This article touches on something that remains very true today: if you want to study genetic disorders you look at closely-related populations. If the condition you want to study is linked to a recessive gene, you want to find (or create) an isolated population with that gene.

In animals/plants this is why inbreeding can serve a useful function. If you want to detect a recessive gene, but either don't have access to genetic testing or are not sure which gene is involved, you in-breed until you see individuals who express the recessive gene. These individuals inherited the recessive gene from both parents. You then know which parental lines carry the gene, which lets you select for or against it.