In recent years, cytogenetic studies of spontaneous abortion products have disclosed the relatively high frequency of aneuploid embryos. These karyotypic anomalies chiefly stem from meiotic errors affecting the distribution of the chromosomes in one of the two gametes fused into a zygote. The gathered information not only implies the remarkable frequency of gonocyte aneusomy, it also reveals the prevalence of certain types of errors. It follows that gametal haploidy is often altered by the loss (nullisomy) or the addition (disomy) of certain members, in particular the X, the Y and chromosomes 15, 16, 21 and 22. It is therefore to be expected that, for some exceptional zygotes, the preservation of euploidy could result from the random union of a disomic gamete with a gamete nullisomic for a same member. To this hypothetical phenomenon, which is however statistically likely and foreseeable, we have ascribed the name of uniparental disomy, owing to the fact that both members of such a pair arise only from one parent, the mother or the father, instead of both. Furthermore, such a mechanism implies the major probability of introducing into the genome pairs of chromosomes with whole sequences of identical alleles, a consequence which we describe by the neologism of isodisomy. Such an extended homozygosity for series of colinear alleles implies from the genetic standpoint risks and advantages akin to those of parental consanguinity. An analogous mechanism could also modulate and modify the consequences of trisomies in which entire segments of two or the three implicated chromosomes, including the supernumerary one, could as well be iso-allelic (isodisomic trisomy or di-isotrisomy). The article briefly states some other predictions steming from the concept of uniparental disomy, whose confirmation should serve as a test of the proposed hypothesis.