Certain forms of human color blindness are inherited as X-linked recessive traits. Homozygous males are color-blind while heterozygous females are usually not. However, heterozygous females sometimes have partial color blindness and there is even one case in which a heterozygous female was color-blind in one eye, and had normal color vision in the other. Beginning with a description of what happens to X chromosomes in females, describe the likely reason for the above-described variability.