354 MENDEL'S LAWS 



chromosome and insures that the two new cells have 

 exactly the same number and kinds of chromosomes. It 

 also follows that every cell in the body of a plant or ani- 

 mal has exactly the same equipment of genes (i.e., heredi- 

 tary units). But since these chromosomes are present in 

 pairs, one member of each pair having been derived from 

 each of the organism's parents, there are actually two 

 genes present for ever}^ separately heritable character, 

 and moreover these paired genes are in opposite members 

 of the same chromosome pair. 



Segregation of Chromosomes and Genes. — In the 

 reduction divisions which precede the formation of spores 

 in plants and gametes in animals the first event of im- 

 portance for the present discussion consists in the mem- 

 bers of each chromosome pair placing themselves side by 

 side in the process called synapsis. After a series of de- 

 velopments (which do not concern us at the present 

 moment) each chromosome pair finally arranges itself on 

 the nuclear spindle. The second significant step is that 

 the two members of each pair separate or segregate from 

 one another and go to opposite poles. This separation of 

 the maternal chromosome from its paternal mate also 

 separates the genes which they contain. Hence if the 

 maternal chromosome of a hybrid pink four o'clock which 

 contains the gene for red flower color goes to one pole 

 the paternal chromosome containing the " white " gene 

 goes to the opposite pole. This behavior of the segre- 

 gating chromosomes thus does two things. First it insures 

 that the spores or gametes produced will get only one of 

 the pair of genes (red or white in four o'clock for example) 

 and not both of them. In the second place it produces the 

 two kinds of gametes in equal number. As we have al- 

 ready seen this is precisely the sort of mechanism required 

 to produce the 1:2:1 ratio in Fa. 



Dominance and the 3: 1 Ratio. — In the four o'clocks 

 the hybrid between red and white is pink and so is readily 

 distinguished from either parent race. In many crosses 



