In Labrador retrievers, two genes determine fur color: the E gene and the B gene. Black fur (B) is dominant to brown fur (b). However, the presence of (ee) will overshadow and create a puppy with yellow fur. Create a Punnett square crossing 2 Labs that are heterozygous for both genes. Then color code your Punnett Square based on the resulting phenotypes. Don’t forget to take epistasis into account! Determine the phenotypic ratio.
The Correct Answer and Explanation is:
Let’s break down the genetic cross involving Labrador retrievers and the two genes that determine fur color: the E gene and the B gene.
- B (black) is dominant to b (brown).
- E (dominant) allows fur color to be expressed, while the ee (recessive) genotype will result in yellow fur, regardless of the B gene alleles. This is an example of epistasis, where the E gene overrides the effects of the B gene.
Parent Genotypes:
We are crossing two Labrador retrievers that are both heterozygous for both genes. Therefore, each parent’s genotype is:
- BbEe (heterozygous for both the B and E genes).
Punnett Square:
To create the Punnett square, we’ll combine the alleles from both parents. Each parent can pass on one allele for the B gene and one allele for the E gene. Thus, the possible gametes are:
- BE
- Be
- bE
- be
Let’s set up the Punnett square with these four possible gametes from each parent:
| BE | Be | bE | be | |
|---|---|---|---|---|
| BE | BBEe | BBEe | BbEe | BbEe |
| Be | BBEe | BBEe | BbEe | BbEe |
| bE | BbEe | BbEe | bbEe | bbEe |
| be | BbEe | BbEe | bbEe | bbEe |
Phenotype Determination:
- BB or Bb (black fur): If there is at least one B allele and at least one E allele (Ee, EE, or ee), the dog will have black fur.
- bb (brown fur): If there are two b alleles and at least one E allele (Ee or EE), the dog will have brown fur.
- ee (yellow fur): If the dog has the ee genotype, regardless of the B or b alleles, the dog will have yellow fur due to epistasis.
Phenotypic Color Mapping:
- Black Fur: Any combination with at least one B allele and at least one E allele (BBEE, BBEe, BbEE, BbEe).
- Brown Fur: bbEE or bbEe.
- Yellow Fur: ee (regardless of B or b alleles).
Results of the Punnett Square:
- Black Fur: 9 outcomes (BBEE, BBEe, BbEE, BbEe).
- Brown Fur: 2 outcomes (bbEE, bbEe).
- Yellow Fur: 4 outcomes (any with ee).
Phenotypic Ratio:
The phenotypic ratio from this cross is:
- 9 Black : 3 Brown : 4 Yellow.
This ratio reflects the combined effects of both the B and E genes, with epistasis from the ee genotype taking precedence and causing yellow fur in a significant proportion of the offspring.