North American Grey Wolves were once always grey, but now some are black. Why? Dive into how the introduction of domestic dogs from Asia changed the makeup of wolves, forever.
Dogs and wolves share much in common
Did you know that it’s because of dogs that some members of the American Gray Wolf population have their famous black coats? Genetics plays an integral role in the coat color of both dogs and wolves, and prior to them meeting, gray wolves in North America had only the light gray coats for which they’re named.
So how did these wolves get their black coats, what does that have to do with dogs, and how exactly does the genetics work? Let’s start the story at the beginning…
The Gray Wolf was in North America when humans migrated there during the last ice age, bringing dogs with them. Soon after their arrival, wolves began to show darker colors, caused by increased melanin in their coat. The trigger for this change was a single gene, called the “K locus”.
Dogs carry a version of the K locus that overrides—or is ‘dominant’ to—wolves’ lighter color patterns, creating the black pigment melanin and darkening the coat color. We now know the K locus is a single gene in dogs’ DNA; the gene makes a protein that sends pigment cells into overdrive, producing lots of melanin. Even today, thousands of years later, black wolves can be seen across North America—the direct result of interbreeding with the domestic dog.
How genetics comes into play
Embark’s geneticists offer lessons to better understand how a dog’s genetics—its genotype—can affect its appearance—its phenotype—and how genotypes interact when dogs breed. Here, you will learn how to calculate the expected physical traits of a litter of puppies using a Punnett square.
Punnett squares are commonly used tools of geneticists and dog breeders, and we will use the K locus to illustrate how they work. The K locus, as we now know, is a gene in dogs’ DNA, which has the ability to confer a black coat in dogs that would otherwise have light-colored coats [Fun fact: the discoverers of this locus used the letter K to stand for kurokami, the Japanese word for ‘black hair’]. But before we get there, remember those two ‘versions’ of the K locus that caused some gray wolves to get a black coat? (See animated video above).
Genes, genotypes, and dominance
In the language of genetics, different versions of the same gene are called alleles. Dogs, like us, each carry two copies of each gene. Which alleles they carry depend on the alleles they inherited from their parents and determine the alleles they can pass on to their puppies.
Because every dog carries two copies of a gene, they can also carry different combinations of alleles. Dogs that carry two copies of the same allele are called homozygous, while a dog with two different alleles is called heterozygous.
At the K locus, we can see the effects of two different alleles, KB, and Ky. Prior to the arrival of domestic dogs, American Gray Wolves all carried two copies of the Ky allele; we write their genotype as KyKy. However, at least some of the dogs brought over when humans first came to North America from Asia carried copies of the KB allele. KB confers a dark coat and is also called Dominant Black. The name ‘Black’ makes sense, but what about ‘Dominant’? If those dogs carried two copies of Dominant Black (KBKB), they would certainly be black—but what happens if a dog is heterozygous KBKy? In this case, that dog looks just like it would if it carried two copies of KB.
Geneticists use the term dominant when a single copy of an allele confers the same phenotype as having two copies. Conversely, the Ky allele is termed recessive for the same reason: heterozygous dogs don’t look at all like homozygous KyKy dogs. This is similar to how the alleles for brown eyes in humans are considered dominant, whereas other eye colors (blue, green, hazel, etc) are considered recessive.
Parents, puppies, and Punnett squares
Here’s where Punnett squares come in! If we know the genotypes of two dogs, we can use the rules of genetics to predict the genotypes (and phenotypes!) of their puppies. Punnett squares are used to calculate breeding results for dogs by showing the expected result for a specific locus when two dogs produce a litter of puppies.
Take a dog that is heterozygous KBKy. When that dog produces a puppy, she will only pass on a single copy of the gene (the puppy’s other copy comes from dad). Which copy gets passed on is random, so about half of the time a puppy will receive KB and half of the time it will receive Ky.
On the Punnett square, we represent each of these possibilities with a row, one for KB and one for Ky. Her mate, who in this case is homozygous for the recessive Ky allele, gets a column for each copy of that allele. To predict the possibilities for the puppies’ genotypes at the K locus, we then combine the possible alleles from mom and from dad in each square. In this case, the puppies will either be KBKy or KyKy—on average, half of the litter will have black coats and half will have light coats!
Test your knowledge
Based on the above article, can you answer the following?
- How many copies of a gene do dogs (and humans!) have?
- How does a geneticist describe a dog that carries two different versions of a gene? What are these versions called?
- How does the Dominant Black KB allele actually change a dog’s coat color? (hint: watch the video)
Punnet Square Learning Tool
- What would the outcome be if a heterozygous KBKy dog mates with another dog who is also KBKy?
- What are the possible genotypes (KBKB, KBKy, or KyKy) of the puppies? What will they look like?
- Imagine you have a black dog but you do not know what it’s genotype is? Set up the Punnett square in a way that will tell you for sure whether the dog is homozygous or heterozygous
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Anderson, T., vonHoldt, B., Candille, S., Musiani, M., Greco, C., Stahler, D., Smith, D., Padhukasahasram, B., Randi, E., Leonard, J., Bustamante, C., Ostrander, E., Tang, H., Wayne, R., Barsh, G. (2009). Molecular and Evolutionary History of Melanism in North American Gray Wolves Science 323(5919), 1339-1343. https://dx.doi.org/10.1126/science.1165448
Schweizer, R., Durvasula, A., Smith, J., Vohr, S., Stahler, D., Galaverni, M., Thalmann, O., Smith, D., Randi, E., Ostrander, E., Green, R., Lohmueller, K., Novembre, J., Wayne, R. (2018). Natural Selection and Origin of a Melanistic Allele in North American Gray Wolves Molecular Biology and Evolution 35(5), 1190-1209. https://dx.doi.org/10.1093/molbev/msy031