Before we can learn about the mechanics of inheritance, it is important to be familiar with the terms defined in figure 9.3 and in the list on page 141 of your text. In addition to these terms, you should know that a trait refers to one facet of the outward appearance (or phenotype) of the organism. An individual that, when self-fertilized, produces only offspring like itself is called true breeding. Working with pea plants (that are capable of this kind of self-fertilization) Gregor Mendel used experiment and observation to work out the rules of inheritance. After years of crossing individual pea plants and observing the results, he postulated that each individual has two copies of each gene, one inherited from the maternal parent and one inherited from the paternal parent. During meiosis, the two alleles segregate from each other and end up in different gametes. These alleles may be the same or different, and, according to Mendel, were either dominant or recessive.

Using the example of flower color (Figure 9.7) we see that when a true-breeding white flowered plant is crossed with a true-breeding purple-flowered plant that the resulting offspring are all purple-flowered. Note that this result refutes the blending hypothesis introduced at the beginning of the chapter. Purple, then is determined by a dominant allele of the gene that controls flower color, and white flower color is recessive. When offspring of the first cross (F1) are mated, the progeny (F2) are 3:1 purple to white. In a real mating, the ratio need not be exactly 3:1 dominant to recessive in the F2 (see Figure 9.5). As the number of F2 offspring increases, though, the ratio should approach 3:1. The genotype ratio in the F2 generation is 1:2:1 AA:Aa:aa. This can be verified experimentally using a testcross (mating with a true breeding white flowered (aa) plant) to determine which purple-flowered plants are Aa and which are AA.

When the alleles determining two different traits (say flower color and plant height (Figure 9.9) are on different chromosome, they assort into gametes independently off each other. To review why this happens, you may want to consult figure 8.6. In terms of inheritance, this independent assortment means that the genotypes of the gametes produced by the F1 individuals in figure 9.9 occur in 1:1:1:1 (AB:Ab:aB:ab) ratios. That means that 1/4 of all gametes have each of the four possible genotypes. Phenotypic ratios in the F2, then are 9:3:3:1 (purple, tall : purple, dwarf : white, tall : white, dwarf).

In addition to the Mendelian dominant / recessive relationship, alleles can interact in other ways. The related examples of incomplete dominance and codominance illustrate this. ABO blood groups are a good example of codominance with a true recessive. In addition to complexity arising from incomplete or codominant inheritance, some genes affect more than one trait (pleiotropy, Figure 9.12) and many traits are influenced by more than one gene. In fact these more complex situations are the rule rather than the exception. The interaction between only two genes (epistasis) is responsible for the inheritance of coat color in dogs (see Figure 9.13). When many genes and environmental factors influence a trait, then individuals cannot be divided into a few discreet categories. These traits are said to show continuous variation (see Figures 9.16 and 9.17). Most interesting human phenotypes are continuously variable and polyfactorial.