Expressing One Allele at a Time Although genetic traits are often dominant or recessive, the impact of the same heterozygous genotype can vary quite a bit between individuals. Deng et al. (p. 193 ), analyzed global gene expression in hundreds of individual mouse cells and found that a substantial fraction of the genes only expressed one of the alleles, chosen randomly, at any given point in time. Such stochasticity in transcription increases the heterogeneity among cells and likely contributes to the phenotypic variance among individuals of identical genotype.