Genetics & Heredity Cheat Sheet

Vocabulary flashcards covering fundamental terms, laws, ratios, and non-Mendelian patterns from the Genetics & Heredity lecture notes.

Allele

Alternative form of the same gene, found at the same locus (e.g., A or a).

Genotype

The genetic makeup of an individual, usually shown by allele symbols (AA, Aa, aa).

Phenotype

The observable physical or biochemical trait produced by a genotype (e.g., tall plant, purple flower).

Homozygous

Having two identical alleles for a gene (AA or aa); true-breeding for that trait.

Heterozygous

Having two different alleles for a gene (Aa).

Dominant Allele

An allele that expresses its phenotype even when only one copy is present (capital letter).

Recessive Allele

An allele whose phenotype is masked in the heterozygote; expressed only when homozygous (lower-case letter).

Law of Segregation

Mendel’s principle stating that allele pairs separate during gamete formation so each gamete carries only one allele of each gene.

Law of Independent Assortment

Mendel’s principle that alleles of different genes segregate independently, producing the 9:3:3:1 ratio in dihybrid crosses.

Monohybrid Cross

A genetic cross tracking one trait (e.g., Aa × Aa) yielding a 1 : 2 : 1 genotypic and 3 : 1 phenotypic ratio.

Dihybrid Cross

A genetic cross tracking two traits simultaneously (e.g., RrYy × RrYy) producing a 9 : 3 : 3 : 1 phenotypic ratio.

Punnett Square

A diagram used to predict offspring genotypes and phenotypes by systematically combining parental gametes.

Genotypic Ratio (Monohybrid)

The 1 AA : 2 Aa : 1 aa distribution expected from Aa × Aa.

Phenotypic Ratio (Monohybrid)

The 3 dominant : 1 recessive distribution expected from Aa × Aa.

Phenotypic Ratio (Dihybrid)

The 9 dom/dom : 3 dom/rec : 3 rec/dom : 1 rec/rec distribution expected from RrYy × RrYy.

Incomplete Dominance

Non-Mendelian inheritance where heterozygotes show a blended phenotype (e.g., red × white = pink flowers).

Codominance

Non-Mendelian inheritance where both alleles are fully expressed in heterozygotes (e.g., AB blood type).

Multiple Alleles

A gene having more than two possible alleles in a population (e.g., ABO blood group).

Epistasis

Interaction where one gene masks or modifies the expression of another gene.

Polygenic Trait

A trait controlled by many genes, producing continuous variation (e.g., human skin color).

Sex-Linked Trait

A trait whose gene is located on a sex chromosome, often the X; shows gender-biased inheritance (e.g., color blindness).

True-Breeding

An organism that is homozygous for the trait and produces offspring with identical phenotypes when self-crossed.

Gamete

A reproductive cell (sperm or egg) carrying a single set of chromosomes and thus one allele for each gene.

F₂ Generation

The second filial generation produced by self- or inter-breeding the F₁ hybrids; displays Mendelian ratios.