Using Punnett Squares and Locus genes to figure out dog coat genetics

I love genetics! There are health problems in my family that I inherited and then passed down to my children. That started me down the rabbit hole of how traits get passed down to offspring. As I started researching how to run my Bernedoodle breeding business, I wanted to learn how dog breeders use genetics to predict puppy litters.

Introduction to Punnett Squares in dog coat genetics

Named after the British geneticist Reginald Punnett, this simple diagram is a cornerstone in genetics, helping breeders and scientists alike understand how different traits are inherited. By mapping the parents' genes, we can see the possible combinations in their offspring. It's like a genetic lottery ticket, and each square reveals a potential outcome for traits like coat color, texture, and even certain health aspects. Today, we're using this remarkable tool to explore the possible puppy outcomes from our future parents Coco and Hollow.

Here are some genetics for our two parents. Don't worry if you don't know what the letters mean.

Hollow:

E-Locus: Ee, K-Locus: ky ky, A-Locus: at at, B-Locus: bb, M-Locus: mm, S-Locus: nS, Coat Type: CC, Furnishings: FF

Coco:

E-Locus: Ee, K-Locus: ky ky, A-Locus: at at, B-Locus: Bb, M-Locus: Mm, S-Locus: Ssp, Coat Type: CT, Furnishings: FF

I'm also going to be referencing this chart from Embark for some of the findings.

Let's follow the chart with each of the parents to see if it's accurate.

• Hollow shows brown with tan points which is accurate.

• Coco shows black with tan points. You then add the Mm (not in the chart) which lightens the black coat to gray with black spots. We talk about the M gene in our post about the Merle coat.

Locus

You probably noticed letters followed by 'Locus' on the chart. Think of a locus as a specific 'address' on a chromosome in our DNA. Much like how a house address tells you where to find a house on a street, a locus tells you where to find a particular gene or genetic sequence on a chromosome. Just as houses on the same street can look different, different versions of a gene (known as alleles) can be found at the same locus, contributing to our unique traits.

Locus Priority

Imagine you're creating a piece of art, and you have a set of colored pencils. Some pencils (loci) have stronger pigments that can cover or dominate the colors of other pencils. In dog genetics, these 'stronger' loci are like dominant genes that mask or override the effects of other genes (the 'weaker' loci). Just as a bold red pencil might cover a light yellow, a dominant gene at one locus can mask the expression of a gene at another locus, determining the dog's traits, like its coat color.

1. Base Layer (K-Locus):

• This is like the first layer of paint on a canvas. The K-Locus determines whether a dog can have black fur (dominant) or not (recessive). It's the foundational layer that sets the stage for the rest of the coat's appearance.

1. Primary Colors (E-Locus and A-Locus):

• These are like the primary colors you add over the base layer. The E-Locus decides if the dog's coat can produce black or brown. If it can then the A-Locus dictates the pattern of this pigment (like solid color, grizzled, etc.).

1. Detailing and Texture (B-Locus, M-Locus, S-Locus):

• Think of these loci as adding details and textures to the painting. The B-Locus determines if the black pigment is diluted to brown. The M-Locus might add lighten the black and brown in spots, creating a dappled effect. The S-Locus adds white markings or spots, like adding highlights to your artwork.

1. Final Touches (Furnishings and Coat Type Loci):

• These are like the final touches an artist adds to bring the painting to life. These loci determine the presence of furnishings (like beards and eyebrows) and the type of coat (curly, straight, wavy). It's the finishing detail that gives each dog its unique and final appearance.
  

Each of these genetic 'layers' contributes to the overall look of the dog, much like how an artist builds up a painting from the base layer to the final details. The dominance order of these loci ensures that each layer affects the final outcome in a specific way, resulting in the diverse and beautiful range of dog coats we see.

Let's take all of this information and see if we can guess how Hollow's and Coco's dog coat genetics can mix together? That's where Punnett squares come in.

Making Coat Predictions with Punnett Squares

We will look at the E-Locus first. The most important finding is the ee. That tells us that there is no black and/or brown pigment and the dog will have a red or tan basecoat. Because it is one possible combination of 4, the odds will be 25%.

25% Red/Tan puppies

If the puppies inherit EE or Ee then we will move onto the K-Locus. Both dogs are ky ky. If you do a Punnett Square you will see that every option is ky ky. The particular combo of ky ky is ignored and the A-Locus will determine the next detail.

The combination of ky ky will be ignored for the A-Locus.

The A-Locus determines if the dog is going to have tan points on their face, eyebrows, chest and legs. Think of a rottweiler or doberman pinscher but with more white. Both dogs have 'at at' so all the puppies who don't have ee (red coat) will have tan points because they will all have at at, just like we say with ky ky. I know it can get a little confusing.

The B-Locus will determine if the base color of the dog will be black or brown. Just one capital B will produce a black coat. bb will produce a brown coat. Remember that ee dogs are red. After taking away 25% for red puppies then what is left will be split between black and brown puppies.

The odds are now 37.5% black and 37.5% brown puppies

Now we will look at the M-Locus. The M is for Merle. I go more into Merle coats here. Merle genes will add a lightener to the base coat. This can turn a black coat gray and a brown coat a lighter tan. The color won't be uniform and will give the puppy a cool spotted look. However, Merle is very hard to see on a red coat. Coco has the Merle gene but Hollow does not so according to the Punnett Square, 50% of the puppies can inherit the Merle gene.

50% of the puppies will have the Merle gene

The S-Locus details how much white a dog will have. Some breeds will have a bit of white in the coat despite what the S-Locus shows. I won't demonstrate a Punnett Square for the S-Locus but I have determined the dogs will have a little more white in their coat than usual.

Hollow and Coco both have dominant genes for furry mustaches and beards. They also will both pass down their curly coats.

Litter Coat Breakdown

After all of this I now have a good idea what to expect. This only determines possibilities and odds. There also may be other genetic factors at play that I haven't considered. Odds also don't equal reality. Coco could have all of her dogs be red and white even though the odds are just 25%.

Probabilities

25% Red and White puppies

18.75% Black and white with tan points puppies

18.75% Brown and white with tan points puppies

18.75% Gray and white puppies with black spots

18.75% Tan and white puppies with dark brown spots

All will have bushy eyebrows, mustaches and beards, and curly coats. They will all probably fall in the mini size category because the parents are mini size.

I'm excited for the variety. This is my first litter and I can't wait to see which puppies will be good choices to be future moms and dads and beautiful companions!

Example of a Black Tri-Color, Brown Tri-Color, Black (Blue) Merle, and Red and White