Genetics Exam 2 Material (9/22/25)

Lethal Allele

An allele that causes the death of an organism, often before birth or maturity, typically altering expected genotypic and phenotypic ratios.

Gene Interaction

How proteins produced by different genes on the same or different chromosomes interact with each other to influence a phenotype.

Product Synthesis (Gene Interaction)

A type of gene interaction where genes work together to create a product.

Masking (Epistasis)

A type of gene interaction where one gene prevents another gene from expressing its phenotype or performing its function.

Suppression (Gene Interaction)

A type of gene interaction where one gene suppresses the effect of another in a different way.

Epistasis

When one gene (epistatic gene) masks the effect of another gene (hypostatic gene) at a different genetic locus.

Epistatic Gene

The gene that does the masking in epistasis.

Hypostatic Gene

The gene whose expression is masked in epistasis.

Recessive Epistasis

A type of epistasis where the masking gene must be in its homozygous recessive form to exert its masking effect (e.g., ee in Labrador coat color masks the B/b gene).

Phenotypic Ratio for Recessive Epistasis

Typically 9:3:4.

Dominant Epistasis

A type of epistasis where the masking gene, when in its dominant form, masks the effect of another gene (e.g., dominant W in squash color masks the Y/y gene).

Phenotypic Ratio for Dominant Epistasis

Typically 12:3:1.

Duplicative Recessive Epistasis (Complementation)

Occurs when a homozygous recessive genotype for either of two interacting genes masks the expression of the dominant allele of the other gene; two dominant alleles (one from each gene) are necessary to produce a specific phenotype.

Phenotypic Ratio for Duplicative Recessive Epistasis

Typically 9:7.

Complementation Test

A test used to determine if two mutations (that produce the same phenotype) are located in the same gene (allelic) or in different genes. Complementation (wild-type offspring) indicates different genes, while no complementation (mutant offspring) indicates the same gene.

Sex-Influenced Traits

Traits controlled by autosomal genes, but their expression is influenced by sex hormones (e.g., male pattern baldness, where heterozygous individuals show hair loss in males but not females).

Sex-Limited Traits

Traits controlled by autosomal genes, but expressed only in one sex, typically due to sex hormones, with 0\% penetrance in the other sex (e.g., beard growth).

Cytoplasmic Inheritance

Inheritance of genetic material located in the cytoplasm, specifically in organelles like mitochondria and chloroplasts, passed almost exclusively from the maternal parent.

Maternal Inheritance

A characteristic of cytoplasmic inheritance, where affected mothers pass the trait to all of their children, but affected fathers do not pass the trait to any of their children.

Genomic Imprinting

A phenomenon where genes from the mother and father are regulated differently, with one copy of a gene (either maternal or paternal) expressed and the other silenced through epigenetic mechanisms.

Conditional Alleles

Alleles whose expression is dependent on specific environmental conditions, most commonly temperature, during development (e.g., Siamese cat coat coloration).

Phenocopy

An environmentally induced phenotype that resembles a phenotype caused by a genetic mutation, but has no genetic basis.

Discontinuous Traits

Phenotypes that fall into distinct, clear-cut categories (either/or), with no intermediate forms (e.g., presence or absence of a widow's peak).

Continuous Traits

Phenotypes that vary across a spectrum, with many intermediate forms, are typically measurable, and often controlled by multiple genes (polygenic inheritance).

Polygenic Inheritance

The inheritance pattern where a single trait is controlled by multiple genes, often leading to continuous variation and a bell-curve distribution of phenotypes.

Pleiotropy

A single gene influencing multiple, distinct characteristics or phenotypes (e.g., a single mutation in cystic fibrosis affects respiratory, digestive, and reproductive systems).

Multifactorial Traits

Traits controlled by many genes (polygenic) and are significantly influenced by environmental factors, making them complex to understand and predict (e.g., hair color influenced by genes and sun exposure).