Question

The kernel color in wheat is a continuous trait determined by two additive genes, each with two alleles, that equally contribute to kernel color determination. The red kernels are determined by two genes and two dominant alleles (R1R1R2R2), and white kernels are determined by two recessive alleles at the same two genes (r1r1r2r2). The duplicate dominant alleles R1 and R2 contribute equally to kernel color and cumulatively control the intensity of the red kernel phenotype. Both these alleles are dominant over the recessive white alleles r1 and r2.

1. A true breeding red plant and true breeding white plant are crossed, and the resulting F1 progeny are selfed. What is the expected phenotypic ratio of the kernel progeny of the F2 offspring?
a. 1:46:4:1.
b. 3:1
c. 9:3:3:1.
d. 27:9:9:9:3:3:3.
2. Determine which statement best describes the relationship between the phenotypes of the R1R1 R2r2 and R1r1 R2R2 genotypes.
A. The phenotype of the R1R1 R2r2 will be darker than the phenotype of R1r1 R2R2 because the alleles R1R1 R2r2 Rn have a stronger additive effect than those in R1r1 R2R2.
B. There is no way to predict the expected phenotype color from the genotypes because the trait is continuous and indiscrete.
C. The phenotypes of the R1R1 R2r2 and R1r1 R2R2 genotypes will be the same because the same number of dominant and recessive alleles are contributing in each genotype.
D. The genotype of the R1R1 R2r2 will be lighter than the phenotype of R1r1 R2R2 because the alleles R1R1 R2r2 have a stronger additive effect than those in R1r1 R2R2.

Answer 1

1) Phenotypic ratio= 9:3:3:1

2) The correct statement is C. The phenotypes of the R1R1 R2r2 and R1r1 R2R2 genotypes will be the same because the same number of dominant and recessive alleles are contributing to each genotype.

Step-by-step explanation:

Available data:

• kernel color in wheat is a continuous trait determined by two additive genes
• Both genes equally contribute to kernel color determination
• Red kernels are determined by two genes and two dominant alleles (R1R1R2R2)
• White kernels are determined by two recessive alleles at the same two genes (r1r1r2r2)
• Dominant alleles R1 and R2 contribute equally to kernel color and cumulatively control the intensity of the red kernel phenotype
• R1 and R2 are dominant over r1 and r2

1)

1st cross: A true-breeding red plant with true-breeding white plant

Parentals) R1R1R2R2 x r1r1r2r2

F1) 100% of the progeny will be R1r1 R2r2

2nd cross: Between two dihybrids

Parentals) R1r1R2r2 x R1r1R2r2

Phenotype) Red Red

Gametes) R1R2, R1r2, r1R2, r1r2

R1R2, R1r2, r1R2, r1r2

By using the forked-line method:

• 1/4 R1R1 x 1/4 R2R2 = 1/16 R1R1 R2R2
• 1/4 R1R1 x 2/4 R2r2 = 2/16 R1R1 R2r2
• 1/4 R1R1 x 1/4 r2r2 = 1/16 R1R1 r2r2
• 2/4 R1r1 x 1/4 R2R2 = 2/16 R1r1 R2R2
• 2/4 R1r1 x 2/4 R2r2 = 4/16 R1r1 R2r2
• 2/4 R1r1 x 1/4 r2r2 = 2/16 R1r1 r2r2
• 1/4 r1r1 x 1/4 R2R2 = 1/16 r1r1 R2R2
• 1/4 r1r1 x 2/4 R2r2 = 2/16 r1r1 R2r2
• 1/4 r1r1 x 1/4 r2r2 = 1/16 r1r1 r2r2

F2)

• 9/16 R1 - R2-, red phenotype
• 3/16 R1- r2r2
• 3/16 r1r1 R2-
• 1/16 r1r1 r2r2, White phenotype

Note: The "-" symbol represents either a dominant or a recessive allele.

Phenotypic ratio: 9:3:3:1

2)

We know that both dominant alleles R1 and R2 contribute equally to kernel color and cumulatively control the intensity of the red kernel phenotype.

Genotypes R1R1 R2r2 and R1r1 R2R2 are carrying the same amount of dominant alleles, all of them are contributing equally to the phenotype, meaning that the cumulative effect is the same in both cases.

If we compare, for instance, R1R1 R2r2 and R1R1 r2r2, we can assume that the first genotype is darker than the second one, because it has 3 dominant alleles ( R1, R1, R2), while the second one only has 2 dominant alleles ( R1, R1).

Dominant alleles contribute equally, but they have cumulative effects, so the more dominant alleles the are in a genotype, the darker is the kernel color.

C. The phenotypes of the R1R1 R2r2 and R1r1 R2R2 genotypes will be the same because the same number of dominant and recessive alleles are contributing to each genotype.