Page 72 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

64 Chapter 3 Extensions to Mendel’s Laws

Figure 3.20 A biochemical explanation for redundant gene the four genotypic classes. Table 3.2 summarizes some of
action. The dominant alleles A and B specify proteins that function in the possibilities, correlating the phenotypic ratios with the
independent pathways to instruct cells to become part of the leaf. The genetic phenomena they reflect.
recessive alleles a and b specify no proteins. Because either pathway is It is important to appreciate that wild-type and mutant
sufficient, only plants that lack both dominant alleles have thin leaves. alleles of genes participating in many different types of
AA, Ab
biochemical pathways may produce any specific F 2 pheno-
typic ratio shown in Table 3.2, such as 9:7 or 12:3:1. Thus,
Protein A
Leaf cell if you observe a certain ratio in a cross, you cannot infer the
precursors or Leaf underlying pathway, although you can exclude some pos-
Protein B sibilities. On the other hand, as you will see in the problems
at the end of this chapter, if you know the pathway’s bio-
BB, Bb chemistry, you can predict accurately the phenotypic ratios
among the progeny of a cross involving the genes that
aa
determine the trait.
Leaf cell
precursors Leaf Incomplete Dominance or Codominance
Protein B Can Expand Phenotypic Variation
We have identified to this point several variations on the
B –
theme of two-gene inheritance:
A –
∙ alleles of different genes can interact additively to
Protein A generate novel phenotypes;
Leaf cell Leaf ∙ one gene’s alleles can mask the effects of alleles at
precursors
another gene (epistasis);
∙ different genes may have redundant functions so that
bb a dominant allele of either gene is sufficient for the
production of a particular normal phenotype.
aa
All but the first of these interactions between different
genes resulted in the merging of two or more of Mendel’s
Leaf cell
precursors four genotypic classes into one phenotypic class. For ex-
ample, when genes are redundant, A− B−, A− bb, and aa B−
have the same phenotype. In examining each of these
bb

TABLE 3.2 Summary of Two-Gene Interactions

F 2 Genotypic Ratios from an F 1 Dihybrid Cross F 2 Pheno-
Gene Interaction Example A– B– A– bb aa B– aa bb typic Ratio
Additive: Four distinct F 2 phenotypes Lentil: seed coat 9 3 3 1 9:3:3:1
color (see Fig. 3.10a)
Recessive epistasis: When homozygous, Labrador retriever:
recessive allele of one gene masks both coat color (see 9 3 3 1 9:3:4
alleles of another gene Fig. 3.12b)
Reciprocal recessive epistasis: When Sweet pea: flower
homozygous, recessive allele of each gene color (see Fig. 3.15b) 9 3 3 1 9:7
masks the dominant allele of the other gene
Dominant epistasis I: Dominant allele of one Summer squash:
gene hides effects of both alleles of the other gene color (see Fig. 3.17a) 9 3 3 1 12:3:1
Dominant epistasis II: Dominant allele Chicken feathers:
of one gene hides effects of dominant color (see Fig. 3.18a) 9 3 3 1 13:3
allele of other gene
Redundancy: Only one dominant allele Maize: leaf
of either of two genes is necessary to development 9 3 3 1 15:1
produce phenotype (see Fig. 3.19b)