BIOL 250 Genetics - Lecture 04: Mendelian Extensions

Vocabulary flashcards covering Mendelian extensions, gene interactions, epistasis, expression modifiers, epigenetics, and statistical genetics.

Complete Dominance

A dominance relationship in which the phenotype of the heterozygote (hybrid) resembes exactly one of the two pure-breeding parents.

Incomplete Dominance

A dominance relationship in which the phenotype of the heterozygote resembles neither parent, producing an intermediate phenotype where the F2 phenotypic ratio of $1:2:1$ directly reflects the genotypic ratio.

Codominance

A dominance relationship in which the heterozygote simultaneously displays distinct traits characteristic of both parents.

Multiple Alleles

The presence of more than two alternative forms of a single gene within a population, such as the $I^A$, $I^B$, and $i$ alleles governing human ABO blood types.

Dominance Series

A sequential ranking of multiple alleles at a single locus ordered from most dominant to most recessive (e.g., $A^1 > A^2 > A^3$).

Pleiotropy

The phenomenon in which a single gene determines several distinct and seemingly unrelated phenotypic traits.

Recessive Lethal Allele

An allele that causes lethality only when present in the homozygous state, altering expected surviving progeny phenotypic ratios (such as yielding a $2:1$ ratio instead of a $3:1$ ratio).

Complementary Gene Action

A gene interaction where multiple genes work together to produce a single trait, requiring at least one dominant allele of each participating gene for full phenotypic expression and yielding a $9:7$ F2 ratio.

Epistasis

A gene interaction in which an allele at one gene locus masks or overrides the phenotypic expression of alleles at a different gene locus.

Epistatic Allele

An allele at one gene locus that overrides or masks the phenotypic expression of an allele at another locus.

Hypostatic Gene

A gene whose phenotypic expression is masked or suppressed by an epistatic allele at a different locus.

Recessive Epistasis

A form of epistasis where an individual must be homozygous recessive for the epistatic gene to mask expression at a second locus, producing a characteristic $9:3:4$ F2 phenotypic ratio.

Dominant Epistasis I

A form of epistasis in which a single dominant allele at one locus masks both alleles of another gene, yielding a $12:3:1$ F2 phenotypic ratio.

Redundancy (Duplicate Gene Action)

A gene interaction in which two distinct genes perform nearly identical functions in a biological pathway, requiring homozygous recessive genotypes at both loci ($aabb$) to produce a mutant phenotype and producing a $15:1$ F2 ratio.

Bombay Phenotype

A rare genetic condition where individuals with a homozygous recessive $hh$ genotype cannot synthesize H substance, preventing A or B antigen expression and causing them to test as blood type O regardless of ABO gene alleles.

Penetrance

The percentage of individuals carrying a specific genotype who actually exhibit the corresponding expected phenotype.

Expressivity

The degree, intensity, or severity with which a particular genotype is expressed in the phenotype of an individual.

Modifier Genes

Genes that alter or fine-tune the phenotypic expression produced by alleles of another gene.

Position Effect

A change in the expression level of a gene caused by a shift in its physical chromosomal location relative to surrounding genomic context.

Discontinuous Traits

Genetic traits that produce clear-cut, 'either-or' phenotypic differences controlled by alternative alleles at one or a few loci.

Continuous Traits

Quantitative traits that show a continuous spectrum of phenotypic variation, determined by multiple segregating polygenic loci interacting with each other and the environment.

Genomic Imprinting

An epigenetic mechanism in which the expression of an allele depends strictly on whether it was inherited from the maternal or paternal parent due to parent-specific gene silencing.

Epigenetics

The study of heritable and stable changes in gene function or regulation that occur without any alteration to the underlying DNA sequence.

Product Law

A probability rule stating that the joint probability of two or more independent events occurring simultaneously is equal to the product of their individual probabilities.

Sum Law

A probability rule stating that the probability of obtaining a general outcome composed of mutually exclusive events is equal to the sum of their individual probabilities.

Conditional Probability

The calculated probability of an event occurring given that another specific dependent condition or event has already occurred.

Binomial Theorem

A mathematical formula $(a + b)^n = 1$ used to determine the probability of a specific combination of alternative outcomes across $n$ independent trials.

Chi-Square ($\chi^2$) Analysis

A statistical test used to evaluate the influence of chance by measuring how well observed experimental data fit expected theoretical ratios.

Null Hypothesis

A statistical hypothesis asserting that there is no real difference between observed values and expected theoretical ratios, attributing any deviation entirely to random chance.