Observing that true-breeding pea plants with contrasting traits gave rise to F 1 generations that all expressed the dominant trait and F 2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. Instead, several different patterns of inheritance have been found to exist.įigure 12.15 The child in the photo expresses albinism, a recessive trait. Since Mendel’s experiments with pea plants, researchers have found that the law of dominance does not always hold true. The recessive trait will only be expressed by offspring that have two copies of this allele ( Figure 12.15), and these offspring will breed true when self-crossed. The recessive allele will remain “latent” but will be transmitted to offspring by the same manner in which the dominant allele is transmitted. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively. Mendel’s law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. This finding contradicted the belief at that time that parental traits were blended in the offspring.
After he crossed peas with contrasting traits and found that the recessive trait resurfaced in the F 2 generation, Mendel deduced that hereditary factors must be inherited as discrete units. Mendel proposed first that paired unit factors of heredity were transmitted faithfully from generation to generation by the dissociation and reassociation of paired factors during gametogenesis and fertilization, respectively. Nevertheless, these laws summarize the basics of classical genetics. As you have learned, more complex extensions of Mendelism exist that do not exhibit the same F 2 phenotypic ratios (3:1). Mendel generalized the results of his pea-plant experiments into four postulates, some of which are sometimes called “laws,” that describe the basis of dominant and recessive inheritance in diploid organisms.
Use the forked-line method and the probability rules to calculate the probability of genotypes and phenotypes from multiple gene crosses.Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis.By the end of this section, you will be able to do the following:
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