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Why Do Some Pups in a Litter Look Different Than the Others?

Puppies in the same litter may have different fathers.

Szakalová Edina--Creative Commons License CC0
Source: Szakalová Edina--Creative Commons License CC0

I have been looking at a number of litters of puppies recently. It is quite striking the way puppies in one litter will look so similar to one another that one gets the impression that they were all clones of one another, while other litters contain pups that display a variety of shapes, sizes, colors, and personalities. This got me thinking about why this is so. The answer turns out to be that it is all a matter of genetics.

The general genetic principle is that the more similar the parents are in appearance, size, and behavior, the more similar the puppies will be. Thus it should not be surprising that litters of purebred dogs will tend to be much more uniform than mixed breed litters.

The reason for this is that pure breeds have been manipulated through selective breeding so that there is much less genetic variability from dog to dog and within a breed. Certain characteristics have been "fixed" so that they are predictable characteristics of the breed and these will include body shape, colors, and basic behavioral predispositions.

For example, in a litter of Golden Retrievers, one should never find a black dog or one with spots or colored patches. This is because of what geneticists call homozygosity, which is just a technical term which means that dogs of the same breed have similar genetic material. So if you breed a pair of Cocker Spaniels you will never find something that looks a looks like a Bulldog in the litter. It is much like having a pot of chicken broth. Every time you dip a ladle into the pot you will find it full of chicken broth and nothing else, simply because it is such a uniform soup.

However, if you are mating dogs of different breeds, the situation changes drastically. Now you are dealing with what geneticists call heterozygosity, which is a broad mixture of different genes. To go back to our soup analogy, we are now looking at a vegetable beef soup, made with large chunks of different vegetables. If we dip a ladle into this mixture, one time we may come up with some chunks of beef, some potatoes, and some peas in our broth, while another time we may get little beef and few potatoes however there may be lots of peas, carrots, and onions in the ladle.

So with mixed breed dogs, all bets are off. As an example, suppose that you cross a Scottish Terrier and a Poodle. Some of the resulting pups may look like Scotties and others like Poodles. There may also be some strange mixtures, like something which looks like a long-legged Scottie, or a square-faced Poodle, or a curly-coated terrier and so forth.

In dogs, however, there is another reason why the pups in a litter may not look or behave alike. That is because all of the dogs in the litter may not have the same father.

For those of us who tend to think in terms of human genetics, this seems very strange; however, one must remember that dogs are a different species and their reproductive systems work in a different way. It can occur in dogs because females produce a number of ova (eggs) at the same time, which is why they have litters of pups rather than one at a time.

Now, at the time that the female ovulates, the ova are still immature and they will continue to mature over the next two to three days after ovulation. Even though they are immature, some have progressed far enough along to be successfully penetrated by sperm, and after ova have fully matured, they will remain available for fertilization for another two to seven days.

If we look at the male side of the reproductive act, we find that canine sperm can remain alive and capable of performing their functions for up to eight days. Now add the behavioral components — namely dogs are polygamous breeders who will mate with anyone who makes themselves available, and females remain receptive to male suitors for a week or more. So if Mommy is a party girl, her pups might look quite different from each other because, although they came from the same litter, they had different daddies. Thus, conscientious breeders of purebred dogs must serve as fairly strict chaperones to make sure that this kind of behavior does not happen.

Stanley Coren is the author of many books including The Left-hander Syndrome.

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