HYBRIDIZING PLANTS 133 



pose that chromosome "e" carries a factor for tallness and 

 chromosome "e"' a factor for dwarf ness. In the zygote and 

 throughout the plant all cells have all 14 chromosomes. The 

 plant becomes tall because of the dominance of the gene for 

 tallness. In the formation of pollen and eggs, meiosis takes 

 place. In every pollen mother cell and every female spore 

 mother cell the a b c d e f g chromosomes pair with the 

 a' b' c' d' t f g' chromosomes. As "e" chromosome pairs 

 with "e"' chromosome, the two alleles for height come to- 

 gether. The homologues are then separated and the allele 

 for tallness is separated from the allele for dwarfness. 



Thus, we have an explanation of Mendel's first law. But, 

 what about his second law.? The second law stipulates that 

 the members of different pairs of factors segregate indepen- 

 dently in gamete formation. This condition would be met 

 with if the pairs of homologues separated without having any 

 effect on each other, that is, at random. When chromosome 

 "e" separated from chromosome "e','' the allele for tallness 

 separated from the allele for dwarfness. At the same time 

 chromosome "a" separated from "a','' "b" separated from 

 "bV "c" from "c"' and so forth. But it is a matter of pure 

 chance as to whether a particular chromosome goes to one 

 pole or to the other. For instance, the daughter cell which 

 contains chromosome "e" with the gene for tallness may also 

 contain chromosomes a' b' c d' f and g'. Any other combina- 

 tion is just as likely because the homologues separate at 

 random. 



For an example, consider the dihybrid in Mendel's eighth 

 experiment (round yellow X wrinkled green) . Let us assume 

 that the round vs. wrinkled alleles are located in chromo- 

 somes "a" and "a"' respectively while the yellow vs. green 

 alleles are located in chromosomes "c" and "c"' respectively. 

 If the round yellow variety is used as the male parent, the 

 pollen will contribute chromosomes a b c d e f g including 

 the two genes for shape and color of seed. The female parent 



