As Mendel stated in his Law of Segregation, each organism has two genes for each trait. The different forms of a gene are called alleles. If both alleles are are identical, the organism is said to be homozygous for that trait. For instance, Mendel's purebred tall plants possessed two tall alleles and are said to be homozygous tall. Likewise, a purebred short plant has two short alleles and is said to be homozygous short.

If the two alleles are different, the plant is said to be a hybrid or heterozygous for that trait. As Mendel noted, when both alleles are present, one allele masks or hides the other. The stronger allele is said to dominant, and the weaker allele that is masked is said to be recessive.

When expressing dominant and recessive alleles, the dominant allele is always written as a capitalized letter, and the recessive allele as the same letter, but lower case.

Based on this definition, how would you express a dominant tall allele? Move your cursor over the correct answer.

How would you express the recessive short allele?