The first definite evidence for chromosome theory came from sex-determination. C.E. McClung in 1902 reported two types of sperms in grasshoppers, one containing an X-chromosome and the other lacking it. Since this was the only difference in sperms and since all eggs were similar with respect to X-chromosomes, it was concluded that X-chromosome must be responsible for determination of sex. Proof of parallelism between chromosomes and Mendelian factors with respect to independent assortment came from Elinors E. Carothers in 1913. She used a particular grasshopper strain, where homologous partners of three pairs of chromosomes could be morphologically distinguished. If these 3 pairs are designated as AA', BB' and CC', the possible arrangements at mctaphase I can be shown as in Figure 9.2. Eight types of combinations in gametes would then be possible in equal proportion from arrangements shown in Figure 9.2. When crosses of thus marked grasshoppers were made with normal grasshoppers, all eight combinations could be identified morphologically through cytological preparations, suggesting an assortment just like that discussed for Mendelian factors in a trihybrid cross in Mendel's Laws of Inheritance.