Genetics Key Terms: Mendel, Inheritance, and Punnett Squares

Gregor Mendel

Father of genetics; studied inheritance in pea plants.

Gene

A unit of heredity found on chromosomes that determines a trait.

Alleles

Alternate forms of a gene (e.g., W or w).

Homozygous

Having two identical alleles for a trait (WW or ww).

Heterozygous

Having two different alleles for a trait (Ww).

Genotype

The genetic makeup (letters).

Phenotype

The physical expression (appearance).

Dominant allele

Masks the expression of the recessive allele (symbolized by a capital letter).

Recessive allele

Expressed only when no dominant allele is present (symbolized by a lowercase letter).

Law of Segregation

Each individual has two factors (alleles) for each trait, which separate during gamete formation.

Law of Independent Assortment

Alleles for different traits separate independently during gamete formation.

Punnett Square

A diagram used to predict the outcome of a genetic cross.

Dihybrid Cross

A cross between individuals that differ in two traits (e.g., WwSs × WwSs).

Incomplete Dominance

The heterozygote shows an intermediate phenotype (e.g., wavy hair).

Codominance

Both alleles are fully expressed in the heterozygote (e.g., AB blood type).

Multiple Alleles

More than two possible alleles for a gene exist in a population (e.g., ABO blood types).

Polygenic Inheritance

A trait controlled by several genes (e.g., skin color, height).

Pedigree

A chart showing how a trait is passed through generations.

Autosomal Dominant Disorder

Only one copy of the dominant allele is needed to express the trait (e.g., Huntington's disease).

Autosomal Recessive Disorder

Two copies of the recessive allele are needed (e.g., cystic fibrosis).