Human beings still should not purposely accept genetic traits that are disadvantageous. If there was a race that had a 50% chance of polio/SCA/diabetes/down syndrome would you think that we should continue breeding with those people? They would be nothing but a strain and it would be morally wrong to continue bringing those avoidable problems into the world. It's a well known case that IQ lowers when white and black races mix. Blacks have a naturally lower IQ than whites, and the offspring of the 2 is lower than the white's and somewhat higher than the blacks. If you look around online and spend more time among average people(i.e. not your typical college/university crowd) you should be able to notice the differences in behavior/intelligence among races of people. https://majorityrights.com/weblog/comments/the_health_consequences_of_race_mixing/