Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four genetically diverse haploid daughter cells from a single diploid germ cell. In sexually reproducing eukaryotes, meiosis is the essential cellular process that produces haploid gametes (sperm and egg cells in animals, or microspores and megaspores that form gametophytes in plants).

In Co-dominance, both alleles of a gene pair in a heterozygote are fully and simultaneously expressed without masking or blending. Consequently, the distinct protein or antigenic products encoded by both alleles are detectable in the heterozygous organism. Classic examples include the ABO blood group system where $I^A$ and $I^B$ alleles simultaneously produce both A and B surface antigens in an $I^A I^B$ heterozygote, and electrophoretically resolved allozymes/isozymes.

Prions (proteinaceous infectious particles), discovered and defined by Stanley B. Prusiner in 1982 (Nobel Prize 1997), are unique infectious agents composed entirely of misfolded protein, devoid of any nucleic acid (neither DNA nor RNA). They cause transmissible spongiform encephalopathies (TSEs) such as scrapie in sheep, bovine spongiform encephalopathy (BSE / mad cow disease) in cattle, and Creutzfeldt-Jakob disease (CJD) in humans by inducing the normal cellular prion protein ($PrP^C$) to refold into the abnormal, insoluble, beta-sheet-rich pathogenic isoform ($PrP^{Sc}$).