For each chromosome contributed by the sperm there is a corresponding
chromosome contributed by the egg, there are two chromosomes
of each kind, which together constitute a pair.
Thomas Hunt Morgan
Thomas Hunt Morgan showed that chromosomes carry genes so forming the basis of heredity.
Born in Kentucky in 1866 to a distinguished southern family whose was the great-grandfather was Francis Scott Key, the author of the "Star Spangled Banner
Morgan's research involved the study of mutations in the fruit fly Drosophila. One day he came across one male fly in one of his culture bottles that had white-eyes. Simply out of curiosity he bred the fly with normal (red-eyed) females. This produced offspring that all had red-eyes. Crossing the offspring (brother–sister matings) produced an F2 generation with some white-eyed flies, all of which were males. From this he postulated that the X-chromosome carries a number of discrete hereditary units he described as genes (a term introduced by Wilhelm Johannsen in 1909). He found a number of other genetic variations that were also sex-linked and usually inherited together, leading him to further conclude that genes were arranged in a linear fashion on chromosomes.
In 1933 he won the Nobel prize for his work that was followed by further Nobel prizes for a number of his students including George Wells Beadle and Hermann Joseph Muller.