Summary - Basic Gene Inheritance

  • Homozygous: 2 copies of the same allele. (AA)
  • Heterozygous: 1 copy each of 2 different alleles. (Aa)
  • Hemizygous: 1 copy of a gene with no alternate allele or gene on the other chromosome. (A_)
  • Dominant alleles mask the expression of recessive alleles.

          IF A = dominant for tall trait

          AND a = recessive for short trait


                  AA = tall plant                  

                  Aa = tall plant                  

                  aa = short plant

  • A Punnett square is used to predict the ratios of the offspring genotypes and phenotypes.
  • To determine if a plant is homozgous or heterzygous for a dominant trait, self-pollinate the plant, plant the seed in progeny rows and observe the offspring phenotypes and their ratios. Homozygous plants will be truebreeding with all offspring expressing the trait. Heterozygous plants will have mixed offspring.