re called P1. And F2s come from F1s. Mendel let the second generation (F1) pollinate normally, and then studied the third generation (F2). He noticed that one in every four plants was as short as the short P1 plant. Mendel did more tests with six more pairs of traits, and got the same result. These were Mendels monohybrid crosses. Based on these observations, Mendel made three rules. The rule of unit factors, the rule of dominance, and the law of segregation. The rule of unit factors states that each organism has two factors that control each of its traits. Genes exist in different forms, these forms, called alleles determine different things. In Mendels case, the height of the pea plant. The rule of dominance states that some traits are dominant, while others are recessive. As in the case with the height, the tall allele was obviously dominant, because if you mixed a tall allele with a short one, the plant would always come out tall. The only way for a recessive trait to affect the organism would be to have two recessive alleles. Meaning, a recessive trait could only be brought out by having two of the same alleles. The law of segregation states that because an organism has two different alleles, it can produce two different gametes. During fertilization, these gametes randomly pair up to produce four different zygotes, each with its own set of alleles. Mendel then made up the idea for phenotypes and genotypes. A phenotype is how an organism looks and behaves. A genotype is an organisms gene combination. If an organism has two alleles that are the same, it is said to be homozygous. Mendel performed another set of crosses known as dihybrid crosses. In this set of experiments he tested the seed of the pea plant. He took round, yellow (both dominant) pea plants and breeded them with green, wrinkled (both recessive) pea plants. Not surprisingly, the F1 offspring were round and yellow, but the F2 plants showed something new: there was one o...
