Figure 4. Protection against non-disjunction may be reduced as women ages.

We propose that protection given by high recombination becomes less efficient with increasing maternal age. Here, we depict oocytes containing only one chromosome, with R recombination events. We suppose that, at each of the three arbitrary age periods (k = 1,2,3), the proportion of oocytes having R recombinations stays the same (i.e. it does not decrease or increase with age). During each age period, several oocytes enter their final stage of maturation and give properly disjoined gametes with one chromosome (Normal) or non-disjoined gametes with zero or two chromosomes (Aneuploid). P_k (R = r) is the probability of proper disjunction in an oocyte with r crossovers for the age period k. E_N and E_A are the mean numbers of recombination in properly disjoined and non-disjoined oocytes, respectively. Under this model, E_N is expected to decrease with k whereas E_A is expected to increase with k (see Text S1).