Chapter 5: Extensions and Modifications of Basic Principles

Vocabulary flashcards covering the concepts of genetic dominance, lethal alleles, gene interactions, epistasis, and sex-related inheritance patterns from Chapter 5.

Allele

Each version of the same gene located at the same locus.

Complete dominance

A type of dominance where the phenotype of the heterozygote is the same as the phenotype of one of the homozygotes.

Incomplete dominance

A type of dominance where the phenotype of the heterozygote falls within the range between the phenotypes of the two homozygotes.

Codominance

A type of dominance where the phenotype of the heterozygote includes the phenotypes of both homozygotes, such as in the $MNO$ blood group.

Penetrance

The percentage of individuals having a particular genotype that express the expected phenotype, such as the $90\%$ penetrance observed in polydactyly.

Expressivity

The degree to which a character is expressed, ranging from a full extra finger to a small skin tag in polydactyly.

Lethal allele

An allele that causes death at an early stage of development, resulting in some genotypes (such as yellow mice $YY$) not appearing among the progeny.

Multiple alleles

The presence of more than two alleles for a given locus within a group of individuals, such as the ABO blood group, although each individual still only carries two alleles.

Genotypic possibilities for multiple alleles

The number of possible genotypes for a locus with $n$ alleles, calculated by the formula $\frac{n(n+1)}{2}$.

Gene interaction

A situation where the effects of genes at one locus depend on the presence of genes at other loci, determining a single phenotype.

Epistasis

A form of gene interaction in which one gene masks the effect of another gene at a different locus.

Epistatic gene

The gene that performs the masking or suppression in an epistatic interaction.

Hypostatic gene

The gene whose effect is masked or suppressed by an epistatic gene.

Recessive epistasis

A type of epistasis (giving a $9:3:4$ ratio) where the presence of homozygous recessive alleles at the epistatic locus masks the expression of the hypostatic locus, as seen in Labrador retriever coat color.

Dominant epistasis

A type of epistasis (giving a $12:3:1$ ratio) where a single dominant allele at one locus masks the expression of alleles at another locus, as seen in the color of squash.

Duplicate recessive epistasis

A type of epistasis (giving a $9:7$ ratio) where recessive alleles at either of two loci can mask the phenotype, as seen in albinism in snails.

Bombay phenotype

A condition caused by a recessive mutation ($hh$) at the $H$ locus that fails to convert an intermediate into compound $H$, preventing the expression of ABO antigens regardless of the genotype at the ABO locus.

Complementation

A test used to determine whether mutations are at the same locus or at different loci by crossing individuals with different recessive mutations.

Sex-influenced characteristics

Autosomal traits where the phenotype is expressed differently in males and females, such as beardedness in goats.

Sex-limited characteristics

Traits encoded by autosomal genes that are expressed in only one sex, meaning the trait has zero penetrance in the other sex.

Cytoplasmic inheritance

The inheritance of characteristics encoded by genes located in the cytoplasm, specifically in mitochondria or chloroplasts, usually inherited from the mother.

Genetic maternal effect

A phenomenon where the phenotype of the offspring is determined not by its own genotype, but by the genotype of its mother.

Genomic imprinting

The differential expression of genetic material depending on whether it is inherited from the male or female parent, often involving DNA methylation.

Epigenetics

Phenomena caused by alterations to DNA that do not change the base sequence but affect how DNA sequences are expressed.

Anticipation

A genetic phenomenon where a trait becomes more strongly expressed or is expressed at an earlier stage as it is passed from generation to generation.

Temperature-sensitive allele

An allele whose protein product is functional only at certain temperatures.

Polygenic characteristics

Characteristics encoded by genes situated at many different loci.

Pleiotropy

A condition where one gene affects multiple, seemingly unrelated characteristics.