376 INTRODUCTION TO EVOLUTION 



"Normal" hamsters vary from gray to grizzly brown in appearance, but 

 black individuals frequently occur. In some regions the black (melanistic) 

 individuals are rare, in other regions they occur with varying frequencies, 

 the extreme being reached in localities having populations in which 

 nearly all individuals are black. 



Genetic experimentation has demonstrated that the color difference be- 

 tween a normal and a black hamster depends upon a sing'.e gene. Thus, 

 inheritance is of the type discussed in the first part of Chapter 15. 

 Melanism (blackness) is dominant to normal pigmentation (Gershenson, 

 1945). Accordingly, gray hamsters are homozygous (see p. 333) for a 

 recessive gene; we may indicate their genetic formula as m/??. Black ham- 

 sters may be homozygous for the corresponding dominant gene (i.e., MM), 

 or they may be heterozygous (i.e.. Mm). It is probable that the color 

 phases of other animals mentioned above depend upon similar genetic 

 mechanisms, but in most cases the genetic analyses necessary to demon- 

 strate the point have not yet been made. 



MENDELIAN INHERITANCE 

 Single-Gene Differences 



What will be the result of mating homozygous black hamsters to gray 

 ones? The homozygous black individuals have the formula MM, the gray 

 individuals the formula mm. Thus the cross becomes: MM X mm. Each 

 germ cell (sperm or ovum) produced by the black parents will contain 

 one of the M genes; each germ cell produced by the gray parent will 

 contain an m gene. The basis of this separation of members of pairs of 

 genes so that each germ cell receives but one member is expressed in 

 Mendel's "law of segregation" (see below). As a result, each offspring 

 produced by the fertilization of an ovum by a sperm will be of the 

 formula Mm, i.e., will be heterozygous. Since black coloration is dominant 

 to gray, all these Fi (first filial generation) offspring will be black (Fig. 

 17.1). 



germ 

 cells 



MM X mm 



Z' germ 

 \ [cells 

 M-^mj 

 Fi offspring Mm, Mm, Mm, Mm; all have black coloration. 



Suppose that the Mm males are now mated to the Mm females. What 

 will be the result in the next, or F-j generation? As we noted in a pre- 



