Morgan's early work with "Drosophila" was on the associations known as 'coupling' and 'repulsion', discovered by English workers in 1909 and 1910 using the Sweet Pea. These were, in reality, the same phenomenon, which was later called linkage. Morgan's first papers dealt with the demonstration of sex linkage of the gene for white eyes in the fly, the male fly being heterogametic (XY).