Each of us likely has a Neanderthal ancestor

What do you get if you cross a sheep and a kangaroo? A woolly jumper. I'm joking, but even if you tried to make such a cross it wouldn't work. The species are just too distantly related. The distance in time since mammals and marsupials last shared a common ancestor translates to large differences at the genetic level. The offspring of a cross of such distant species are weak or inviable. However, more closely related species can form hybrids that are as strong as a mule.

This month we were treated to the rare sight of a cross between a sheep and a goat – sometimes called a “geep” – on a farm in Co Kildare. This unusual animal can be listed alongside other strange hybrids such as the “liger” (a cross between a lion and a tiger), the cama (camel and llama), and, of course, the mule (a cross between a male donkey and a female horse). Such hybrids are usually sterile, and are thus a biological dead-end.

It is such barriers to interbreeding, even though they are sometimes porous, that help us distinguish even similar-looking animals as separate species. Conversely, we recognise that dogs, no matter how variable in size, shape, colouration and behaviour, are all one species because they readily interbreed.

Geneticists have a long history of performing crosses to understand the nature of heredity and genes. Mendel discovered genes by performing carefully planned crosses of pea plants. In the 20th century, Thomas Hunt Morgan crossed varieties of fruitfly and observed the patterns of inheritance of traits.

His work, carried out in the so-called “fly room” of Columbia University in New York, led to the discovery that genes are arranged on chromosomes a bit like beads on a string. Morgan’s crosses also allowed his team to “map” genes – figuring out on which chromosome and in what order they lie.

The work of early geneticists such as Mendel and Morgan is all the more remarkable when you realise that they never had the opportunity to see genes, only to observe their effects. Their carefully designed experimental crosses allowed them to see using logic.

The logic of genetic crosses of different species also led to the unexpected discovery that some genes have a different effect depending on whether they are inherited from your mother or your father. A clue to the existence of this phenomenon is seen in crosses of horses and donkeys.

If a female horse is crossed with a male donkey, the offspring is a mule. However, the reciprocal cross of a male horse and a female donkey produces a hinny. Mules and hinnies have physiological differences as well as differences in temperament, despite the fact that they share the same genes.

Of mice and mules
Crosses of mice strains revealed an unusual genetic inheritance pattern that explains the mule-hinny conundrum. Some genes are only turned on when they are paternally inherited, and others are only turned on when they are inherited from the mother. These silenced genes are said to be "imprinted" with their origin.

This is a peculiar phenomenon, the evolutionary origins of which remain unclear. One of the best-studied examples of imprinting is the gene IGF2 (insulin-like growth factor 2) which promotes growth during foetal development. Like any other gene, one copy of this gene is inherited from each parent, but in this case only the paternal copy is switched on.

Imprinted genes such as this explain why a mule and a hinny are different. The set of imprinted genes in the horse is different to those in the donkey, so that the set of active genes inherited is different from a paternal horse and a maternal horse, even though the same genes are all there.

Patterns of inheritance and genetic crosses can also be charted in pre-history. When the Neanderthal genome was sequenced, comparison with DNA collected from people living all over the globe today revealed that 1.5-2 per cent of the DNA of people outside Africa is derived from Neanderthals.

How did this DNA get there? Most likely, each of us has a distant Neanderthal ancestor. So, even though Neanderthals are distinguished by physical characteristics, apparently setting them apart from modern humans, the genetic barrier between them and their contemporary modern humans was either weak or non-existent. Successful interbreeding was not only possible, it happened.

So, what do you get when you cross a Neanderthal and a Stone Age man? Well, it could be you.

Aoife McLysaght is a professor in genetics at Trinity College Dublin