by Lauren Bailey
You may have looked at your parents and thought about the genetics behind your own hair colour, but have you ever considered your dog’s? Man’s best friend comes in all different colours, including one of the most beloved breeds, the Labrador Retriever. Everyone knows about black labs, chocolate labs, and yellow labs, but Labrador Retrievers can actually come in an even broader array of colours, including silver, charcoal, and champagne. While these dogs all look distinct, it’s entirely possible to breed two black labs together and end up with a chocolate lab or breed a black lab and a chocolate lab together and end up with a yellow lab! Labrador coat colour all comes down to three important genes: the B gene, the E gene, and the D gene.
When studying genetics, we often use the term genotype and phenotype. Genotype is the genetic makeup, while the phenotype is the characteristics we can physically observe, such as hair or eye colour. Sometimes animals with two different genotypes can actually have the same phenotype. This is because genes can be dominant or recessive, and dominant genes “override” the recessive gene, making the individual what is called a carrier of the recessive gene. Therefore, an animal that has two dominant genes has the same phenotype as an animal with a dominant and recessive gene. We don’t know the genotype of every person or animal we encounter, so a lot of times these distinctions can only be found once we have kids, such as two brown-eyed parents having a kid with blue eyes.
The same can be said for Labrador Retrievers. The first gene, the B (black) gene, is responsible for black pigmentation. If this gene is dominant (“B”), the puppy will have black pigment, and if the gene is recessive (“b”), the puppy will have brown pigment. Just like with humans, when two dogs breed, the puppies receive an allele from each parent. Any dog that has a BB or Bb genotype will have black fur, but if the puppy receives two recessive b alleles, it will have brown fur, making a chocolate lab. Although you wouldn’t be able to tell if two black labs had BB or Bb genotypes by looking at them, if you bred them and got a brown puppy, it would mean that they are carriers of the recessive b allele. So that’s why two black labs can have a chocolate puppy, but two chocolate labs can’t have a black puppy.
But what about yellow labs? The E (extension) gene is responsible for allowing the deposit of pigment into the fur, which means it can mask the phenotype of another gene in a process called epistasis. The dominant E gene (“E”) allows the pigment to be deposited normally into the fur, but its mutated recessive gene (“e”) prohibits the depositing of pigment. If you’re a redhead, you’re already familiar with this, because a mutation of the same pigment-depositing receptor is what causes red hair in humans! Black and chocolate labs always have an EE or Ee genotype, but yellow labs are the product of an ee phenotype and pigment is unable to deposit into the fur.
Interestingly, the E gene only controls fur pigment deposit. If you have a yellow lab, you can tell if they have a BB or bb genotype by the colour of their nose. Yellow labs with the BB genotype have dark noses, while yellow labs with the bb genotype have pink noses. This is also why chocolate labs always have a pink nose: they have no black pigment!
The D (dilute) gene is controversial because Labrador Retrievers originally didn’t have diluted colours, and some experts think this is because they were bred with another dog species that does carry the D gene — or it might just be a recent mutation that emerged as Labrador Retrievers became more popular and were bred more. Pure black, chocolate, and yellow labs either have the DD or Dd genotype, meaning their coat colours aren’t diluted. But when these dogs have the dd genotype, black labs become charcoal, chocolate labs become silver, and yellow labs become champagne. Combining all of these genes together, we can write out each dog’s genotype and use Punnett squares like the one below to get an idea of what its puppies would look like. For example, a black lab would be B_EEdd and a black-nosed champagne Labrador’s genotype is a Bbeedd. What would the puppies look like if you bred those two dogs together?
Understanding how genotypes influence phenotypes helps us better understand the people (and pets!) around us and why they are the way they are. Next time you see a yellow lab with a pink nose, you can call it a bbeeD_ instead!