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.
Males are never carriers but express the one allele on the X chromosome.
All alleles on a chromosome form a linkage group that stays together except when crossing over. Linked alleles do not obey Mendel's laws because they tend to go into the gametes together. Crosses involving linked genes do not give same results as unlinked genes. Heterozygote forms only two types of gametes and produces offspring with only two phenotypes. Crossing-over causes recombinant gametes and at fertilization, recombinant phenotypes.