an organism has two unlike factors (called alleles) for a characteristic, one may be expressed to the total exclusion of the other (dominant vs recessive). Test of the hypothesis A good hypothesis meets several standards. It should provide an adequate explanation of the observed facts. If two or more hypotheses meet this standard, the simpler one is preferred. It should be able to predict new facts. So if a generalization is valid, then certain specific consequences can be deduced from it. In order to test his hypothesis, Mendel predicted the outcome of a breeding experiment that he had not carried out yet. He crossed heterozygous round peas (Rr) with wrinkled (homozygous, rr) ones. He predicted that in this case one-half of the seeds produced would be round (Rr) and one-half wrinkled (rr). To a casual observer in the monastery garden, the cross appeared no different from the P cross described above: round-seeded peas being crossed with wrinkled-seeded ones. But Mendel predicted that this time he would produce both round and wrinkled seeds and in a 50:50 ratio. He performed the cross and harvested 106 round peas and 101 wrinkled peas. Mendel tested his hypothesis with a type of backcross called a testcross. An organism has an unknown genotype which is one of two genotypes (like RR and Rr) that produce the same phenotype. The result of the test identifies the unknown genotype. Mendel did not stop there. He went on to cross pea varieties that differed in six other qualitative traits. In every case, the results supported his hypothesis. He crossed peas that differed in two traits. He found that the inheritance of one trait was independent of that of the other and so framed his second rule: the rule of independent assortment. Today, it is known that this rule does not apply to some genes, due to genetic linkage. See also Dihybrid cross Test cross References External links Interactive hybrid experiments What is Monohybrid cross? YouTube video King, Rita. M (2003). Biology Made Simple, A Made Simple Book, Broadway Books, NY, page 42 Classical genetics