Original: In 1931 , Stern was the first to demonstrate crossover of homologous chromosomes in Drosophila melanogaster , only weeks after Barbara McClintock and Harriet Creighton had done so in maize ( corn ) .
Simplified: In 1931 , he was the first to demonstrate crossover of homologous chromosomes in Drosophila melanogaster , only weeks after Barbara McClintock and Harriet Creighton had done so with maize .