Genes Affecting Form 315 



Genes Affecting Form 



Genes have been found which affect the ratio of growth of 

 organs or parts of organs and in that way determine the final 

 size and shape that an organ will have. For example, Sinnott 

 showed that if two squash plants were crossed, one of which 

 bore essentially round fruit known as "sphere" and the other 

 a flattened type known as "disc," all the Fi plants had disc- 

 shaped fruit, whereas the F2 segregated into a ratio of three 

 discs to one sphere. Studies indicated that one locus was in- 

 volved and that the gene for disc was completely dominant 

 over its allele for sphere. Such a statement, however, considers 

 only the end product and does not begin to show what is the 

 true relationship of these alleles as functional units and why 

 the effect in one case is a disc-shaped and in the other case a 

 sphere-shaped fruit. Other types of squashes and gourds are of 

 still a different shape. Although the difference is sometimes 

 due to more than one gene, the important facts in all these ques- 

 tions of the development of form are that, irrespective of the 

 number of genes involved, the development of the fruit is under 

 genie control and it may be possible to determine at least some 

 of the steps by which two different genotypes produce two dif- 

 ferent phenotypes. 



The difference in shape between the disc and sphere cucurbit 

 fruits is caused by a difference in shape which is present as soon 

 as the ovary primordia are. In other words, these two genes act 

 very early in the life of the ovary, producing differences in the 

 two ovaries as soon as the ovaries can be identified. Only very 

 small changes occur during all subsequent development of the 

 ovary and fruit, so that final differences in shape are due to ini- 

 tial differences in shape. Such a discovery advances our knowl- 

 edge slightly, but merely pushes the problem back a little. In- 

 stead of asking now why the gene difference produces a dif- 

 ference in fruit shape, we ask why the gene difference produces 

 a difference in the shape of the ovary primordia. 



Initial shape differences do not explain all the differences in 

 the shape of the various races of gourds, for some gourds which 

 differ greatly in shape when mature have ovary primordia that 

 are practically indistinguishable. If two races have identical 

 ovary primordia, they will have fruit of the same size and shape 



