Genes are located on chromosomes. Independently proposed in 1902 by Theodor Boveri and Walter S. Sutton. Accounts for the similarity of chromosomal behavior during meiosis and fertilization. Chromosomes and alleles of each pair separate during meiosis so gametes have one-half. Fertilization restores diploid chromosome number and paired condition for alleles in zygote.
Autosomes are nonsex chromosomes that are the same number and kind between sexes.
Sex chromosomes differ in number and kind between males and females. Besides genes that determine sex, sex chromosomes carry genes for traits unrelated to sex. Experiments involved fruit flies with XY system similar to human system. An allele for eye color on the X but not Y chromosome supports the results of the cross. Behavior of allele corresponds to chromosome, confirming chromosomal theory of inheritance.