Mendelian Genetics — Study Materials Flashcards

Front

Allele

Back

A variant form of a gene at a particular locus. Diploid organisms carry two alleles per gene, which can be identical or different and influence the **genotype** and **phenotype**.

Front

Gene

Back

A unit of hereditary information that encodes a trait, usually via a specific DNA sequence. Genes can have multiple **alleles** that influence trait variation.

Front

Homozygote

Back

An individual with two identical alleles at a locus (e.g., $RR$ or $rr$). Homozygotes breed true for that allele's trait when selfed or crossed with the same genotype.

Front

Heterozygote

Back

An individual carrying two different alleles at a locus (e.g., $Rr$). Heterozygotes can display the dominant phenotype while carrying a hidden recessive allele.

Front

Law of Segregation

Back

Mendel's first law stating paired alleles separate during gamete formation so each gamete carries one allele. This explains the $F_2$ genotypic ratio of $1:2:1$ and phenotypic ratio of $3:1$ in monohybrid crosses.

Front

Law of Independent Assortment

Back

Mendel's second law: alleles of different genes segregate independently during gamete formation when genes are unlinked. A dihybrid heterozygote ($RrYy$) yields a $9:3:3:1$ phenotypic ratio in the $F_2$.

Front

Monohybrid cross

Back

A cross that examines the inheritance of a single gene. Crossing two heterozygotes ($Rr$ x $Rr$) typically yields a $3:1$ phenotypic ratio if one allele is dominant.

Front

Dihybrid cross

Back

A cross that follows two independent genes simultaneously (e.g., $RrYy$ x $RrYy$). If genes assort independently, the $F_2$ phenotypic ratio is $9:3:3:1$.

Front

Testcross

Back

A cross of an individual with unknown genotype to a homozygous recessive to reveal the unknown genotype by offspring phenotypes. A 1:1 offspring ratio indicates the unknown parent was heterozygous.

Front

Pure line

Back

A strain that produces offspring identical to the parent for a trait, indicating homozygosity at the locus. Mendel used pure lines to ensure clear contrasting parental phenotypes.

Front

Phenotype

Back

The observable characteristics of an organism (e.g., round or wrinkled seeds). Phenotype results from the interaction of genotype and environment.

Front

Genotype

Back

The genetic constitution of an organism for a trait (e.g., $RR, Rr, rr$). Genotype determines potential phenotypes based on allele interactions.

Front

Dominant allele

Back

An allele whose phenotype is expressed in heterozygotes. Dominance describes the relationship between alleles at the same locus, not the allele's frequency.

Front

Recessive allele

Back

An allele whose phenotype is masked in heterozygotes by a dominant allele and only expressed in homozygotes. Many loss-of-function mutations behave recessively.

Front

Punnett square

Back

A diagrammatic tool used to predict offspring genotypes and phenotypes from parental gametes. It organizes possible gamete combinations and reveals expected ratios.

Front

Trihybrid cross

Back

A cross involving three independent genes (e.g., $RrYyVv$). If independent, phenotypic classes expand predictably; a heterozygous trihybrid produces $2^3=8$ gamete types.

Front

Probability product rule

Back

The rule stating that the probability of two independent events both occurring is the product of their individual probabilities. Used to calculate combined genotype probabilities in crosses.

Front

SBE1 (round allele)

Back

The wild-type allele of the starch-branching enzyme gene that produces round seeds; the mutant loss-of-function allele ($r$) gives wrinkled seeds due to altered starch branching.