How recombination works
Here’s a quick refresher: humans have 23 pairs of chromosomes. Half of our chromosomes come from our mother and the other half come from our father. The same goes for dogs, who have 39 pairs of chromosomes. Inheritance isn’t as simple as an even split in chromosomes, though. Before chromosomes are passed on to egg and sperm cells, recombination takes place.
During recombination, a chromosomal pair in the parent (which was inherited evenly from the grandmother and grandfather) splits in two. While the split happens, the two chromosomes have an opportunity to exchange DNA. Different pieces of the maternal and paternal chromosomes exchange places with each other, but the amount that’s exchanged is random. The resulting chromosome in the offspring could look like a 60-40 split between the grandmother and grandfather or a more drastic ratio like 90-10.
It’s not clear when the large-bodied allele evolved. The researchers found that an ancient wolf that lived in Siberia around 53,000 years ago carried one copy of this version. Other ancient wolves and modern grey wolves tend to have two, suggesting that the large-bodied allele might have been beneficial to wolves.
Ancient dogs, domesticated from wolves in the past 30,000 years, differed in size to some extent. But the current extreme size differences — the largest breeds are up to 40 times bigger than the smallest — emerged in the past 200 years, as humans established modern breeds.
“We’re not talking about a mutation that makes a wolf chihuahua-sized,” says Karlsson. “We’re talking about one of many mutations that tends to make you a bit smaller.”
The researchers think that the allele linked to small bodies is, evolutionarily, much older than the large-bodied version. Coyotes, jackals, foxes and most other canids they analysed had two copies of the ‘small’ version, suggesting that this version was present in a common ancestor of these animals.
Ostrander and colleagues including geneticist Jocelyn Plassais at INSERM-University of Rennes, France, analysed the genomes of more than 1,400 canids, including ancient dogs, wolves, coyotes and 230 modern dog breeds.
Your Dog’s Breed
Whether Fluffy has three puppies or ten depends for a good part on her breed. Some breeds are notorious for producing large litters, while several others are more on the conservative side. As a general rule of thumb, you should expect smaller litters in smaller dog breeds and larger litters in larger dog breeds.
However, consider that some small breeds may still yield large litters; the Pekingese, for instance, may yield up to 10 puppies in a litter.