Peter Joshi - The same or different?

Kat - Every human on the planet is distantly related in some way, but people who live in the same geographical area tend to be more genetically similar than those living a long distance apart. Now scientists have discovered that people who inherit the same versions of certain genes from mum and dad, are likely to be shorter and do less well at school than those who get different versions from each parent. To find out more, I spoke to University of Edinburgh researcher Peter Joshi, one of the hundreds who contributed to the study.

Peter - We inherit one copy of each gene from our mother and one copy of each gene from our father. Sometimes we inherit the same copy from both parents. What we look at is, in the context of the genome, relatively short stretches of about 1.5 million letters at a time and see whether or not those 1.5 million letters had been inherited the same from your mother and your father. Somebody in that situation doesn't have genetic diversity in that part of the genome. And vice versa, somebody that has inherited different genetics over those 1.5 million letters is genetically diverse.

Kat - And we're just talking not necessarily about really bad gene faults. We're just talking about you know, sometimes the variation that makes us unique and different.

Peter - Yes, absolutely. So the traits that I study are what geneticists called complex traits and they're exactly about what makes us all unique and different, and a little bit different from each other. What we see is that there's lots and lots of genes that have small effects and it's the combined effects of those and the environment that all make us individual people.

Kat - Tell me about the study. Who were you looking at? What were you actually analysing?

Peter - What we're doing is using modern genomic techniques to measure genetic diversity. The way that we're doing that is that we're using 350,000 people in total, and to make sure that the study is very robust, we used a hundred different populations across the world with all of that information and then collating it centrally in Edinburgh. We basically looked and measured the genetic diversity of each of those individuals.

Kat - So, you've got hundreds of thousands of people from populations across the world, how are you analysing their genomes? What are you looking at?

Peter - What we do is we look at the genetic diversity of individual people within that population and compare them with other people within the population. At the same time, look at their cognitive ability, height or blood pressure. Within that population, see whether or not there's an association between genetic diversity and say, educational attainment. Having done that, we then combine all the results of the different studies to see whether or not the effect is robust and reliable across lots of populations.

Kat - So, when you started looking at the levels of diversity about whether people had two copies of a gene or two different copies of a gene, what did you start to find?

Peter - What we were doing is we were looking at the whole of the genetic code of each individual and we're scanning along that genetic code and checking what proportion of that genetic code is identical from the mother and the father. What we typically find is that about 0.1 per cent of the genetic code is an inherited identical from the mother and the father. But that number varies from individual to individual with some people that might be none at all and in some other people that might be 3 or 4 times that. What we find is, that if you've inherited 2 or 3, 3 or 4 times the norm in terms of lack of diversity, that reduces educational attainment, cognitive ability, and height.

Kat - So, people who've got more similar genes from their parents, they don't do as well at school and they're shorter?

Peter - Yes, but it's also important to recognise that these effects are small. But on an individual level, it wouldn't be measurable. That's why we needed 350,000 people to robustly demonstrate that it was a real effect.

Kat - So, that height and broad measure of intelligence, but what about something like your risk of illness, because we know that the risk of diseases is also encoded in our genes?

Peter - It has been known for a long time that rare particularly strong genetic diseases like cystic fibrosis are subject to this effect. But what we hadn't shown was that these more complicated traits like educational attainment, which has got a lot of environmental factors in it as well obviously, like height, showed these effects. In fact, what we found for the risk of heart disease that we found no evidence in fact that genetic diversity affected your risk of heart disease. The mechanism that we think might be involved is that there will be things associated with development that might underpin growth and therefore your height. And so, it's not height itself in a way that is being controlled in this way, but it's the sort of underlying biological systems and two bad copies might affect height in that way and cognitive ability. If that happens, if these traits are favoured by evolution then we see this overall effect whereas for traits that are not subject to evolutionary pressures, we don't see that in quite the same way. It's plausible that heart disease hasn't been under the amount of selection pressure from evolution as you might first think, simply because people tend to get heart disease after they've had their families and its reproductive success that counts in Darwinian evolution.

Kat - Peter Joshi from the University of Edinburgh, and that study was published in the journal Nature this week.