Dog Genetics - Level 1
A Dog Genetics Primer
Learn the basics of canine genetics and how genes affect dog height, coat type and more.
By D. Caroline Coile, Ph.D.
No other species of mammal has the diversity that the domestic dog displays. Whether the dog at your side is a hairless Chinese Crested, long-legged Greyhound, massive Neapolitan Mastiff or flat-faced Pug, they all share the same basic DNA with each other and with their wolf ancestors, but with great variation in certain genes.
The latest studies determine that dogs were first domesticated in the Middle East from wolf ancestors about 15,000 years ago. Humans probably selected for gregariousness and docility, and eventually for physical and behavioral traits that they found helpful or fanciful. By Roman times Greyhound-type dogs, mastiff-type dogs, spaniel-type dogs, terriers and lapdogs were already distinct.
Ancient lap dogs?
Archaeologists have found the remains of small dogs dating back 12,000 years in the Middle East. And a recent study discovered a gene (called IGF1) found in all small dogs is closely related to one found in Middle Eastern wolves.
Don't confuse small dogs with short-legged dogs. Dwarfed dogs like Dachshunds and Corgis share a different genetic mutation (called the fgf4 retrogene) that suggest they all come from an ancient common ancestor as well. Such major physical changes that arise from single mutations suggest that much of the evolution of domestic dogs occurred rapidly rather than as a gradual accumulation of tiny changes. Researchers have mapped the entire dog genome, and have currently identified more than 1,600 genes that have been altered by artificial selection.
But before you can understand how various genes made the dog by your side (or in your lap), some background on basic dog genetics: Dogs have 39 pairs of chromosomes, with one member of each pair inherited from each parent. At the same locations along each chromosome are groups of base pairs that form genes. The genes on both chromosomes may be the same (a condition called homozygous), or they may be different (heterozygous).
Dominant vs. recessive
In many cases, a trait is determined by the presence of a single gene. If a dog needs only one copy of a particular gene to express a trait, that gene is said to be dominant. If a dog must have two copies, that gene is said to be recessive. The dog world is filled with examples of dominant and recessive genes. Do you have a chocolate Lab? If so, he has two copies of the "b" gene, which is a recessive gene that makes any black pigment turn brown. What about a black Lab? He may have two copies of the dominant "B" gene, or he may have one copy of the "B" gene and one copy of the "b" gene. Because "B" is dominant to "b," you can't tell which just by looking at him.
But in some cases you can tell whether a dog is homozygous or heterozygous just by looking at him. Take the case of the merle color pattern, that mottled mix of dark and light pigment found in Australian Shepherds, Collies and some other breeds. The presence of a single "M" gene causes the merle pattern. But the presence of two "M" genes causes the merle pattern but with excessive amounts of white – as well as the strong possibility of severe visual and auditory disorders.
This is a case of incomplete dominance, since you can tell an "mm" dog (non-merle), "Mm" dog (normal merle), and "MM" dog (merle with excessive white) from one another. This is also why you will never find a true-breeding line of merles – at least, not from responsible breeders. To produce merles ("Mm") without risking the health problems of double-merles ("MM"), you must only breed merle ("Mm") to non-merle ("mm").
In many cases, genes at one location affect the expression of genes at another location. Back to our black and chocolate Labs – where do yellow Labs fit in? Genes at the E location affect whether any dark pigment at all can be formed in the fur. A dog with "EE" or "Ee" allows dark pigment, so a Lab with either would be either black or chocolate.
But "ee" restricts the formation of dark pigment, so these dogs will be yellow whether they would otherwise be black or chocolate. But here's a hint: yellow Labs with black noses carry the "B" gene, and those with liver noses don't. And if you really want to know for sure, there's a DNA test for that.
Many traits in dogs are discreet: The dog is either black or brown, or merle or non-merle. But many more are continuously distributed over a range of values. Unlike clothing sizes, dogs don't just come in small, medium and large; they come in everything in between. Many such traits are determined by the interaction of many genes in which the number of particular genes determine how extreme a particular trait is. Hip dysplasia is another trait that appears to be polygenic rather than under the influence of just one or two genes.
D. Caroline Coile, Ph.D., is an award-winning author of 26 books and hundreds of articles about dogs.
<< Go back to class schedule and course materials >>
Give us your opinion on Dog Genetics 101
Login to get points for commenting or write your comment below
Get New Captcha