Distribution of expected counts given 8 independent equiprobable trials, each with a probability of 0.5. This figure show the probability that somatic tissue in an adult will have a particular allele ratio, assuming stochastic inactivation of either the maternal or paternal X chromosome at the 8-cell stage of embryogenesis and no positive or negative selection over time based on X chromosome gene expression. The x-axis indicates the number of either maternal or paternal alleles that are active. For example, 8 could indicate that all of the maternal alleles are active. In that case, 8 paternal alleles would be inactive, and the allele expression ration would be 100:0. Similarly, 7 would indicate that 7 maternal and 1 paternal gene are active. In that case, the allele ration would be 87.5:12.5. The probability of each ratio is as follows: 0:8, 0.004; 1:7, 0.031; 2:6, 0.109; 3:5, 0.219; and 4:4, 0.273.