A lot of heritable diseases are recessive traits - dominant genes are easier to select against.
Two people from the same background/race etc. are far more likely to carry the same recessive genes for a disease than two from different backgrounds.
This is why inbreeding produces unhealthy individuals. Two individuals that share a parent/grandparent etc will be very likely to inherit identical recessive disease genes from the parent.
Two people of different backgrounds will have offspring that are somewhat more likely to be healthy and have fewer problems.
Two people from the same background/race etc. are far more likely to carry the same recessive genes for a disease than two from different backgrounds.
This is why inbreeding produces unhealthy individuals. Two individuals that share a parent/grandparent etc will be very likely to inherit identical recessive disease genes from the parent.
Two people of different backgrounds will have offspring that are somewhat more likely to be healthy and have fewer problems.