Many years ago, I decided to breed a litter of cocker spaniels. I had a solid-black female with a good pedigree, and after some searching, I found a black male with a little spot of white on his chest with a similarly good pedigree.

Chas delivered six puppies — five solid black puppies and one blond. Blond? How had that happened?

Canine Genetics - How to Make a DogI had made certain that she “interacted” with no other dogs, but here was the evidence. She had a blond puppy. Once I did a little reading, however, I determined she was not a free spirit (or at least, not allowed to be a free spirit). The research led me to the field of genetics.

I can hear it now — flashbacks to middle school: “Why do I have to learn algebra? I’ll never use it.” Well, admit it — you do, and often. Balancing your checkbook requires algebra; comparison shopping requires algebra, and there’s much more.

I learned a little bit about genetics in a biology class in high school, but clearly did not retain the information thinking that I would never need it again. Little did I know, I would be researching it again years later. Once you understand the basic principles of genetics, you can use it too — maybe not every day and maybe not in depth, but you’ll know enough to get by when you need it.

Canine Genetics: How to Make a Dog

This is a very, very simplified explanation (because that’s the only kind I can give). Hopefully, you’ll take away from it some understanding of the problems that can occur with the offspring of matings of both purebred dogs and mixed breeds as a result of the genes they inherit.

Genes and chromosomes

Genes live in the chromosomes (which are in the nucleus of the cell), and each chromosome joins another chromosome to make a pair. All living things have chromosomes — each dog has the same 38 pairs of chromosomes in each of the millions of cells in its body, and the makeup of the cell’s chromosome pairs is the same in almost all of its cells.

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Canine Genetics
Photo: euthman

Genes control how the dog looks, how it sounds, any genetic diseases it might be predisposed to, its life span, its potential to go blind, its potential for conditions like hip displasia, and to some extent its personality — everything you can think of.

Of course, the environment affects this also. For instance, you can have a dog with a gene for a long life span who gets hit by a car when he’s three years old. He had the gene, but the environment affected its influence.

The 39th chromosome

Note that I said “… the same in almost all of its cells” a couple of paragraphs above. The 39th chromosome is different, and its difference is what we’re concerned with. (The blood cells are also a little different, but that’s not important here.)

The 39th chromosome controls gender and inherited genetic makeup, and it’s not active anywhere other than in the cells used for reproduction. This chromosome does not contain chromosome pairs but rather a random mix of half of each chromosome pair in the rest of the cells. In males, this is the sperm, and in females, the egg.

At conception, the father’s sperm will combine with the mother’s egg to make offspring whose cells contain one-half of the mother’s randomly selected chromosomes and one-half of the father’s randomly selected chromosomes. The combination of each parent’s half becomes the chromosome pairs in its offspring’s cells, its offspring’s genetic makeup, and it’s a roll of the dice as to what the genetic makeup of each puppy will look like as a result of that combination. The combination, unique in each puppy, causes the differences seen in siblings.

Dominant and recessive genes

The other basic principle is that some genes will overwhelm other genes. The genes that overwhelm are called dominant; the genes that are overwhelmed are called recessive. If a chromosome pair contains one chromosome with dominant genes and one with recessive genes, the chromosome with the dominant genes is the one that controls. The recessive genes are still there. They just don’t show.

Canine Genetics
Photo: pixel0908

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When referring in writing to genes, the names of genes are abbreviated. Dominant genes are in all caps while recessive genes are in lowercase. To indicate the makeup of a particular chromosome pair, the genes are written together: BLK-bld would be the makeup of the gene for black coat color with a recessive blond gene. bld-bld would be the makeup of two genes for blond coat color. Blond dogs cannot have the gene for black coat color because they aren’t black — black is dominant over blond.

Remember, this is a very basic explanation. In reality, there are all sorts of dominant-recessive issues. In addition, few, if any, chromosomes affect only one thing, and few things are controlled by only one chromosome. But we’re taking baby steps here, and this generality will do.


You want a puppy. In fact, you’ve already named your new puppy Coco. But you want a blond puppy, and since black is the dominant color, two black dogs will have only black puppies, right?

Not necessarily. Look at Chas’ litter above. To figure out what coat colors will be available in this litter, you need to go back to Coco’s grandparents.

Coco’s grandparents

Coco’s parents, the two black dogs to be mated, each had a sire who was black with no blond recessive and a dam who was blond. In other words, Coco’s father’s father (Coco’s paternal grandfather) was black with a genetic makeup of BLK-BLK, and his father’s mother (Coco’s paternal grandmother) was blond with a genetic makeup of bld-bld. This held true for Coco’s mother. Her father (Coco’s maternal grandfather) was black with no blond recessive, a genetic makeup of BLK-BLK, and her mother (Coco’s maternal grandmother) was blond with a genetic makeup of bld-bld.

