Morgan used fruit flies as experimental materials in his genetic experiments, and all his important discoveries were made from fruit flies. Some people say that God created fruit flies for Morgan. Drosophila is a small fly, only a few millimeters long. It likes to fly on rotten fruit, so it is called fruit fly. In fact, it likes the wine fermented by rotten fruits, so many fruit flies will be attracted in front of the wine fermentation pool. The ancient Greeks called fruit flies "drunkards".

Morgan's laboratory initially used fruit flies to study whether the acquired traits could be inherited. He cultivated fruit flies for many generations in a dark environment. According to Lamarck's theory that acquired traits can be inherited, his vision should be getting worse gradually. But the result is not like this. Morgan thinks the experiment is futile.

Morgan made an important genetic discovery with Drosophila, starting with a white-eyed Drosophila, from which he discovered sex-linked inheritance. Wild fruit flies have red eyes, but at 19 10, Morgan's wife found a male fruit fly with white eyes. According to genetic theory, this is a genetic mutation. As this white-eyed male fly mated with an ordinary red-eyed female fly, the first generation of fruit flies were all red-eyed. According to Mendel's theory, red eyes are dominant and white eyes are recessive. The first generation of fruit flies mated to produce the second generation. As a result, female fruit flies are all red-eyed, while male fruit flies are half red-eyed and half white-eyed. If the ratio of red-eyed Drosophila to white-eyed Drosophila is 3: 1, it conforms to Mendel's law. But why do male fruit flies always have white eyes?

Morgan also conducted a backcrossing experiment to mate the first generation of red-eyed females with the first generation of white-eyed females. Results When fruit flies were born, both sexes had red eyes and white eyes, which also conformed to Mendel's law.

Based on these experimental results, Morgan thought deeply and put forward a hypothesis that the gene that determines the eye color of fruit flies is located on the X chromosome in the sex chromosome. A pair of sex chromosomes of male Drosophila is composed of X chromosome and Y chromosome. The Y chromosome is very small, and there are few genes on it, so as long as there is a white eye gene on its X chromosome, it will show white eyes. The sex chromosomes of female Drosophila melanogaster are a pair of X chromosomes, because white eyes are recessive traits, and only those genes with white eyes on both X chromosomes will show white eyes. According to this assumption, the above experimental results can be satisfactorily explained.

The white-eyed gene exists on the sex chromosome, and its genetic law is related to sex, which is called "sex-linked inheritance".

The inheritance of color blindness and hemophilia is also a sex-linked inheritance. Color blindness patients are mostly men, and few women. The children of male color-blind patients are generally not color-blind, but their grandchildren are color-blind People have been puzzled by this phenomenon in the past, and the concept of sex-linked inheritance has made people understand the mystery.