No one said genetics aren't real or that there a aren't differences between ethnicities, families, individuals. None of that has anything to do with race or breed though. Dog breeds only exist because humans selected them. Certain mutations may be more common in certain breeds, but certain dogs are more likely to have genetic disorders so there is no real necessity to use the breed categories. A dog from a certain family of dogs is going to have a certain probability of inheriting some genetic disease but the breeds were made up by pet owners to identify the dogs visually. This doesn't provide you with a high enough degree of accuracy to predict the genetics of an individual dog, so while you might be able to say that a certain disease is more common in certain breeds, it doesn't really say very much. It's also true that in human beings, giant humans are more likely to have a genetic disorder which caused them to grow so much due to a tumor in the pituitary gland.
We don't say these giant humans are a breed of humans, but if we mated these giants together then eventually over time these humans would all be giants, and would inherit the same disease. But we don't currently consider giant humans to be a "race" or a "breed", so why is that? Robert Pershing Wadlow and Andre the Giant had the same exact disease, and both were not ordinary in size. What constitutes a breed or a race other than some arbitrary set of traits which some humans determined constitutes a breed or a race?
In humans, northern Europeans get cystic fibrosis at higher rates, Ashkenazic Jews carry the Tay-Sachs disorder at higher rates, and people of African descent are more likely to have sickle-cell. Those are single-gene disorders, which are relatively easy to explain through mutation and historical accidents, like a particular population living with malaria, which selects against "normal" hemoglobin genes.
No doubt this is true but this speaks to ethnicity not race. African tribes share genes because of the fact that over thousands of years many families lived in the same location and share the same ancestors. European tribes share the same genes for a similar reason. Gene pools are geographically distributed but race has nothing to do with geography or genetics.
For example, an African immigrant who just arrived from Africa might not be genetically similar to a black American. Why is that? The black American has been in America for much longer, may have mixed with Native American, or European or many different races, and now has genetic vulnerabilities distinct to Americans. What I mean to say is that American is a gene pool of it's own, just as African is a gene pool of it's own, and European is a gene pool of it's own, and it goes down to geography where certain states, countries, etc, might have different mating patterns and gene pools.
For this reason, and because of constant migration, race based medicine is not going to ever be as accurate as individualized medicine. The only way to know if a black person has sickle cell is to check for it individually. Just looking at them and assuming based on some probability that they might have it is not 100% accurate while if you treat each patient individually it is 100% accurate. So for this reason race isn't scientifically useful and at best only provides a rough estimate. You can for example determine that certain diseases are more common to certain races based on the fact that most black people live in Africa, or most white people live in Europe, or most ancestors share certain genes, but as people migrate away from Europe or Africa into other places, this becomes harder to predict.
For example, Barack Obama is half African, so there is a probability that he is lactose intolerant and has sickle cell, but this isn't something we can just assume. To know with 100% accuracy we have to test his genetics individually. Why? Because while he is black, it doesn't mean his genes are all from Africa. Even in the case where his genes all were from Africa it wouldn't mean his genes were all from the tribes which have that disease or which lack the mutation to digest milk.
In summary I agree with most of what you say. Single gene mutations can be found and in certain ethnicities certain diseases are common. My point is race doesn't tell us anything scientifically useful because it's a limit on the accuracy which doesn't exist if you treat each patient individually.
References
RE: Race and IQ, The Alt-Right, and the Post-Racial future of humaity