Genetics: Linkage, Non-Mendelian Inheritance, Gene Interactions, and Quantitative Traits

Comprehensive practice flashcards covering genetic linkage, 2-point and 3-point mapping, dominance, epistasis, organellar inheritance, maternal effects, epigenetics, and quantitative genetics.

Gene Linkage

The condition in which two or more gene loci reside close to each other on the same chromosome and therefore tend to segregate together into the same gamete during Meiosis I unless separated by crossing over.

Recombination Frequency

The proportion of recombinant progeny formed relative to all progeny, calculated as: $\text{Recombination frequency} = \frac{\text{number of recombinant progeny}}{\text{total number of progeny}} \times 100\%$.

Map Unit (mu)

A standard unit of measurement used to quantify genetic distance on a chromosome, where $1\,\text{map unit (mu)} = 1\,\text{centiMorgan (cM)} = 1\%$ recombination frequency.

Coupling Configuration (Cis)

The arrangement in a double heterozygote in which the wild-type (or dominant) alleles of two linked genes are situated on one chromosome, and the mutant (or recessive) alleles are situated on the homologous chromosome (e.g., $p^+ b^+ / p\,b$).

Repulsion Configuration (Trans)

The arrangement in a double heterozygote in which each homologous chromosome carries one wild-type (or dominant) allele and one mutant (or recessive) allele (e.g., $p^+ b / p\,b^+$).

Genetic Map

A chromosome map that depicts the relative positions and distances between genetic loci based on recombination frequencies, measured in map units ($\text{mu}$) or centiMorgans ($\text{cM}$).

Physical Map

A chromosome map that reflects the actual physical distances between base pairs on a DNA molecule, measured in base pairs ($\text{bp}$), kilobases ($\text{kb}$), or megabases ($\text{Mb}$), where $1\,\text{bp} = 3.4\,\text{\AA}$.

Double Crossover (DCO)

Two simultaneous crossover events between two outer loci that restore parental flanking combinations while altering only the middle gene locus in the recombinant chromatids.

Three-Point Testcross

A testcross between an individual heterozygous for three linked genes and an individual homozygous recessive at all three loci, used to determine gene order along a chromosome and map distances simultaneously.

Consanguinity

Mating between genetically related individuals, designated in human pedigree charts by a double horizontal line connecting the biological parents.

Complete Dominance

A condition in which the heterozygous genotype produces a phenotype identical to that of the homozygous dominant genotype, because one copy of the wild-type allele produces sufficient product.

Incomplete Dominance

An inheritance pattern in which the heterozygous individual exhibits a novel phenotype intermediate between the homozygous dominant and homozygous recessive phenotypes.

Codominance

An inheritance pattern in which the heterozygote simultaneously and distinctively expresses the phenotypic characteristics of both alleles, such as the $AB$ phenotype in the ABO blood group system.

Recessive Lethal Allele

An allele that causes early developmental mortality only when homozygous, altering expected Mendelian ratios in live-born progeny of a monohybrid cross to a $2:1$ ratio of heterozygotes to surviving homozygotes.

Penetrance

The percentage or proportion of individuals with a specific genotype who actually exhibit the associated expected phenotype within a population.

Expressivity

The degree, severity, or extent to which a specific genetic trait or phenotype is physically manifested in an individual carrying that genotype.

Epistasis

A genetic phenomenon in which the alleles at one gene locus mask or suppress the phenotypic expression of alleles at a distinctly different gene locus.

Epistatic Gene

The gene whose allele performs the masking effect over another locus in an epistatic interaction.

Hypostatic Gene

The gene whose phenotypic manifestation is masked or suppressed by an epistatic allele at another locus.

Recessive Epistasis

A form of gene interaction wherein homozygous recessive alleles at one locus ($ee$ or $aa$) mask the expression of alleles at a second locus, altering the classic dihybrid cross ratio to $9:3:4$.

Dominant Epistasis

A form of gene interaction wherein a single dominant allele at one locus masks the expression of all alleles at a second locus, producing a modified dihybrid phenotypic ratio of $12:3:1$.

Bombay Phenotype

A rare genetic condition in which individuals homozygous for a recessive mutant allele ($hh$) at the Fut1 locus fail to produce Compound H (fucosyltransferase activity), leading to a blood type O phenotype regardless of ABO locus genotypes.

Complementation Test

A cross performed between two independently isolated homozygous recessive mutant organisms to establish whether the mutations reside in the same gene (allelic; fail to complement) or in different genes (complement each other to produce wild-type progeny).

Compound Heterozygote

An individual carrying two different recessive mutant alleles of the exact same gene locus, thereby exhibiting the mutant phenotype without being homozygous for either allele.

Cytoplasmic Inheritance

The transmission of traits encoded by extranuclear DNA present in organelles like mitochondria and chloroplasts, characterized by maternal transmission via the egg cytoplasm and reciprocal cross differences.

Maternal Effect

An inheritance pattern in which the offspring's phenotype is determined entirely by the nuclear genotype of its mother (through substances pre-loaded into egg cytoplasm prior to meiosis), rather than by the offspring's own genotype.

Genomic Imprinting

An epigenetic phenomenon causing differential expression of genetic material depending solely on whether an allele is inherited from the male or female parent, mediated by parental-specific silencing.

DNA Methylation

An epigenetic modification involving the enzymatic addition of methyl groups ($-\text{CH}_3$) to cytosine bases in DNA, typically leading to transcriptional silencing without changing the nucleotide sequence.

Temperature-Sensitive Allele

An allele encoding a protein that functions normally at a permissive temperature but unfolds (denatures) and loses enzymatic function at a non-permissive (higher) temperature.

Pleiotropy

A genetic condition in which a single gene mutation has multiple, seemingly unrelated phenotypic effects in an organism, such as in phenylketonuria (PKU).

Polygenic Trait

A characteristic whose phenotype is governed and created by the combined actions and interactions of multiple distinct genes located across the genome.

Sex-Influenced Trait

An autosomal genetic trait whose phenotypic manifestation or allele dominance pattern differs significantly between biological sexes due to physiological hormonal environments.

Sex-Limited Trait

An autosomal genetic trait encoded by genes present in both sexes but expressed exclusively in one sex, showing zero penetrance in the other sex (such as lactation in dairy cattle).

Discontinuous Characteristic

A qualitative trait that exhibits a few discrete, separate phenotypic classes without intermediate forms (e.g., tall versus dwarf plants).

Quantitative Trait

A characteristic that displays continuous variation across a population, typically resulting from polygenic inheritance and environmental influences, and often distributed as a bell curve.

Meristic Characteristic

A quantitative polygenic characteristic measured strictly in whole integers rather than continuous measurements, such as litter size or leaf count.

Threshold Characteristic

A polygenic quantitative trait that does not vary continuously in outward presentation, being measured simply by presence or absence (such as disease liability) once a critical liability threshold is crossed.

Additive Genetic Effects

Quantitative allele interactions where each contributing allele incrementally adds a discrete amount to the total phenotypic value, excluding dominance and epistasis.

The (2n + 1) Rule

A rule in quantitative genetics stating that if $n$ independently assorting additive loci with equal effects determine a trait, the expected number of distinct phenotypic classes in the $F_2$ generation equals $2n + 1$.

Heritability

The proportion of total phenotypic variation in a population that is attributable to genetic differences among individuals.