Chapter 8: Exceptions to Mendel's Laws

Vocabulary flashcards covering non-Mendelian inheritance patterns and gene interactions from Chapter 8, including incomplete dominance, codominance, pleiotropy, epistasis, penetrance, expressivity, and genetic heterogeneity.

Incomplete Dominance

An inheritance pattern in which the heterozygote is intermediate in phenotype between either homozygote (for example, wavy hair resulting from curly and straight alleles).

Codominance

An inheritance pattern in which the heterozygote expresses both phenotypes simultaneously, resulting in a unique phenotype (for example, roan coloration in cows).

Multiple Alleles

A condition where a trait has $>2$ alleles within a population, although any individual diploid organism possesses only two alleles.

Phenylketonuria (PKU)

A metabolic disorder caused by the body's inability to break down phenylalanine, which exhibits multiple alleles with $>400$ alleles known in the population.

Pleiotropy

A phenomenon that occurs when a single gene affects or influences more than one phenotype (such as in albinism and sickle-cell anemia).

Epistasis

A gene interaction in which the alleles of one gene affect or alter the phenotypic expression of another gene.

Polygenic Inheritance

An inheritance pattern where a single phenotypic trait is controlled by two or more genes (for example, skin color, which is influenced by over $150$ identified genes).

Penetrance

The frequency or percentage of individuals with a given genotype who actually express the corresponding phenotype.

Complete Penetrance

The condition in which every individual who inherits a specific allele combination fully expresses the associated phenotype.

Incomplete Penetrance

The condition where some individuals who carry a specific genotype do not express the associated phenotype.

Expressivity

The degree or variation in allelic expression when an allele is penetrant in an individual.

Polydactyly

A genetic condition causing extra digits on appendages that exhibits variable expressivity, ranging from extra digits on all appendages to just one, and varying from full-sized digits to stubs.

Genetic Heterogeneity

A phenomenon where mutations in different, independent genes cause the exact same phenotype.

Leber Congenital Amaurosis

A visual disorder that exhibits genetic heterogeneity because several different genes are involved in causing the disease.

Lethal Allele Combinations

An allele combination that causes early death before reproduction can occur, as exemplified in Mexican Hairless Dogs.