STUDY HINTS

Regular segregation and the assortment of alleles in a heterozygote produce the familiar genotypic and phenotypic ratios that you investigated in Problem Set 4. In a real sense these ratios are also “hypotheses,” in that they are the expectations appropriate to a particular genetic situation. For example, a cross between two heterozygotes, Aa × Aa, yields a 3:1 phenotypic ratio among the offspring. Turning this around, if one finds a 3:1 phenotypic ratio in a family, it is reasonable to hypothesize that the parents were both heterozygotes. Ratios are therefore an important key to establishing the genetic basis of an unfamiliar trait.

Sex-linkage, multiple alleles, gene interactions, and maternal and cytoplasmic effects are natural complications that can modify these underlying patterns and ratios. The secret to solving these types of problems is to be familiar with the clues that are often embedded in modified ratios. A distinct difference between male and female phenotypic ratios leads one to consider the possibility of sex-linkage and/or a maternal or cytoplasmic involvement. Lethality would lead to truncated ratios (e.g., 1:2), whereas the gene interactions such as epistasis would merge certain genotypic classes into the same phenotypic class. Textbooks often describe a large number of these modified ratios, but for convenience we have summarized some of the most commonly encountered ones in Table 6.1.

In addition to traits that can be traced to nuclear genes and their interactions, phenotypes are often dependent upon cytoplasmic interactions.