B.8B_Genetics_OnLevel_03

But how do we explain the yellow Labrador puppies? None of the allele combinations result in the yellow color. There happens to be a second gene that affects coat color in Labradors. The two alleles are E and e. This gene does not actually code for a specific coat color. This gene acts like a switch that turns the coat color genes ON or OFF. When a Labrador puppy inherits two recessive forms of this gene, ee, the coat color gene is turned OFF. This results in a yellow Labrador puppy. If a dominant allele, E, is inherited, the coat color gene remains ON. So, the genotypes EE and Ee will result in the black or brown coat color coded by the B and b alleles.

There are two possible combinations of the E alleles for black and brown dogs – EE or Ee. Neither Ebony nor Shadow have the combination of ee because their coat color gene is not turned OFF. Let’s look at the possible genotype outcomes in a monohybrid cross of two black Labs.

Outcome #1: If both Ebony and Shadow are EE, all their offspring are also EE. Their coat color gene is ON for black and brown coat colors. There is no possibility for yellow coat color.

Outcome #2: If one parent is EE and the other is heterozygous Ee, all offspring are EE or Ee. Their coat color gene is ON for black and brown coat colors. There is no possibility for yellow coat color.

Parents:  EE x Ee 

Outcome #3:  If both parents are Ee, their offspring can have any genotype EE, Ee, or eeThe coat color gene can be ON for some puppies, resulting in the possibility of black or brown puppies.  The coat color gene can be OFF for some puppies, resulting in the possibility of yellow puppies.  This is the only combination of parent genotypes that can result in black Labs, chocolate Labs, and yellow Labs.  There are Labs of all three colors in the litter, so both Ebony and Shadow must have the heterozygous genotype (Ee). 

Parents:  Ee x Ee 

Because the two genes work in combination with each other, let’s look at all the possible coat color outcomes when inheriting both traits together. The table indicates the possible puppy coat colors for black Lab parents. Note that each parent must have at least one E allele because the coat color gene was not turned OFF. Also, each parent must also have at least one B allele for black coat color.

The only possible genotype for both Ebony and Shadow is EeBb. This is the only combination that can result in puppies of all three coat colors. There are several possible genotypes for black Labs that have all black puppies, like the black Labs that belong to the children’s cousins. At least one of the parents must have the genotype EEBB. As you can see in the table, EEBB is the only genotype that will produce only black puppies regardless of the other parent genotype. The other parent can be EEBB, EEBb, EeBB, or EeBb.

The study of genetics is interesting and helps to explain the mechanisms of inherited traits. Certain traits, like chocolate and yellow coat colors in Labrador retrievers, can remain hidden for generations. Understanding basic genetic principles unravels the mystery of how these traits show up in future generations. The family will have a new mystery to solve if a silver Labrador puppy shows up in one of Ebony and Shadow’s future litters!

Source:

Schini, Scott. “Genetics of Labrador Coat Color.” Maple Leaf Veterinary Care Center, 14 Aug. 2018, https://mapleleafvet.com/genetics-of-labrador-coat-color/.