438 INTRODUCTION TO EVOLUTION 



Dobzhansky's Genetics and the Origin of Species, 1951, for more complete 

 discussion of mutation pressure and genetic equilibrium.) 



In this connection we may note that a new mutation may not produce 

 a detectable effect until several generations following the actual occurrence 

 of the change in the gene. This is true when a dominant gene mutates to 

 form a completely recessive one. Let us imagine a population of black 

 hamsters all homozygous MM. What will be the fate of a single mutation 

 which occurs in this stock? Suppose that in this case the mutation occurs 

 in a sperm-forming cell in a male. As a result one or more of his sperms 

 contains gene m instead of the gene M possessed by all his other sperm 

 cells. If an m-containing sperm functions in fertilization, it must necessarily 

 fertilize an M-containing ovum (there are no others). When this occurs an 

 individual of the formula Mm is produced. This individual, like its parents, 

 is black; the "new" gray gene has still not produced a visible effect. The 

 Mm individual must mate with an MM individual of the opposite sex. As 

 we saw on page 429, such a mating (Mm X MM) is expected to result in 

 offspring that are half Mm, half MM. Again, these offspring are all black 

 although half of them are heterozygous. Still we have no gray hamsters! 

 How can we obtain gray hamsters? These can only arise if an Mm female 

 mates with an Mm male. As we saw earlier, one-fourth of the offspring of 

 such a mating are expected to be gray (mm) (p. 379). Thus the actual 

 occurrence of a recessive mutation must necessarily be separated from the 

 production of an individual showing the visible effects of that mutation 

 by at least two generations. 



Generation I Heterozygous Homozygous 

 Black Hamster Black Hamster 

 Mm X MM 



Generation II MM MM Mm X Mm 

 Black Black Black j Black 



Generation III MM Mm Mm mm 



Black Black Black Gray 



It will be noted that in order to produce a gray individual in the fewest 

 possible generations we have made use of a brother-sister mating in Gen- 

 eration II. Such matings are not uncommon among lower animals. In mod- 

 ern human societies the nearest approach to them consists of first-cousin 

 marriages. These could also bring a recessive gene "to light" in the mini- 

 mum number of generations if the Mm individual in Generation I had a 

 brother or sister of the same constitution, and an Mm offspring of the 

 latter married one of the Mm individuals shown in the diagram. 



