Mendel's model for the F1 generation is summarized in the table at the right. The model states that each trait is controlled by a pair of hereditary packets we now call genes. One packet comes from each parent. The alleles (= forms) of the gene for height are the same in true breeding plants ( T T and t t parent plants ). Cross breeding T T with t t plants produces T t plants in the first or F1 generation. The F1 plants receive a T allele from the tall parent and a t allele from the short parent. The F1 plants are tall because the T allele is expressed and "cover up" the t allele. So the T (tall) allele is called dominant and t (short) allele is called recessive.
