POSSIBLE COMBINATIONS OF CHROMOSOMES 



When a zygote is formed, the paternal chromosomes combine with the corresponding 

 maternal chromosomes. When reduction division takes place as gametes are formed, 

 the chromosomes become separated at random. Where there are four pairs of 

 chromosomes, 16 combinations are possible — 2", n being the haploid number 



reduction division and fertilization agree with the simple formulas based on 

 Mendel's experiments. But the theory that the chromosomes are the deter- 

 miners raises new problems (see illustration opposite). 



Are Chromosomes Determiners? If each inherited character, or trait, 

 were determined by a particular chromosome, the number of chromosomes in 

 the germ cells would strictly limit variation among individuals. The tremen- 

 dous variation among human beings, for example, would have to be explained 

 by the combination and recombination of twenty-four pairs of chromosomes. 

 Theoretically the number of combinations possible in any species is V", x 

 being the number of pairs of chromosomes. In a species which had, let us say, 

 only three pairs of chromosomes, the number of combinations possible would 

 be 2^, or 8. In tobacco or in human beings, with twenty-four pairs of chromo- 

 somes, the largest possible number of combinations would be 16,770,216. 

 And this number would include thousands of cases in which two individuals 

 were identical except for one or a few details. 



We are forced to assume that each chromosome must bear several, or even 

 many, determiners. Indeed, there is so much evidence on this point that for 

 many years students have been speaking not of determiners in the chromo- 



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