134 PLANT GROWTH 



will contribute chromosomes a' b' c' d' e' f g including the 

 corresponding alleles. The hybrid plant will have 14 chro- 

 mosomes in every cell. One of these will carry a gene for 

 yellow, another will carry the allele (green) and will be its 

 homologue. A third chromosome will carry a gene for round, 

 and a fourth will carry the allele wrinkled. The other ten 

 chromosomes will carry still other genes not considered here. 

 At meiosis, "a" pairs with "a'," "c" with "cV etc. But the 

 homologues separate at random. Considering these two 

 pairs of chromosomes only, the following combinations are 

 possible: ac, ac', a'c, aV. Cells of these four classes should 

 occur in approximately equal numbers. 



Let us represent the gene for yellow by Y and its allele 

 by y, and the gene for round by R with its recessive allele 

 by r. The chromosome classes above would then have the 

 following genie constitution: YR, Yr, yR, yr. These four 

 classes apply equally well to both pollen and the eggs. If we 

 self-fertilize the hybrid, we combine pollen and eggs at ran- 

 dom. The following combinations of F2 zygotes then are 

 possible: 



YR 



Yr 



yR 



yr 



