Question

You are examining two traits of a plant called Trait A and Trait B. Each trait is associated with a different gene on a different chromosome. Trait A shows: the same phenotype for both the heterozygous and homozygous dominant allele combinations a different phenotype for the homozygous recessive allele combination. Trait B shows: three different phenotypes, each associated with only one of the three possible allele combinations. A) Describe the type of inheritance associated with Trait A and describe the type of inheritance associated with Trait B. In your own words explain how you arrived at each of your answers. B) Your plant is displaying the Trait A phenotype associated with a homozygous dominant or heterozygous allele combination. In your own words, explain how you would determine which allele combination is present in your plant.

Answer 1

1. Let us assume
Recessive allele = a
Dominant allele = A
The presence of a single dominant allele is sufficient to cause the dominant phenotype.
The phenotype of homozygous dominant (AA) = The phenotype of heterozygous dominant (Aa)
There are two possible phenotypes.
i. AA = Aa
ii. aa
Trait A exhibits complete dominance.

In the second case, let us assume that
Recessive allele = b
Incompletely dominant allele = B
The presence of a single dominant allele is not sufficient to exhibit the dominant phenotype.
The phenotype of homozygous dominant (BB) ≠ The phenotype of heterozygous dominant (Bb)
There are three possible phenotypes.
i. BB
ii. Bb
iii. bb
Trait B exhibits codominance or incomplete dominance

2. To determine the genotype of an unknwon individual with a dominant phenotype, a testcross is performed. In a test cross, the unknwon individual is cross with the recessive individual for the same phenotype.
i. If the unknown individual is heterozygous dominant, half the testcross progeny would exhibit the recessive phenotype.
Test cross: Aa X aa
Progeny: Aa a
Phenotypic ratio = 1:1

ii. If the unknown individual is homozygous dominant, all the testcross progeny would exhibit the dominant phenotype.
Test cross: AA X aa
Progeny: Aa
Phenotypic ratio = 100% dominant phenotype