pierceessentials3e

Genetic Symbols

As we have seen, genetic crosses are usually depicted with the use of symbols that designate the different alleles. The symbols used for alleles are usually determined by the community of geneticists who work on a particular organism, and therefore there is no universal system for designating those symbols. In plants, as noted earlier in this chapter, lowercase letters are often used to designate recessive alleles and uppercase letters to designate dominant alleles. In animals, the most common allele for a characteristic—called the wild type because it is the allele usually found in the wild—is often symbolized by one or more letters and a plus sign (+). The letter or letters chosen are usually based on a mutant (less common) phenotype. For example, the recessive allele that encodes yellow eyes in the Oriental fruit fly is represented by ye, whereas the allele for wild-type eye color is represented by ye⁺. At times, the letters for the wild-type allele are dropped and the allele is represented simply by a plus sign. Superscripts and subscripts are sometimes added to distinguish between genes; for example, El^R represents an allele in goats that restricts the length of the ears. A slash may be used to distinguish the two alleles present in an individual genotype. For example, the genotype of a goat that is heterozygous for restricted ears might be written El⁺/El^R, or simply +/El^R. Sometimes it is useful to designate the possibility of several genotypes. An underline in a genotype, such as A_, indicates that any allele is possible. In this case, A_ might include both AA and Aa genotypes.

CONNECTING CONCEPTS

Ratios in Simple Crosses

Now that we have had some experience with genetic crosses, let’s review the ratios that appear in the progeny of simple crosses in which a single locus is under consideration and one of the traits exhibits dominance. Understanding these ratios and the parental genotypes that produce them will enable you to work simple genetic crosses quickly, without resorting to the Punnett square. Later in this chapter, we will use these ratios to work more complicated crosses that include several loci.

There are only three phenotypic ratios to understand (Table 3.2). The 3:1 ratio arises in a simple genetic cross when both of the parents are heterozygous for a dominant trait (Aa × Aa). The second phenotypic ratio is the 1:1 ratio, which results from the mating of a heterozygous parent and a homozygous parent. To obtain this 1:1 ratio, the homozygous parent in this cross (Aa × aa) must carry two recessive alleles to produce progeny of which half display the recessive trait. A cross between a homozygous dominant parent and a heterozygous parent (AA × Aa) produces progeny displaying only the dominant trait.

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TABLE 3.2 Phenotypic ratios for simple genetic crosses (crosses for a single locus)

Phenotypic ratio	Genotypes of parents	Genotypes of progeny
3:1	Aa × Aa	¾A_:¼ aa
1:1	Aa × aa	½ Aa:½ aa
Uniform progeny	AA × AA	All AA
	aa × aa	All aa
	AA × aa	All Aa
	AA × Aa	All A_

The third phenotypic ratio is not really a ratio: all the progeny have the same phenotype. Several combinations of parents can produce this outcome (see Table 3.2). A cross between any two homozygous parents—either between two parents of the same homozygous genotype (AA × AA or aa × aa) or between two parents with different homozygous genotypes (AA × aa)—produces progeny all having the same phenotype. Progeny of a single phenotype can also result from a cross between a homozygous dominant parent and a heterozygote (AA × Aa).

If we are interested in the ratios of genotypes instead of phenotypes, there are again only three outcomes to remember (Table 3.3): the 1:2:1 ratio, produced by a cross between two heterozygotes; the 1:1 ratio, produced by a cross between a heterozygote and a homozygote; and the uniform progeny produced by a cross between two homozygotes. These simple phenotypic and genotypic ratios and the parental genotypes that produce them provide the key to understanding crosses for a single locus and, as you will see in the next section, for multiple loci.

TABLE 3.3 Genotypic ratios for simple genetic crosses (crosses for a single locus)

Phenotypic ratio	Genotypes of parents	Genotypes of progeny
1:2:1	Aa × Aa	¼ AA: ½ Aa: ¼ aa
1:1	Aa × aa	½ Aa: ½ aa
Uniform progeny	AA × AA	All AA
	aa × aa	All aa
	AA × aa	All Aa