If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Genetics vocabulary and Punnett squares

Review your understanding of genetics vocabulary and Punnett squares in this free article aligned to NGSS standards.

Key points:

  • An organism’s traits are affected by its genes. Genes can have variations, called alleles. Different alleles of a gene can cause different versions of a trait.
  • Many organisms have two alleles for each gene. One allele is inherited from each parent. An organism’s combination of alleles is called its genotype.
  • Genotypes can be homozygous or heterozygous for a gene. A homozygous genotype has two of the same alleles. A heterozygous genotype has two different alleles.
  • A phenotype is an observable version of a trait.
  • An organism’s phenotype is the result of its combination of alleles. Alleles can be dominant or recessive. Dominant and recessive alleles affect an organism’s phenotype in different ways.
    • A dominant allele determines the phenotype, even if the genotype also contains a recessive allele. A dominant allele is usually shown as an uppercase letter.
    • A recessive allele only determines the phenotype if there is no dominant allele present. A recessive allele is usually shown as a lowercase letter.
  • A Punnett square is a model that represents a cross, or breeding event, between two organisms. A Punnett square helps predict the likelihood of certain offspring genotypes and phenotypes, given the genotypes of two parents.
A Punnett square shows a square with four boxes filled in with genotypes. Across the top of the square from left to right are uppercase G and lowercase g. Along the left side of the square from top to bottom are uppercase G and lowercase g. The filled-in boxes of the square show the following genotypes. Top-left box: uppercase G and uppercase G. Top-right box: uppercase G and lowercase g. Bottom-left box: uppercase G and lowercase g. Bottom-right box: lowercase g and lowercase g. The top-left, top-right, and bottom-left boxes contain a green pea pod. The bottom-right box contains a yellow pea pod.
A Punnett square modeling a cross between two pea plants. The Punnett square shows that when two parents with the Gg genotype are crossed, their offspring can have the genotypes GG, Gg, or gg. The GG and Gg genotypes correspond to green pods, and the gg genotype corresponds to yellow pods. Image created with Biorender.com.

Want to join the conversation?