Mendelian Genetics, Probability Laws, and Epistatic Interactions

Vocabulary practice flashcards covering Mendelian inheritance laws, probability rules (sum and product rules), multi-gene crosses, the forked-line method, and epistatic interactions.

Unit Factors

Gregor Mendel's term for the discrete particulate units of inheritance that occur in pairs, dissociate during gamete formation, and reassociate during fertilization without blending.

Gene

A specific sequence of DNA that encodes a protein; inherited from parents as two discrete units (one maternal, one paternal) that do not blend and remain unchanged unless a mutation occurs.

Law of Dominance

Mendel's rule stating that in a heterozygous individual possessing two different alleles, the dominant allele completely masks or hides the phenotypic expression of the recessive allele.

Law of Segregation

Mendel's first law stating that paired hereditary particles (alleles) separate equally and physically from one another into different gametes during anaphase of meiosis I, ensuring each gamete carries only one allele.

Law of Independent Assortment

Mendel's second law stating that pairs of homologous chromosomes and their associated alleles align and separate into gametes independently of other pairs during metaphase of meiosis I.

Probability

A numerical measure describing the likelihood of a chance event, expressed as a fraction, decimal, or percentage ranging from $0$ (an event that cannot occur) to $1$ (a certain event).

Product Rule

A probability law stating that the overall probability of two or more independent random events occurring together in exact order equals the product of their individual probabilities ($P(A \text{ and } B) = P(A) \times P(B)$).

Sum Rule

A probability law stating that when multiple mutually exclusive events or pathways can lead to the same chance outcome, the overall probability is the sum of their individual probabilities ($P(A \text{ or } B) = P(A) + P(B)$).

Test Cross

A formal breeding method devised by Mendel in which an individual with a dominant phenotype but unknown genotype is crossed with a homozygous recessive individual to detect unknown heterozygotes.

True-Breeding

Organisms that are homozygous for a given trait, carrying two identical hereditary particles, such that self-fertilization or crossing with an identical parent always produces offspring with the same phenotype.

Dihybrid Cross

A mating between individuals heterozygous for two independently assorting genes (e.g., $YyRr \times YyRr$), yielding an $F_2$ phenotypic ratio of $9:3:3:1$ and nine distinct genotypes.

Forked-Line Method

A branching diagram approach that determines genotypic and phenotypic frequencies of multi-gene crosses by calculating individual monohybrid probabilities and combining them via the product and sum rules.

Trihybrid Cross

A genetic cross tracking three independently assorting gene pairs simultaneously (e.g., $AaBbCc \times AaBbCc$), yielding $64$ possible genotypic boxes in a Punnett square and an $F_2$ phenotypic ratio of $27:9:9:3:9:3:3:1$.

Epistasis

A non-independent gene interaction where one gene masks, alters, or influences the phenotypic expression of a separate gene, producing modified Mendelian dihybrid ratios.

Mouse Pigmentation Epistasis

A recessive epistatic interaction where homozygous recessive alleles ($cc$) at a pigment production locus prevent any color from forming (causing albinism), masking the agouti ($A$) and solid black/gray ($a$) alleles to produce a $9:3:4$ ratio.

Summer Squash Fruit Color Epistasis

A dominant epistatic interaction where a dominant allele ($W$) at the first locus inhibits all color production to yield white squash, masking the yellow ($Y$) and green ($y$) alleles of the second locus and generating a $12:3:1$ ratio.

Shepherd's Purse Seed Shape Epistasis

A duplicate gene epistatic interaction where a dominant allele at either of two loci ($A$ or $B$) produces a triangular seed shape, with only double homozygous recessive plants ($aabb$) exhibiting the ovoid phenotype in a $15:1$ ratio.