Thomas Morgan expanded Mendel's principles through his studies of Drosophila (fruit flies). He hypothesized that a mutation expressed predominantly in males must be carried on the X chromosome. In females heterozygous for the mutation, a second X chromosome masks the mutation. Morgan realized that, contrary to Mendel's principle of independent assortment, some alleles do not segregate independently. Alleles that are located near each other on the same chromosome are likely to be inherited together, because entire chromosomes are separated into gametes during meiosis.