Figure 1. Changes in allele frequency in underdominant mutations. q: allele frequency of the variant chromosome; p: allele frequency of the ancestral chromosome; q: changes in allele frequency; s: coefficient of selection against heterozygotes; m: segregation ratio of the variant chromosome at meiosis. q={pq[s(4q–2m–1)+2m–1]}/2(1–2spq) (Graphs are based in the formulas from Hedrick (1981).)

Figure 2. Effect of genetic drift on the probability of fixation of underdominant mutations. The graph shows a comparison between the probabilities of fixation for a neutral mutation with Mendelian segregation (s=0; m=0.5) and underdominant mutations with (s=0.1; m=0.6) and without (s=0.1; m=0.5) drive favoring the new variant. s: coefficient of selection against heterozygotes; m, segregation ratio of the variant chromosome at meiosis. (Graphs are based in the formulas from Hedrick (1981).)

Figure 3. Effect of natural selection on the unstable equilibrium frequency. (a) Adaptive selection in favor of the new homokaryote. (b) Meiotic drive. m: segregation ratio of the variant chromosome at meiosis; q_e: unstable equilibrium frequency; s: coefficient of selection against heterozygotes; s¢: coefficient of selection in favor of the new homokaryote.

Figure 4. Change in allele frequency for underdominant (s=0.1) mutations in the presence of drive in favor (m=0.6) and against (m=0.4) the new chromosome variant. The curve in the absence of meiotic drive is also shown. m: segregation ratio of the variant chromosome at meiosis; q: allele frequency of the variant chromosome; p: allele frequency of the ancestral chromosome; q: changes in allele frequency. q={pq[s(4q–2m–1)+2m–1]}/2(1–2spq).

Figure 5. Meiotic drive in Robertsonian translocations. (Top) Alternative products of balanced segregation at MI, first meiotic division. (Bottom) Insets depict the different orientation of the centromeres that lead to each type of segregation product. Arrows represent the alternative types of segregation favored by human and mouse.