342 INTRODUCTION TO CYTOLOGY 



the (factors for the) two characters forming each allelomorphic pair 

 separate and pass to different gametes (or spores). Thus the chromo- 

 somes and the characters alike form a duplex group in the body cells and 

 a simplex group in the gametes (or spores) : the chromosomes, like the 

 characters, form new combinations at fertilization and are segregated 

 when the gametes (or spores) are formed. In the diagram the letters 

 ABCDabcd stand equally well either for chromosomes or for characters. 

 In view of these facts it appears extremely probable that chromosomes 

 and Mendelian characters have a definite causal relationship of some 

 kind: it is scarcely conceivable that the exact and striking parallelism 

 that they show can be without significance. 



The precise nature of this correspondence between chromosome be- 

 havior and character distribution can be even more clearly shown by a 

 consideration of the history of a single homologous pair of chromosomes 

 in a typical Mendelian cross. If a pure white (albino) guinea pig be mated 

 to an individual of a pure black strain the offspring are all black; black 

 is completely dominant over white (Fig. 132). If these black hybrids 

 are bred among themselves they produce in the F 2 generation three black 

 animals to one white, or, more precisely, one pure black to two black 

 hybrids to one pure white. Let us now follow a single pair of chromo- 

 somes of each of the original animals through these two generations. 



At the left in Fig. 133 are represented the two animals, pure black 

 and pure white, their chromosomes being drawn in solid black and outline 

 respectively. In the black animal the two chromosomes pair at synapsis 

 and separate to the two daughter cells at the first maturation mitosis, 

 and split longitudinally at the second, so that each of the gametes re- 

 ceives a single chromosome representing a longitudinal half of one of the 

 original pair. A similar process occurs in the white individual. Unions 

 between the gametes of the two animals now result in the F\ hybrids, 

 each of which has one chromosome from its black parent and one from 

 its white parent (not counting the chromosomes of other pairs). When 

 these hybrids form gametes, as is seen at once in the diagram, the pa- 

 ternal and maternal members of the chromosome pair separate, with the 

 result that half the gametes receive one of them and half the other. 

 There are thus two kinds of spermatozoa and two kinds of eggs, one kind 

 carrying the paternal chromosome and the other carrying the maternal 

 one. Chance combinations now result in a generation (F^) of animals, 

 one-quarter of which have derived both chromosomes of the pair in 

 question from the black grandparent, one-half of which have derived 

 one chromosome of the pair from each grandparent, and one-quarter of 

 which have derived them both from the white grandparent. Moreover, 

 these animals are respectively pure black, hybrid black, and pure white, 

 in the proportion of 1:2:1. Thus it is seen that there is a direct paral- 

 lelism, not only between chromosome sets and character groups, but also 



