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Introduction to Mendelian Genetics

Author(s): Lisa M. Meffert, PhD

Moving on to Two Traits at a Time

Increasing now to two traits, we will keep the designations of "T" and "t" for tall and short pea plants, respectively, and then add "R" and "r" for round and wrinkled seeds, respectively. In this example, we will cross a male that is heterozygous for tallness and rounded seeds with a female that has the same genotype (TtRr). A Punnet square will, again, help us to work out all possible genotypes (and phenotypes) of the resultant offspring.

The male can produce four types of sperm, each with one allele for each trait. The four types of sperm for a double-heterozygote male are TR, Tr, tR, and tr.

Because the allele for one gene (or characteristic) is independent of (does not dictate) the allele of the other gene, the alleles are said to assort independently. The is Mendel's second law of inheritance: The Law of Independent Assortment. This law would be violated, for example, if the "T" allele was always associated with the "R" allele (and thus eliminating the possibility of Tr and tR sperm).