Final answer:
Curt Stern provided the experimental evidence for the crossing over among X chromosomes in female Drosophila through his research in 1931, where he meticulously connected changes in the offspring’s genetic makeup to corresponding chromosomal crossovers.
Step-by-step explanation:
Experimental evidence that crossing over occurs between the X chromosomes of female Drosophila was provided by Curt Stern. In his 1931 research, Stern observed X-linked phenotypes in Drosophila which indicated that crossing over was indeed happening. Stern's work meticulously connected the deviations in offspring allele combinations from the parental types to an associated exchange of an X chromosome segment. This discovery was paramount to our understanding of genetic inheritance and recombination.
Stern used flies with structurally distinctive X chromosomes to detect the recombination products, taking advantage of X-linked traits such as eye color, which had been previously mapped to the X chromosome by Thomas Hunt Morgan in 1910. The importance of this experiment lies in the visualization of crossing over before DNA sequencing and molecular tools became available. Thus, homologous chromosomes were observed exchanging segments in meiosis, a fundamental aspect of how genetic diversity is generated in sexual reproduction.