386 



INTRODUCTION TO EVOLUTION 



Two pairs of contrasting characteristics in guinea pigs are ( 1 ) black 

 versus white and (2) short hair versus long. Individuals heterozygous for 

 both pairs of characteristics are black and have short hair. This shows 

 that the gene for black (B) is dominant to the gene for white (b), and the 



A B 



FIG. 17.5. Chromosomal basis of the independent assortment of genes in meiosis. 



gene for short hair (S) is dominant to the gene for long hair (s). 

 Fig. 17.5 is a diagram of meiosis in a doubly heterozygous, black, short- 

 haired male. As with the hamster, the diagram is simplified by showing 

 only two pairs of chromosomes, a long pair and a short pair. It is assumed 

 that the male inherited blackness and short-hairedness from his mother, 

 the opposite characteristics from his father. Thus the gene B is in the 

 maternal (white) "long chromosome" and the gene S is in the maternal 



