B.8B_Genetics_Approaching

Puppies! The family was so excited to finally see the first litter of puppies from their black Labrador retrievers, Ebony and Shadow. The children were surprised that the seven puppies were not all black. There were four black Labs, one chocolate Lab, and two yellow Labs. The children were confused because their cousins have black Labradors. Their puppies are always black. One of the children asked the question “How can black dogs have brown and yellow puppies?” Let’s look at the genetics of Labrador coat color to explain the puppy possibilities.

There is one gene that codes for coat color in Labradors. The gene has two forms, or alleles – B and b. B is the dominant allele. It codes for black coat color. The other allele, b, is recessive. It codes for chocolate (brown) coat color. The puppies inherit one form from each parent. If a puppy inherits at least one black allele it will have a black coat. Black is dominant over brown.

There are two possible combinations of the alleles for black coat color – BB (homozygous black) or Bb (heterozygous black). The combinations show the genetic makeup for the trait. Another term used for genetic makeup is genotype. Ebony and Shadow must have one of these genotypes. Neither dog has genotype bb because that codes for brown coat color. Let’s look at all possible outcomes in a monohybrid cross of two black Labs like Ebony and Shadow. A monohybrid cross is one that involves only one trait.

Outcome #1: If both Ebony and Shadow are BB, then all offspring are also BB. They all inherit black coat color.

Outcome #2: If one parent is BB and the other is Bb, then all offspring are either BB or Bb. They all inherit black coat color.

Parents:  BB x Bb

Outcome #3: If both parents are Bb, their offspring can be black or brown. Their genotypes are BB, Bb, or bb. There are black and brown puppies in the litter, so both Ebony and Shadow must have the heterozygous black genotype (Bb).

Parents:  Bb x Bb 

But how do we explain the yellow Labrador puppies? None of the combinations result in the yellow color. There happens to be a second gene that affects coat color in Labradors. The two forms of this gene are E and e. The gene does not actually code for a 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 E is inherited, the coat color gene stays ON. So, EE and Ee will result in the black or brown coat color coded by B and b.

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

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. Yellow coat color is not possible.

Parents:  EE x Ee 

Outcome #3:  If both parents are Ee, their puppies can have any genotype – EE, Ee, or ee. The coat color gene can be ON for some puppies. Their coat color can be black or brown. The coat color gene can be OFF for some puppies. Their coat color is yellow. 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 

The two genes work together. So, let’s look at all possible coat color outcomes when both traits are inherited together. The table shows 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 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. The children’s cousins had black Labs that always have black puppies. There are many possible genotypes for black Labs that have all black puppies. 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 all black puppies with any other parent. The other parent can be EEBB, EEBb, EeBB, or EeBb.

The study of genetics can be interesting. It helps to explain how traits are inherited. Certain traits, like chocolate and yellow coat colors in Labrador retrievers, can remain hidden for generations. Understanding basic genetics can solve 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/.