Thomas Hunt Morgan was a biologist who became a very early geneticist at the turn of the century at Columbia University in the United States. He studied fruit flies for inherited traits and characteristics due to the fact that fruit flies have a very short breeding cycle and therefore new generations could be produced very quickly for study. One particular trait that he studied was the eye color of the flies. Red was the dominant color but every now and again a white eyed variant would appear. Morgan discovered that breeding a white eyed male with red eyed females produced only red eyed offspring. But the next generation of offspring produced some white eyed males (but no white eyed females). This eye color proved to be a sex gene linked recessive trait. Hunt's initial fruit fly experiments proved some of Mendel's theories about heredity but Hunt also discovered gene linkage and the concept of crossing over. He found that genes were packaged in chromosomes and some chromosomes were linked to sexual characteristics. Moreover, genes that are closely linked together have a higher probability of crossing over (or mixing genetic fragments) than genes that were not linked together. And keep in mind that this all years before the discovery of DNA. So this set into motion the study of modern genetics.