How similar do two animals have to be to have babies?

Thanks for sending that one in, Girts!

Domestic dogs - Canis lupus familiaris - may appear quite different from breed to breed, but are all in fact the same species. Provided they can, anatomically speaking, do the business, 2 dogs should be able to produce viable offspring.

Foxes belong to the same family as dogs - canidae - but a different genus: vulpes. As a general rule, animals who don’t belong to the same species aren’t compatible parents. But why is this? I’ve been speaking to Gary England, Professor of Comparative Veterinary Reproduction at the University of Nottingham…

Gary - So could there be a dog-fox hybrid? And the significant factor in whether they could reproduce will be around their chromosome number. So the number of chromosomes must align well enough so that when the egg and the sperm come together, a viable embryo can form.

James - Chromosomes contain all of the genetic material - the DNA - that's required to code for the development, growth, function and reproduction of an animal. They come in pairs: dogs have 39 pairs of chromosomes, totalling 78.

Gary - If you think about the biology of fertilisation, obviously we have to get some chromosomes from the female and some chromosomes from the male. So what has to happen is during the process of formation of the sperm and of the egg, the chromosome number has to halve. And that's a process called meiosis. Each of those offspring cells contains half of the number of chromosomes. So it contains one of the pairs of chromosomes. So when the egg and the sperm then join together, when fertilisation occurs and the embryo forms, those chromosome pairs can come back together, half of them from the sperm, half of them from the egg. And so we end up again with 39 pairs of chromosomes or 78 chromosomes in total.

James - This is what leads to the development of a successful embryo between mating dogs. But what about foxes? 

Gary - If we assume that the question is about the red fox, the typical fox, the European fox, that is in the different genus. It's in the genus of vulpes. There's actually 12 species of fox altogether. The fox has only got 17 pairs of chromosomes. So the 17 pairs of chromosomes obviously doesn't match very well with the 39 pairs of chromosomes that we see in the domestic dog. So if we tried to have a red fox hybrid, we've got a big mismatch in the chromosome number. So it's very unlikely that the chromosomes will pair up and form a viable embryo.

James - So Girts, for two animals to produce offspring, they need to have a compatible number of chromosomes so that they align during fertilsation. Because chromosome numbers vary greatly between species, often even when they belong to the same family taxonomically speaking, you won’t get viable embryos in most cases of cross species copulation.