Canine Genetics - How to Make a Dog

Coco’s parents

Now that we have the genetic makeup of Coco’s grandparents, we can determine the genetic makeup of Coco’s parents. Remember that each grandparent supplied half of the chromosomes for their respective offspring (Coco’s parents). Since Coco’s father received a gene for black coat color from his father and one for blond coat color from his mother, his genetic makeup is BLK-bld, and he has a black coat since BLK is dominant over bld. Ditto for Coco’s mother.

Canine Genetics - How to Make a Dog

Coco’s litter

How can we predict the coat colors in Coco’s litter? Since one-half of each puppy’s chromosome makeup comes from half of its father’s chromosomes and one-half of its chromosome makeup comes from half of its mother’s chromosomes, and since black is dominant over blond, three-quarters of the parents’ puppies will be black and one-quarter will be blond.

Canine Genetics - How to Make a Dog

Coco’s littermates who received the BLK-BLK combination from their parents will be black. Likewise, the puppies who received the BLK-bld or bld-BLK combination will be black since black is dominant over blond. The puppies who received the bld-bld combination will be blond. So the litter is three-quarters black and one-quarter blond.

You choose Coco

Of course, this is only an approximation of the breakdown of dog's coat color in any single litter. Over many litters from this same mating, however, this ratio should hold true. And there may be more than four puppies in the litter in which case the ratio will still be approximately three-quarters black and one-quarter blond.

Genes and inherited conditions

Not only do genes control relatively unimportant things like coat color, they also control genetic conditions that can cause disability or death. Now that you know how genes combine to produce certain traits, you can use this knowledge to understand some of the problems of purebred and mixed-breed dogs.

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In a small geographical area, the mating of two purebred dogs will necessarily have to include matings to close relatives because close relatives are all that are available. This is also true in the matings of show-quality dogs with other show-quality dogs — a very small group — to reproduce the traits that won in the ring.

In these cases, the chances of breeding dogs carrying undesirable recessive genes are far greater. There are fewer mates to choose from, and the genetic makeup of the mating dogs will have less variety because of the limited number of genetic combinations available. It’s not impossible for all dogs in a small population to carry the same recessive genes, which they’ll then pass on to their offspring.

Canine Genetics
Photo: Me in ME

The same thing applies to mixed breeds except that the number of mates they can choose from is larger because they’re not limited to mating within their breed. In a small geographical area though, it carries the same kind of risk as that for purebred dogs.

As an example, go back to the coat color chart above except for black coat color, substitute “normal condition” (NC, which is a dominant gene) and for blond coat color, substitute “fatal condition” (fc, which is a recessive gene). We have to assume that the fatal condition doesn’t appear until after the first conception and whelping.

Just as in the coat color chart, we’re going to pick a sire and a dam who each have a dominant gene for NC and a recessive gene for fc. How will their litter turn out?

One-quarter of the puppies will have a NC-NC gene combination. They’ll be healthy and have no chance of passing on the fc gene to their offspring.

One-quarter of the puppies will have a NC-fc gene combination and one-quarter will have a fc-NC gene combination. In other words, one-quarter received the NC gene from their father and the fc gene from their mother, and one-quarter received the fc gene from their father and the NC gene from their mother. Except for statistical purposes, it doesn’t matter who the puppy gets the fc gene from. It has it and can pass it to its offspring.

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And one-quarter of the puppies will have the fc-fc gene combination, having received the recessive fc gene from each of their parents. They will eventually die of the fatal condition. If they have offspring before they die, their offspring will all have the recessive gene for the fatal condition since that’s all this puppy can contribute.

Canine Genetics
Photo: Kevin Timothy

While dangerous, these matings can be okay if done very carefully by someone who knows what they’re doing when it comes to genetics. Even so, the resulting litter will contain more than the average number of culls (puppies that aren’t acceptable for one reason or another). Culls are usually destroyed.

As I said, this is a very, very simplistic explanation of how genes from the parents control everything about their offspring and how the offspring can inherit genes for conditions the offspring doesn’t have but which can be passed to their offspring for generations to come.

If you're interested in breeding your dog, you should seriously consider this information and look into your dog's genetic history before you make your decision. You should also think about genetics when you are shopping for a puppy. Make sure to meet the parents and ask to see vet records. If the breeder will give you the information, you may even want to call the veterinarian and ask some questions yourself.

It's better to be diligent about your puppies health and well-being now. If you're not, you could end up with a very ill dog and a lot of expensive bills in the future.

Catherine is a writer and editor living in Memphis, TN. After working for a state supreme court justice, she retired to write and work with her dogs, six papillons and a rescue. She has been a foster parent for abused children, using her love of dogs to teach them the benefits and responsibilities of loving something dependent on them.