In America, the collie is considered one breed and interbreeding between the two coat varieties is allowed (and will still produce a purebred collie). Understandably though, people still wonder how smooth and roughs are both born to in the same litter.

The best thing to remember is that the gene that produces a smooth coat is dominant and the gene for a rough coat is recessive. In genetics, two genes are needed to produce a single trait. The combinations therefore can be as follows

Dominant + Dominant
Dominant + Recessive
Recessive + Recessive

Anytime a dominant gene is in a trait, that dominant feature will appear. For instance, the first two combinations will produce a smooth collie. However, the collie with that dominant trait can also be a carrier of a recessive trait, and can pass it along to their children.

In the above table, here is how that will play out in real life:

Dominant (Smooth) + Dominant (Smooth) = Smooth Collie (can’t produce rough children)

Dominant (Smooth) + Recessive (Rough) = Smooth Collie (carries a rough gene, can have rough children)

Recessive (Rough) + Recessive (Rough) = Rough Collie (can’t produce smooth children)

Based on genetics, you can see that if a rough collie and a smooth collie were mated, they can produce may be able to produce rough and smooth children if the smooth parent is a carrier of a rough recessive gene. However, if two roughs are bred together, a smooth will not be born since neither of them carries the dominant smooth coated gene!

It is important to always remember that the coat variety takes backseat when it comes to beautiful lines, great carriage, sound conformation, and a healthy collie.

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