12 FROM FISH TO PHILOSOPHER 



another without change during the union of the repro- 

 ductive cells from the parents. 



Through the mechanism of sexual reproduction 

 (which is nearly universal in both animals and plants) 

 the oflFspring receives genetic factors from both parents, 

 and thus differs genetically from either parent, though it 

 does not necessarily possess any new genes. In this type 

 of reproduction, variation is limited to shuffling and re- 

 shuffling of the parental genes. By repeated brother-and- 

 sister mating it is possible to obtain males and females 

 the genetic composition of which is so similar that the 

 offspring is highly constant in character, giving so-called 

 *pure-bred' strains— in other words, strains showing mini- 

 mal variation in body characters. This type of breeding, 

 however, rarely occurs in nature, because the father and 

 mother are generally of different genetic strains, and un- 

 der these circumstances the potentialities for variation 

 afforded even by the simple reshuffling of genes are al- 

 most unbelievably large. For example, with only 23 

 chromosomes in the himian paternal and maternal germ 

 cells, the total number of different combinations of 

 chromosomes in the potential offspring of one man and 

 one wom^i is over 70 thousand billions, or 26,000 times 

 the population of the earth. But every chromosome is 

 composed of many genes, each of which may have sev- 

 eral variations: on the assumption that there are a thou- 

 sand distinctive genes in the human chromosomes (and 

 there are probably many more), and that each gene has 

 only two variations (and there are probably more) , it has 

 been calculated that the number of potential combina- 

 tions of genes to be derived from one man and one 

 woman is 2 to the 1000th power, a figure greater than 

 the number of electrons in the world. With random 

 mating such as occurs in nature, it can therefore be 

 safely said that no two individuals in any species are ever 

 exactly ahke. This even apphes to identical twins, which 

 may, but do not necessarily, start out with an absolutely 

 identical genetic pattern but are now thought to undergo 



