Answer: E.
Explanation: The fact that about 80% of the alleles in thoroughbred horses can be traced back to a small number of ancestors suggests a lack of genetic diversity in the population. Inbreeding and selective breeding practices have led to a reduction in the number of alleles present in the population, resulting in a limited gene pool. This reduced genetic diversity is likely to result in a higher frequency of specific alleles and a lower number of polymorphic alleles, as seen in the case of thoroughbred horses.