoded by autosomal genes that are more readily expressed in one sex. For example, an autosomal dominant gene may have higher penetrance in males than in females, or an autosomal gene may be dominant in males but recessive in females.

Sex-limited characteristic

Characteristic encoded by autosomal genes and expressed in only one sex. Both males and females carry genes for sex-limited characteristics, but the characteristics appear in only one of the sexes.

Cytoplasmic inheritance

Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by one parent, most cytoplasmically inherited characteristics are inherited from only one parent.

Genetic maternal effect

The phenotype of the offspring is determined by the genotype of the mother.

Genomic imprinting

Differential expression of a gene depending on whether it is inherited from the male or female parent.

Epigenetics

Phenomena due to alterations in DNA that do not include changes in the base sequence; often affects the way in which DNA sequences are expressed. Such alterations are often stable and heritable in the sense that they are passed to descendant cells or individuals.

Multifactorial characteristic

Characteristic determined by multiple genes and environmental factors.

Temperature-sensitive allele

An allele that is functional at only certain temperatures.

Phenocopy

Phenotype produced by environmental factors alone that is the same as the phenotype produced by a genotype.

Discontinuous characteristic

Characteristic that exhibits only a few, easily distinguished phenotypes. An example is seed shape, in which seeds are either round or wrinkled.

Continuous characteristic

Characteristic that displays a large number of possible phenotypes that are not easily distinguished, such as human height.

Quantitative characteristic

Continuous characteristic; displays a large number of possible phenotypes or is encoded by multiple genetic factors.

Polygenic characteristic

Characteristic encoded by genes at many loci.

Pleiotropy

Ability of one gene to affect multiple characteristics.