HYBRIDIZING PLANTS 131 



Mendel's understanding of the problem of the inheritance 

 of certain characters was genetically complete. But, as we 

 have mentioned before, he did not know about chromosomes. 

 The chromosomes are important because they are the physi- 

 cal basis of heredity. Chromosomes were described in Chap- 

 ter 4 on cell division. In normal cell division each chromo- 

 some splits longitudinally and the halves separate into the 

 two daughter cells. Since this splitting is accurate, the two 

 halves are identical and the daughter cells are genetically the 

 same. Obviously, such a procedure would not afford a 

 mechanism for the separation of the members of pairs of 

 factors. You will remember that Mendel's first law stipu- 

 lates that the members of pairs of factors must separate into 

 sister gametes. If the factors are borne in the chromosomes, 

 then there must be a special kind of cell division in the for- 

 mation of gametes. This is exactly what takes place. The 

 special kind of cell division is called reduction division or 

 meiosis in contrast to normal cell division or mitosis. 



The term "reduction division" comes from another peculi- 

 arity of this division. In sexual reproduction two sex cells 

 (gametes) unite to form a zygote. Each of the sex cells 

 contributes a complete set of chromosomes (n) . The zygote 

 and the individual developing from the zygote have two sets 

 of chromosomes (2n). If there were no reduction in the 

 number of chromosomes, the next generation would have 

 four sets, and so on. It is evident that a reduction in the 

 number of chromosomes is necessary if all individuals of a 

 kind are to have the same number of chromosomes. Meiosis 

 reduces the number from 2 sets to one set (from 2n to n) . 



Reduction division occurs at only one stage in the course 

 of a generation. This stage is just prior to gamete formation. 

 To illustrate such a division, let us take the garden peas as 

 an example. In peas, the chromosome number is seven. 

 That is, there are seven chromosomes in a set (n = 7) and 

 the pea plant actually has 2n = 14 chromosomes in every 



