Mendel studied the transmission of genes in peas(pisum sativum) that were placed on different chromosomes. During meiosis there are two types of recombinations that make sure the offspring is different from the parent. First intrachromosomial recombination (crossing-over)where genetic material is exchanged between chromosomes. And interchromosomial recombination when homologous chromosomes are separated.
In Mendel's experiments only interchromosomial recombination was considered, but when Thomas Hunt Morgan experimented with Drosophilla Melanogaster he didn't get the same results because he focused on genes that were placed on the same chromosome. In this situation crossing over influenced the results.
