Thursday, 3 May 2012

What's In A Name?

Wouldn't it be good if we had a record of how genes are spread through a population going back some 500 years or so? It would be a bit like watching ripples spread out when we drop a stone in a pond or genes flowing through a population over time.

What we need is some sort of marker which got recorded fairly frequently, unlike hair or eye colour, which, if they got recorded at all before we had photography or passports, were far too patchy and were probably only written about then if they were unusual and belonged to a person of some importance. We all know about Queen Elizabeth I's red hair, but what colour was Henry II's or Thomas Beckett's hair? Who know what colour Alfred the Great's eyes were?

As it happens, we have just such a marker, which has been recorded in England since 1837 and since about 1500 or even earlier in some places. We have the Register of Births, Deaths and Marriages and, before that the records of baptisms, marriages and deaths maintained by churches, albeit in a non-standard format and often badly stored so that many have fallen foul of mildew, mice, water, fire and fading ink so that getting back beyond 1600 is often a matter of luck.

But what has that got to do with genes?

Until recently, the law in England required a child to take the surnames of the father if the parents were married, or the mother if not, so, even though birth out of wedlock was common, families were large and the majority of births were in marriage. This means that males tended to inherit their y chromosomes along with their surnames. (For the uninitiated: males have an x and a y chromosome; females have two x chromosomes. Sperm always has either an x or a y chromosome and ova always have an x. The gender of the baby is determined by the chromosome in the successful sperm, so men determine the gender of the baby and boys will always have their father's y chromosome). Not that the y chromosome is of any special interest in this because breeding success, and especially producing more males, may have nothing to do with it. It is a convenient metaphor, however.

What all this means is that, since boys always have a copy of their father's y chromosome and they usually have their father's surname, the y chromosome is 'marked' by the surname and the surname has been recorded, so, in effect the y chromosome has been marked.

This is not an exact mapping for two reasons:
  • Children of unmarried mothers take their mother's name, not their fathers. While the mother may well have taken her father's name, she didn't take his y chromosome.
  • Children may not have the father they think. It's a wise child who knows his own father.
Now, suppose something about a man's genes made him more likely to have male children or more healthy children, or just more children? What we would expect is that, over a few generations from a more-or-less even spread of surnames, his surname would become more common in a small area to begin with and to spread out from that focal point at each generation. But, because of the diluting effect of the two factors above, this effect would die away as it spread.

Is this what we see?

These surname distribution maps for names in my family tree are taken from the 1891 census. They are from Ancestry.co.uk:
It is quite clear from this random sample that surnames are not randomly distributed across the country but tend to be much more common in some areas than in others. Major centres are also surrounded by zones of diminishing frequency, just as I hypothesised above.

One of these maps is especially interesting and not just because I'm related to probably all the people in it. The Pratley surname has been the subject of a One-name Study by Michelle Hawke, an Oxfordshire genealogist. From Michelle's website:
The origins of the Pratley surname are clear to see, but not so easy to explain. The very first Pratley was originally William Spratley, who came to Leafield, Oxfordshire from a town near Banbury in around 1620. During his life in Leafield, the surname in the records fluctuates between Spratley and Pratley, but after his death in 1660, the family permanently took the surname Pratley...

The first few generations of Pratleys are easy to trace from parish registers and wills. William married twice, producing a dozen children, four of whom were boys who went on to continue the Pratley line. After a few generations, each branch of the family became quite distinct - not only were Pratleys marrying other Pratleys, but Pratley gamekeepers were arresting Pratley poachers! But it was still more than a hundred years before more than a few Pratleys left Leafield and started families in other villages.

By this time they filled all strata of working society - from paupers living on parish relief to farmers of a couple of hundred acres. At first they migrated to neighbouring villages and counties, but with the advent of the railways, and a couple of notable events in Pratley history, they spread across England and off to America, Australia, Canada, South Africa and New Zealand.

Today there are more than a thousand Pratleys around the world - not a large number as surnames go, but not bad for just three hundred and fifty years! And there are still Pratleys living in Leafield today, all these years after William first arrived.
April 30, 1832 First Pratley Baptism.
Baptisms for Leafield were performed at Shipton-Under-Wychwood until
A church was built at Leafield. L in the margin indicates from Leafield
We can see how, from a single 'seeding', the Pratley name, and with it, the Pratley 'gene for success', which I'll use as a general term for whatever factor it might be for any particular surname because it might well be any number of possible factors, spread from it's nucleus in the Wychwood forest. Now, anyone like me who is descended from families who were present in Oxfordshire at least before 1500, and who isn't a recent new-comer to the area is almost certainly related to one or more Pratleys. If you are a 'native' of North West Oxfordshire, from the general Witney - Chipping Norton - Woodstock area, and you think you aren't related to a Pratley, you or one of your ancestors, have probably been lied to.

If my theory is correct, that this local success of a surname is due to something in the male genes which either produces more healthy children or, even better, more sons, we should see this in the record.

And guess what?

A quick count of the number of Pratley births registered between 1837 and 1887 in the Witney and Chipping Norton Registration Districts. of 688 registered births, 53% were of males and 47% of females. I got this from a search on FreeBMD.org.uk

But if that were true, you might expect the 'Pratley gene' to spread very rapidly because each new centre, as Pratleys moved out of the area, should have set off a new explosion. And this is just what did happen when George Pratley, a forester from Leafield in the Wychwood Forest upped and moved to Perthshire in Scotland in 1829, taking his foresting skills, his wife Jane and his two surviving children with him to work on the Scottish estate of his employer, who also owned the Cornbury Estate in the Wychwood Forest. As this map shows, Edward and his Wife Jane soon founded a new colony of Pratleys, producing another nine children .

But this doesn't always apply and the tendency is for the 'gene for success' to lose it's vigour as time goes by. This apparent reduction in 'vigour' is caused by the two things I mentioned earlier - marital infidelity, and the law of matronymics for children of unmarried mothers. This means that some 'Pratley' genes will be in people with other surnames and some Pratleys will not have the Pratley genes.

This isn't the only interpretation of course, and it is equally possible that William Pratley (or Spratley) had the good fortune to marry a woman who carried a gene for making more breeding males. However, I think it is easy to see how a gene which is firmly linked to something which gives breeding success, will spread out from an initial 'seeding' or advantageous mutation.

This of course is exactly what we would expect to see if a new advantageous mutation arose in a local area of a wide-spread population. However, this is not the only mechanism by which alleles spread and change their frequency in the gene pool. Genetic drift plays a part and so does the 'founder effect' when a small population becomes isolated from the main population. The founders are statistically unlikely to have the same proportion of different alleles as the parent population and, with a small population, the effects of chance will be more marked. In these populations, and especially if there are local environmental pressures different to those acting on the main population, shifts in allele frequency can be marked.

Using the surname analogy again, a 'founder effect' can be seen in the population of Pitcairn Islands in the South Pacific, which was populated by the Bounty mutineers and a few islanders, mostly women, from Tahiti. The leader of the mutiny, Fletcher Christian had married a Tahitian girl named Maimiti with whom he had two sons before he died, Charles and Thursday October (No! Really!). Fletcher and all but one of the mutineers, John Adams, either died of illness or were murdered in in-fighting. A very large proportion of the Pitcairn Islanders now have the surname Christian or Adams and like me and the thousands of Oxonians with our little pieces of Pratley in us, so they have a little bit of John Adams or Fletcher Christian in them.


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