Non-Mendelian genes in Chamydomonas reinhardtii are inherited in a uniparental (UP) fashion. Most zygotes and their progeny receive UP genes only from the mt + or maternal parent. However, a few exceptional zygotes are also found in which the mt - or paternal UP genome is transmitted. Most of the exceptional zygotes are biparental in that their progeny segregate UP genes transmitted by both parents. As a result, biparental zygotes have been extensively used to study the rules governing UP inheritance. The frequency of biparental zygotes can be greatly increased if the maternal parent is irradiated with ultraviolet light prior to mating. Based principally on studies with ultraviolet-induced biparental zygotes, Sager has argued that a vegetative cell contains two copies of the UP genome and that the progeny of a biparental zygote receive a copy derived from each parent. Results reported in this paper with spontaneous and ultraviolet-induced biparental zygotes do not support the two copy model, but argue for a mulitple copy model with most of the copies normally being transmitted by the maternal parent. A multiple copy model which accounts for both Sager's results and ours is presented.