IT IS NOT THE wistful gaze towards the horizon, nor the fitted elegance of her attire, that draws the eye to the image of Chiara de Cabarrus standing next to a portrait of her ancestor, Lady Mary Craven: it is the striking physical likeness between the women — the matching curves and hollows of their cheeks, the parallel lines of their noses, the neat smallness of their mouths. It is as if the recipe for producing the facial proportions of her great-great-great grandmother has been handed down to Chiara, a 29-year-old artist: which, in a way, it has. The two women, though separated by five generations, are blood relatives and Chiara is endowed with roughly 3 per cent of her ancestor’s genes. Of Chiara’s approximately 30,000 genes, around 940 have come from Lady Mary.

While aristocratic families are more likely to bestow material heirlooms on their descendants, biological legacies are a model of social equity. What is true for Chiara is also true for every person: each of us shares the same proportion of genes with certain relatives. Our immediate inheritance is from our parents: 50 per cent of genes come from the mother, the same from the father. Of course, parents merely pass on to their children a proportion of what they themselves have inherited. So another way of looking at it is that every person can be “quartered” in terms of their four grandparents, with each grandparent contributing 25 per cent.

The sharing of genes occurs horizontally across family trees, as well as vertically. Identical twins share all their genes. We share, on average, 50 per cent of our genes with full siblings; 25 per cent with half siblings. While there may be little to unite us with our first cousins, we generally have one-eighth of our genes in common. And, like Chiara, we share around 940 genes with each of our great-great-great grandparents.

Geneticists do not know which genes are responsible for physical appearance. We know that hair and eye colour have a genetic basis and that some genes dominate others, which is why brown-eyed parents tend to have brown-eyed children, but science cannot yet determine which genes sculpt face shape or carve the nose.

Genetic inheritance is random in the sense that it is impossible to predict precisely which genes will make up the 50 per cent contributions from the parents. Because of this, it is theoretically possible, though extremely unlikely, for full siblings to have no overlapping genes and therefore show virtually no genetic similarity. In some cases, says Bryan Sykes, professor of human genetics at Oxford University and an expert on the applications of genetics to genealogy, it is as if a person has inherited almost all their looks from one parent.

“Most people look a bit of a mixture of their parents but you sometimes see children who are carbon copies of either their mother or father,” says Sykes, who runs Oxford Ancestors, a company providing the DNA dimension to genealogy. “I occasionally see mothers and daughters out shopping who, if they were the same age, would be identical twins. That, for me, weakens the idea that we are always a mixture. But we simply don’t know how the genetics works.”

The doppelgänger effect is compelling evidence that some people’s looks can be dominated by the genes from one parent. Sykes also points out that some stretches in an individual’s genetic material can be deleted, or silenced. If those inactivated genes contributed to facial characteristics, it might tip the balance towards looking more like one parent than the other.

The 19th century geneticist Francis Galton thought the nose was the most inheritable facial feature, and Sykes agrees: “When you look at people in restaurants, the best indicator of whether it’s a family is to look at the shape of the nose. That’s something that portrait painters always bring into sharp relief. And, of course, while your cheeks might get fatter, the nose hardly changes with age or weight. I suppose if you’re an aristocrat, the danger is that it can become red through drinking too much vintage port.”

There is a famous precedent for the inheritance of a facial feature: the Habsburg jaw. The distinctive, protruding jaw — mandibular prognathism syndrome — afflicted 34 generations of European nobility, particularly the Austro-Hungarian monarchy. The gene conferring this feature was dominant: if a child inherited the associated gene from one parent and a normal gene from the other, he or she would be guaranteed to develop the condition. Although painters of the day would surely have been shrewd enough to adopt a somewhat blinkered view of this unattractive trait, the trademark jaw and lip remain noticeable in artworks of the time. Perhaps unsurprisingly, this genetically fascinating dynasty seems not to have gone in for portraits in profile.

Because the rules of genetic inheritance are oblivious to social standing, Sykes says, the upper classes are no more likely to look like their ancestors than any other sector: “The difference is that most of us don’t have paintings of our forebears to look at.”

Still, the noble people are more likely to be the beautiful people. Sexual selection — the search for appropriate partners with whom to procreate — means that wealthy, male aristocrats can attract the most comely companions. “I would say that aristocrats tend to be better-looking,” Sykes admits. “That’s because of the allure of status and wealth. You only have to go to Harrods to see beautiful women accompanying men whose only attraction is their wallet.”

VERY TOUCHING, VERY SAD

Lily and Christian Buchanan and Irma and Lisch Buchanan