Two pure strains of corn with white kernels were crossed. All the F1 progeny had

Question

Two pure strains of corn with white kernels were crossed. All the F1 progeny had red kernels; the F2 progeny consisted of 91 red and 69 white. Explain the inheritance of pattern for kernel colour by defining allele symbols and giving genotypes and phenotypes of all 3 generations.

Explanation / Answer

Epistasis - the interaction between two or more genes to control a single phenotype The interactions of the two genes which control comb type was revealed because we could identify and recognize the 9:3:3:1. Other genetic interactions were identified because the results of crossing two dihybrids produced a modified Mendelian ratio. All of the results are modifications of the 9:3:3:1 ratio. Example 1: 15:1 Ratio Phenotypes: Kernel Color in Wheat For this type of pathway a functional enzyme A or B can produce a product from a common precursor. The product gives color to the wheat kernel. Therefore, only one dominant allele at either of the two loci is required to generate the product. Thus, if a pure line wheat plant with a colored kernel (genotype = AABB) is crossed to plant with white kernels (genotype = aabb) and the resulting F1 plants are selfed, a modification of the dihybrid 9:3:3:1 ratio will be produced. The following table provides a biochemical explanation for the 15:1 ratio. Genotype Kernel Phenotype Enzymatic Activities 9 A_B_ colored kernels functional enzymes from both genes 3 A_bb colored kernels functional enzyme from the A gene pair 3 aaB_ colored kernels functional enzyme from the B gene pair 1 aabb colorless kernels non-functional enzymes produced at both genes If we sum the three different genotypes that will produce a colored kernel we can see that we can achieve a 15:1 ratio. Because either of the genes can provide the wild type phenotype, this interaction is called duplicate gene action. Example 2: 9:7 Ratio Example: Flower color in sweet pea If two genes are involved in a specific pathway and functional products from both are required for expression, then one recessive allelic pair at either allelic pair would result in the mutant phenotype. This is graphically shown in the following diagram. If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure line, homozygous recessive plant with white flowers, the F1 plant will have colored flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but epistatic interactions of the C and P genes will give a modified 9:7 ratio. The following table describes the interactions for each genotype and how the ratio occurs. Genotype Flower Color Enzyme Activities/TH> 9 C_P_ Flowers colored; anthocyanin produced Functional enzymes from both genes 3 C_pp Flowers white; no anthocyanin produced p enzyme non-functional 3 ccP_ Flowers white; no anthocyanin produced c enzyme non-functional 1 ccpp Flowers white; no anthocyanin produced c and p enzymes non-functional Because both genes are required for the correct phenotype, this epistatic interaction is called complementary gene action.

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