POPULATION GENETICS AND EVOLUTIONARY CHANGE 443 



Weinberg equilibrium; 3MM : 6Mm : 3mm. To represent them, three pairs 

 of red beads, six pairs consisting of one red and one blue bead, and three 

 pairs of blue beads were placed in a box and thoroughly mixed. Then the 

 pairs were withdrawn at random, two at a time. Two pairs withdrawn to- 

 gether constituted a "mating," arranged at random. The "matings" actu- 

 ally obtained at one trial were: MM X Mm; MM X mm; MM X Mm; 

 Mm X Mm; Mm X mm; Mm x mm (Fig. 19.1). 



ConstituHons of the "poiren1-s" 



"Matings" of "poirenfs" determined by choince' 



^O ^xc2) ^xC) C)xC) C)x(23 C)x(£) 



"Offspring," two to each "moiting," determined by chance 



A A A A A A 



^ ^ «3C) ^€) €)C2D #DC) «DC) 



FIG. 19.1. Model of genetic drift, employing red and blue beads. Production of one 

 "generation" only is shown. "Matings" of "parents" occurred by chance. Chance also 

 determined the nature of the two "offspring" resulting from each "mating." 



Then chance was allowed to operate in determining the two offspring to 

 arise from each "mating." For example, from the Mm x mm "mating" two 

 types of offspring are possible. Mm and mm. The mm parent must con- 

 tribute an m gene; this could be coupled with either an M gene or an m 

 gene from the Mm parent. To permit chance to operate in deciding 

 whether in each case the latter would contribute M or m, a bowl contain- 

 ing equal numbers of red and blue beads was used. One bead was with- 

 drawn from the bowl at random; if the bead was red, that meant that the 

 Mf7i parent contributed an M gene (and the offspring was therefore Mm); 

 if the bead was blue, that meant that the Mm parent contributed an m 

 gene (and the offspring was therefore mm). This drawing was done twice, 

 once for each of the two "offspring." The bowl of beads was used in the 

 same way to determine the contribution of a heterozygous parent in the 

 other matings listed. When both parents were heterozygous, as in the fourth 

 "mating" listed, two beads were withdrawn from the bowl at a time, since 

 both parents might contribute either an M (red bead) or an m (blue 

 bead), the offspring being either MM (two red beads), Mm (one red 



