Lecture 19: Mendel and the Gene Idea

Vocabulary practice flashcards covering Mendelian genetics, the laws of segregation and independent assortment, monohybrid and dihybrid crosses, and foundational genetic terminology.

Blending inheritance

The historical hypothesis that offspring are a uniform mixture of the phenotypes (traits) of each parent, which ultimately eliminates genetic variation over time.

Particulate inheritance

Gregor Mendel's model of inheritance stating that parents pass on discrete heritable units (genes) that retain their identities across generations rather than blending.

Gene

A discrete heritable unit passed from parents to offspring that determines a specific biological trait.

Pisum sativum

The scientific name for the garden pea plant used by Gregor Mendel in his inheritance experiments due to its ability to self-fertilize or cross-pollinate.

True-breeding

Organisms that consistently produce progeny with the same phenotype as the parent when self-fertilized, independent of environmental variations.

Monohybrid cross

A genetic cross between parental organisms that differ in only one trait or character being tracked.

$F_1$ generation

The first filial generation; the hybrid offspring resulting directly from a cross between the parental ($P$) generation.

$F_2$ generation

The second filial generation; the offspring resulting from the self-fertilization or intercrossing of individuals from the $F_1$ generation.

Allele

An alternative version or form of a gene that accounts for phenotypic variation in an inherited trait.

Dominant allele

An allele that fully determines the organism's appearance or phenotype when present, masking the expression of an alternative allele in a heterozygote.

Recessive allele

An allele that has no noticeable phenotypic effect on appearance when paired with a dominant allele, requiring two copies to be expressed.

Genotype

The underlying genetic makeup or specific combination of alleles present in an organism (e.g., $PP$, $Pp$, or $pp$).

Phenotype

The observable physical and physiological traits or appearance of an organism, such as purple or white flower color.

Homozygous

Having two identical alleles for a given gene, denoted as either homozygous dominant ($PP$) or homozygous recessive ($pp$).

Heterozygous

Having two different alleles for a given gene, denoted as a hybrid combination ($Pp$).

Law of Segregation

Mendel's first law stating that the two alleles for a heritable character separate during gamete formation and end up in different gametes.

Punnett square

A diagrammatic grid used to depict all possible combinations of gametes and calculate predicted genotypic and phenotypic ratios among offspring.

3:1 ratio

The typical phenotypic ratio observed in the $F_2$ generation of a monohybrid cross between heterozygous ($Pp$) parents, consisting of $3$ dominant phenotypes to $1$ recessive phenotype.

1:2:1 ratio

The genotypic ratio observed in the $F_2$ generation of a monohybrid cross: $1$ homozygous dominant ($PP$) : $2$ heterozygous ($Pp$) : $1$ homozygous recessive ($pp$).

Dihybrid cross

A cross between parental varieties that differ in two distinct characters or pairs of alleles (e.g., seed color and seed shape).

Law of Independent Assortment

Mendel's second law stating that two or more genes assort independently of one another—each pair of alleles segregates independently of any other pair—during gamete formation.

9:3:3:1 ratio

The expected phenotypic ratio in the $F_2$ generation of a dihybrid cross ($YyRr \times YyRr$), demonstrating independent assortment of two distinct traits.