0.41 Mendel’s experiments enbio  (Page 2/13)

Once he was sure he had true-breeding plants, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F ₁ hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait had completely disappeared in the F ₁ generation.

Importantly, Mendel did not stop his experimentation there. He allowed the F ₁ plants to self-fertilize and found that 705 plants in the F ₂ generation had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait. This is called a reciprocal cross    —a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross. For the other six characteristics that Mendel examined, the F ₁ and F ₂ generations behaved in the same way that they behaved for flower color. One of the two traits would disappear completely from the F ₁ generation, only to reappear in the F ₂ generation at a ratio of roughly 3:1 ( [link] ).

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these dominant and recessive traits, respectively. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits become latent, or disappear in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid offspring. An example of a dominant trait is the violet-colored flower trait. For this same characteristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F ₂ generation meant that the traits remained separate (and were not blended) in the plants of the F ₁ generation. Mendel proposed that this was because the plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of their two copies to their offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic, or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

Section summary

Working with garden pea plants, Mendel found that crosses between parents that differed for one trait produced F ₁ offspring that all expressed one parent’s traits. The traits that were visible in the F ₁ generation are referred to as dominant, and traits that disappear in the F ₁ generation are described as recessive. When the F ₁ plants in Mendel’s experiment were self-crossed, the F ₂ offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, confirming that the recessive trait had been transmitted faithfully from the original P parent. Reciprocal crosses generated identical F ₁ and F ₂ offspring ratios. By examining sample sizes, Mendel showed that traits were inherited as independent events.