A common question we get here at Wisdom Health is why puppies from the same litter can look so different and have different ancestry trees in their WISDOM PANEL™ reports. There are a couple reasons for this. One reason relates to how genes are inherited, and the other has to do with the reproductive systems and mating habits of dogs. Let’s look at both.

Inheriting Genes

In dogs (and any other animal), a parent passes down 50% of their genetic makeup to their offspring. However, the segments of DNA that the offspring inherits from each parent are largely random. This process, known as genetic recombination or genetic reshuffling, means that the actual genetic composition of siblings varies.

In purebred dogs, this variation doesn’t produce dramatic differences within a litter since both parents are genetically similar. In mixed breed dogs—which are genetically diverse—the variation in genetic composition can produce different breed percentages, physical traits and predispositions. Just as humans have different physical characteristics from their brothers or sisters, puppies from the same litter can have variation in their coat color, coat length, head size, ear shape and more.

The exception to this is siblings that are identical twins and share the same genetic makeup. Twins are extremely rare in dogs and there is only one case that has been scientifically confirmed.

Reproduction and Mating

Female dogs produce multiple eggs at one time, which is why they typically give birth to litters rather than single puppies. These eggs are available for fertilization in the uterus for around ten days. And, dogs are polygamous breeders—meaning they can mate with any available dog during the period when their eggs are viable. This combination of physiology and behavior can result in puppies from a single litter having different fathers.

In this instance, the genetic variation among siblings in the litter is even greater. On average, siblings who share the same parents are approximately 50% genetically related. But if a litter was fathered by more than one male, the puppies who have different fathers are half-siblings and are only approximately 25% genetically similar. This can produce very different breed ancestry results—and dramatically different looking puppies—within a single litter.

For more information on how traits are inherited in dogs, take a look at our previous blog post.