Thomas Hunt Morgan

Two years work wasted, I have been breeding those flies for all that time and I've got nothing out of it.

The egg of every species of animal or plant carries a definite number of bodies called chromosomes. The sperm carries the same number. Consequently, when the sperm unites with the egg, the fertilized egg will contain the double number of chromosomes.

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.