Gene
A short section of DNA on a chromosome that codes for a specific protein, determining a characteristic.
Allele
Different versions of the same gene. For example, the gene for seed shape has a 'round' allele and a 'wrinkled' allele.
Dominant Allele
An allele that is expressed in the phenotype even when only one copy is present (e.g., Rr shows dominant trait). Shown as a capital letter.
Recessive Allele
An allele that is only expressed when two copies are present (homozygous recessive, e.g., rr). Shown as a lowercase letter.
Genotype
The genetic makeup of an organism — the combination of alleles it carries (e.g., RR, Rr, or rr).
Phenotype
The observable physical characteristic that results from the genotype (e.g., round seeds or wrinkled seeds).
Homozygous
Having two identical alleles for a gene: homozygous dominant (RR) or homozygous recessive (rr).
Heterozygous
Having two different alleles for a gene (e.g., Rr). The dominant allele determines the phenotype.
Punnett Square
A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross between two parents.
Monohybrid Cross
A cross between two organisms looking at the inheritance of a single gene with two alleles.
F1 Generation
The first generation of offspring from a cross between two true-breeding (homozygous) parents.
3:1 Ratio
The expected phenotype ratio when two heterozygous organisms are crossed (Rr × Rr), giving 3 dominant : 1 recessive.