Basic Terms Used in Inheritance Studies

Alternative forms of the same gene present at the same locus on homologous chromosomes and controlling the expression of a character are called alleles or allelomorphs.

An individual possessing identical alleles for a particular gene (e.g., RR or rr) is called homozygous.

An individual possessing dissimilar alleles for a particular gene (e.g., Rr) is called heterozygous.

The genetic constitution or allelic composition of an organism for a particular character is called its genotype.

The observable external expression of a character resulting from the interaction of genotype and environment is called the phenotype.

The numerical proportion of different genotypes obtained in a cross is called the genotypic ratio.

The numerical proportion of different phenotypes expressed in the offspring of a cross is called the phenotypic ratio.

A genetic cross between two individuals differing in a single pair of contrasting characters is called a monohybrid cross.

A cross between an F₁ hybrid and either of its parental forms is called a back cross.

A cross between an F₁ hybrid and a homozygous recessive parent to determine the genotype of the hybrid is called a test cross.

Two crosses in which the same parental genotypes are used but their sex roles are reversed are called reciprocal crosses.

An F₁ hybrid produced from a cross involving three pairs of contrasting characters is called a trihybrid.

An F₁ hybrid produced from a cross involving more than three pairs of contrasting characters is called a polyhybrid.