B.8B_Genetics_Approaching_02

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