The chromosome theory of heredity says mendel"s heritable factors are located on chromosomes. Mendel"s law of segregation is explained by homologous chromosomes assoring into separate haploid gametes. Mendel"s law of independent assortment is explained because each pair of homologs segreagates without affecting how any other pair segregates. Sib mated f1, 3:1 ratio of red:white in f2. 100% of f2 females had red eyes. 50% of f2 males had red eyes. 50% of f2 males had white eyes. Male ies had xy and female ies had xx chromosomes. Morgan"s conclusion: white eyes was a sex- linked trait. X-linked inheritance differers from mendelian inheritance, since males have only one allele while females have two. Cross white eyed females or red males. Rarely get w- female or w+ male. ~1/500 to 1/1000 were exceptions because there were white eyed females and red eyed males. Bridges saw that exceptions could occur from chromosome segregation errors.