132 PLANT GROWTH 



• 



cell. These 14 chromosomes comprise two sets of 7 each and 

 each set was derived from a different sex cell. Suppose the 

 pollen came from a pure tall plant, and the egg from a dwarf 

 plant. Then one of the chromosomes in one of the sets would 

 bear a gene conditioning tallness, and there would be a corre- 

 sponding chromosome in the other set bearing a gene con- 

 ditioning dwarfness. These two chromosomes are said to be 

 homologues. They correspond to each other, and each bears 

 the same assortment of genes arranged in the same order. 

 The genes for round and wrinkled seeds would be borne in a 

 different pair of chromosomes. The genes for yellow and 

 green would be borne in still another homologous pair, and 

 so forth. Genes which condition alternate conditions such 

 as yellow and green, round and wrinkled, tall and dwarf, are 

 said to be alleles, and alleles always have the same relative 

 position in a chromosome. 



We have already seen that the gene for tallness must 

 separate from the gene for dwarfness in meiosis. We have 

 now seen that these two alleles occupy different chromo- 

 somes. The essential feature of meiosis, then, is the separa- 

 tion of the homologues. In order for this to happen consis- 

 tently, the homologous chromosomes pair off during the 

 meiotic prophase. The two homologues of a pair come to lie 

 side by side in such close approximation that they resemble 

 a single chromosome. Then the two homologues separate, 

 each going to a different pole just as the halves of a chromo- 

 some go to different poles in normal cell division. In this 

 way the two sets of chromosomes are separated and the chro- 

 mosome number reduced by half. There is no division of 

 chromosomes, but rather a separation of homologues. 



Now, in our garden pea we had seven pairs of chromo- 

 somes. In the cross we are considering, tall X dwarf, we got 

 seven chromosomes from the pollen and seven from the egg. 

 Let us represent the chromosomes in the pollen asabcdefg 

 and the seven in the egg as a' b' c' d' e' f g'. And let us sup- 



