THE FUNDAMENTALS OF MENDELISM 33 



This "independent assortment" was later found not to be a general 

 rule, although it holds for many factors. Bateson and Punnett^ showed 

 that with some loci in the sweet pea a heterozygote AB.ab {A affects 

 flower colour, B shape of pollen grains), made by crossing A ABB with 

 aabb, forms more gametes of the sorts AB and ab from which it was 

 itself derived than of the other two possible kinds Ab and aB. Thus 

 there were fewer recombinations of factors than would result from pure 

 chance. If the two dominants A and B were originally together, they 

 tended to remain together ("coupling"), while if they were originally 

 apart they tended to remain apart ("repulsion"). The general name for 

 the phenomenon is Unkage; and the strength of Unkage between two 

 factors is measured by the percentage of recombinations among the 

 total number of gametes. Thus a strong linkage gives a low percentage 

 of recombinations, while no linkage at all gives the chance expeaation 

 of recombinations, which is 50 per cent. 



The progeny of any cross can be worked out by determining the 

 ratio of the various sorts of gametes formed by the parents and multi- 

 plying together the two gametic series. This gives the ratio of the various 

 classes of fertihzed eggs on the assumption that fertilization is at ran- 

 dom; if there is any selective fertilization (p. 55), each class must be 

 multiplied by a coefficient expressing the relative probability of the 

 type of fertilization involved. The ratio of the classes of adult organisms 

 depends on the way in which the genes concerned interact in develop- 

 ment; on relations of dominance and recessiveness, on the appearance 

 of double heterozygotes, etc., and on whether any of the genes lower 

 the viability of the organisms which thus tend to die off before attaining 

 adult age. 



4. The Chromosome Theory 



Mendel's postulated factors are present in pairs in the zygotes, but 

 are single in the gametes. The parallel between this behaviour, and the 

 behaviour of the chromosomes to which attention had been drawn by 

 Weismann, was noticed soon after Mendel's work was rediscovered. ^ It 

 was suggested that the factors might be chromosomes or parts of 

 chromosomes. This suggestion has turned out to be correct, and we 

 must examine the behaviour of the chromosomes to see how it explains 

 the behaviour of Mendel's postulated factors. 



^ Cf. Bateson 1930. 



2 Montgomery 190 1, Sutton 1902. For a full history of these discoveries, 

 see Wilson 1928. 



