Final answer:
Sex-linked traits were discovered in Thomas Hunt Morgan's laboratory through research with fruit flies, which informed our understanding of X-linked recessive disorders observed in human genetics, such as color blindness, hemophilia, and muscular dystrophy.
Step-by-step explanation:
Sex-linked traits were fundamentally discovered and understood through studies in Thomas Hunt Morgan's laboratory. Morgan's research began in the early 20th century and used fruit flies as a model to understand how traits are passed from one generation to the next. These discoveries shed light on how certain disorders, such as red-green color blindness, hemophilia, and muscular dystrophy are inherited in humans, which are linked to the X chromosome. These conditions are considered X-linked recessive disorders, often observed more in males because males have only one X chromosome, and just one copy of the recessive gene is enough to express the trait.
In the case of females, two copies of the recessive gene, one from each parent, are needed to express the trait. If they have one recessive and one dominant allele, they become carriers without typically showing symptoms, but they can pass the condition on to their sons or carrier status to their daughters.