One of the many fun things about owning a mixed breed dog is guessing what breeds make up their ancestry. But while it may be fun to speculate, it’s very hard to accurately identify the breeds present in a dog just by looking at him. In fact, visual identification—even by professionals—is only accurate about 25% of the time. That begs the question of why it’s so difficult. To get an idea of why visual breed identification is so challenging, it helps to have a basic understanding of how physical traits are inherited.

How genes are inherited

When a dog is conceived, he or she receives two copies of DNA—one from the father and one from the mother. Which pieces of DNA a dog inherits from each parent is completely random. Within each strand of DNA are genes. Genes determine traits such as coat color, ear type, tail style, and more. The random nature by which these genes are inherited explain why puppies (or humans, for that matter) who share the same mother and father can look very different.

Dominant vs recessive traits

Of the traits a dog inherits, some are dominant and some are recessive. Dominant traits require only one copy of the gene for the trait to be expressed. So, if either the mother or the father pass on a dominant trait, that trait will be seen in the offspring. Recessive traits require two copies—one from each parent. That means recessive traits can be hidden in the genetic make-up of a dog for generations, and not be seen until two copies of that gene are inherited by a puppy.

How traits determine a dog’s coat

To give you an idea of how inherited traits work together, let’s look at a dog’s coat. There are several genes that contribute to a coat’s appearance. For example, coat color comes from the cells that produce pigment. Black, brown and yellow are the basic foundations of color, but it’s not quite that simple. There is a dominant black gene and a recessive black gene, though dominant black is by far the most common. There are are also two forms of yellow. Clear yellow/red is recessive and sable (yellow and dark banded hairs) is dominant. Brown is always recessive. You can start to see why there are so many different color combinations in a dog’s coat, even within a single litter!

During development, pigment cells start at the top of a dog’s back and spread down his body to his legs and tail, and from the back of the head around the face. White spotting in coats occurs when a dog doesn’t have enough pigment cells to cover the entire body. Lack of pigment cells combined with the way pigment cells spread help explain why so many dogs have white on their stomachs, legs and tails, or a “blaze” down the middle of the face and muzzle.

The genes a dog inherits also determine coat length and type. Long hair is typically a recessive trait, while short hair is typically dominant. Curly hair, on the other hand, is a semi-dominant trait. That means if a dog has two copies of the trait it will have very curly hair, but if there is only one copy the coat will be wavy in appearance. (Though the wave may not be noticeable on short-coated dogs). That’s why when you breed a curly-haired Poodle and a straight-haired Labrador, you get the wavy-coated Labradoodle.

The combinations are endless

These are just a few examples of physical traits. Others include leg-length, body size, head shape—the list goes on! A dog’s DNA controls all of these traits and their various combinations. Hopefully this helps start to paint a picture of all the different factors that lead to a dog’s outward appearance. And why it’s so tough to visually identify a dog’s breed make-up!