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The prefix mono refers to single and di refers to two. An example of this type of cross is between two individuals monohybrid for a particular trait, for instance, two pea plants that are Tt and Tt, where T represents the allele for tall and t represents the allele for short. The individual is said to be hybrid or heterozygous because the two genes for a trait are different. When these plants are crossed, the results are the typical 3:1 ratio of Tall to short offspring. One offspring is TT(tall) two are Tt(tall) and one is the recessive tt(short). In a dihybrid cross, we examine two traits in an individual, where the individual is heterozygous for both traits. In this example, perhaps the pea plants are also hybrid for seed coat. They could each be Rr--where R stands for round seed coat, and r stands for the recessive wrinkled coat. In this case, a cross between two dihybrid individuals who are TtRr and TtRr will warrant a punnet square with 16 boxes. The ratio of offspring will be 9:3:3:1 in a dihybrid cross. Nine offspring have the two dominant traits--tall and round, three have one dominant and one recessive trait--tall and wrinkled, three have the other dominant and recessive traits--Round and short and only one offspring gets both recessive traits short and wrinkled.
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