I can tell a lot about someone by asking them in private, when it's just me and them, "So... what do you think of genetic differences between races?" Almost everyone, in private, will agree that there's some - but some people are really, really uncomfortable saying it. Some people, especially very diplomatic people, hedge and qualify first. When I make it known that I'm not particularly judgmental, that I've had friends and lovers of all colors and from all over the place, but that I also have a healthy respect for science and reality - well, the diplomatic people tend to open up a lot more.
Basically I'll put it like this - people of difference descendencies have been observed to have different average heights, weights, body fat percentages, average musculatures, bone densities. Different kinds of blood cells even - if you look at sickle cell anemia, blood cells shaped a little differently than normal make a person more resistant to malaria, but more prone to anemia. Malaria of course, being orders of magnitude more deadly and dangerous than anemia, which is a more long term unhealthiness thing. And sure enough, sickle cells are more present as an adaptation in people descending from formerly high malaria areas - http://en.wikipedia.org/wiki/Sickle-cell_disease#Genetics
Moving on - there's clearly slight variances and differences between people of different descendants. The idea that there's no cognitive differences just seems incredibly unlikely. Now, I think "IQ" or "intelligence" is drastically oversimplified and bunk. But things like abstract recall - how fast can you remember something? Pattern recognition. Dexterity and coordination. And so on - why wouldn't these things be adaptable and heritable? Wouldn't some parts of the world being able to remember a skill after not using it for a long period of time be more valuable than others? (places with harsh winters, for instance?) The ability to think quickly and adapt be better in different places? And if they were, wouldn't you expect natural and sexual selection to make the local population more like that?
This is almost all moot in many practical ways - the only thing that bothers me is that since people don't want to acknowledge that these differences exist, there's been very little work to learn teaching styles that would apply to different people, how different medicine and diet regimes would work better, and so on.
Different groups of people tend towards different lactose intolerance and alcohol intolerance levels - this is only recently being considered in nutrition and diet because the implications of it - different food good better for different races - is not a good political position these days. It would have been easy to test for lactose sensitivity among different groups of people and make recommendations 20-50 years ago, but no one wanted to touch it. Instead, "milk for everyone!" was the official position, which is nonsense. Dairy is terrible for the people it's bad for.
Finally - I'm very curious about what the effects of mixing blood in children would be, and there's been very little research that I know of. From my understanding of genetics, mixed blood children should be particularly strong. My background is mixed Western/Eastern European, and I'd prefer a wife of different bloodlines than my own, as I think it's a very positive thing to do for my children. Obviously you can't control who you fall in love and connect with completely, but if I choose to live in Ankara or Osaka instead of London, that greatly increases the chance my kids are half-Turkish or half-Japanese. I think that'd make them stronger, healthier, and less prone to some of the negative things in my bloodlines. But how much so? When parents have different cognitive backgrounds, which are dominant? For instance, if one parent has much higher natural testosterone levels, is that dominant? That means the children would have higher energy, be more resistant to some diseases, be more willing to act independently and defy consensus, but also have more proneness to violence, anger, and aggression. I generally have high testosterone throughout my family - we've made good soldiers but some of us have had pretty bad tempers. What happens if I marry a woman with a lower natural predisposition towards testosterone? How about different cognitive abilities? We've generally got bad eyesight and bad spatial relations in my family. You'd expect good eyesight should be a dominant characteristic, no? But how much so?
I'd love to see research on this, and I think we will, because you can only hold back science and knowledge through moral fashion for so long before it breaks through.