Human Genetics

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Discontinuous variation

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Phenotypes that fall into two or more distinct, nonoverlapping classes.

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Continuous variation

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A distribution of phenotypic characters that is distributed from one extreme to another in overlapping, or continuous, fashion.

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Complex traits

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Traits controlled by multiple genes and the interaction of environmental factors where the contribution of genes and environment are undefined.

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Polygenic traits

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Traits controlled by two or more genes.

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Multifactorial traits

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Traits that result from the interaction of one or more environmental factors and two or more genes.

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Regression to the mean

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In a polygenic system, the tendency of offspring of parents with one of the extreme phenotypes to exhibit a phenotype that is closer to the population average.

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Genetic variance

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The phenotypic variance of a trait in a population that is attributed to genotypic differences.

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Environmental variance

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The phenotypic variance of a trait in a population that is attributed to differences in the environment.

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Heritability

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An expression of how much of the observed variation in a phenotype is due to differences in genotype.

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Correlation coefficients

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Measures of the degree to which variables vary together.

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Monozygotic (MZ) twins

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Twins derived from a single fertilization event involving one egg and one sperm; such twins are genetically identical.

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Dizygotic (DZ) twins

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Twins derived from two separate and nearly simultaneous fertilization events, each involving one egg and one sperm. Such twins share, on average, 50 percent of their genes.

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Concordance

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Agreement between traits exhibited by both twins.

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Leptin

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A hormone produced by fat cells that signals the brain and ovary. As fat levels become depleted, secretion of leptin slows and eventually stops.