Mendel's Law 



33 



I. Why do just two of the four chromosomes carry 

 the determiner for tallness rather than all of them or 

 only one of them ? Just here it would be difficult to 

 explain why no more than two of the four chromosomes 

 are represented as carrying the same determiner. This 

 will be explained later. It is easy to answer, however, 

 why the determiner is being carried by more than one 

 chromosome. When gametes are formed the chromo- 



Dwarf Parent 



Gametes 



Fig. 3. — Diagram illustrating behavior of chromosomes in Mendel's 

 cross of tall and dwarf peas. Large rectangular figures, nuclei of 

 zygotes or mature individuals; large circles, gametes; small circles 

 within zygotes and gametes, chromosomes; letters on chromosomes, 

 determiners {T, tallness; D, dwarfness). 



some number is reduced one-half. Since every gamete 

 from a pure tall plant carries the determiner for tallness 

 there must have been at least two chromosomes carrying 

 the determiner before the gametes were formed. 



2. Do these two chromosomes carry any other deter- 

 miner than that for tallness? In a tentative way this 

 question may be answered in the affirmative, but a 

 fuller discussion of the situation must be deferred. 

 There is much experimental evidence that indicates 

 that more than one determiner is carried on a single 



