Final answer:
The early pioneer in studying sex-linked genes was Thomas Hunt Morgan, who used white-eyed fruit flies to understand inheritance patterns of X-linked characteristics, essential to explaining disorders more prevalent in males.
Step-by-step explanation:
The most well-known early pioneer in studying sex-linked genes was Thomas Hunt Morgan and colleagues. The mutant trait that he used was white eyes in the lab organism Drosophila melanogaster, commonly referred to as the fruit fly.
Sex-linked genes are those located on the sex chromosomes, and in humans, these are represented as X and Y chromosomes. Thomas Hunt Morgan's research with Drosophila melanogaster contributed significantly to the understanding of how these genes are inherited and expressed in organisms. Using fruit flies, Morgan was able to demonstrate that genes located on the X chromosome exhibit unique patterns of inheritance, since males (XY) and females (XX) have different sex chromosomes. This was in contrast to the non-sex-linked traits, or autosomal traits, that Gregor Mendel studied in peas.
Discoveries in sex-linked inheritance explain why certain disorders, such as red-green color blindness and hemophilia, are disproportionately observed in males. This is because males have only one X chromosome and thus one copy of each X-linked gene, making them more susceptible to expressing recessive traits governed by these genes.