26 Chromosomes and Genes 



from nature and because the shape of the wing it produces looks 

 more efficient and more suitable than the wing of any other 

 gene combination. 



Gene interaction is by no means confined to wing shape in 

 Drosophila. Eye color and other traits in this fly and many 

 characters in many other plants and animals have proved to be 

 the result of the interaction of many genes rather than of the 

 action of one gene alone. In fact, so many examples of inter- 

 action have been discovered that one wonders whether there 

 are any true cases of unit characters. Certainly, in the broad 

 sense the unit character idea is philosophically untenable for the 

 organism is a unit and not a mosaic of independent structures. 

 Since all the parts of a body act together for the benefit of the 

 body as a whole, it is difficult not to believe that all the genes 

 must act together also. Although all genes probably affect all 

 parts of the body at least slightly or in an indirect manner, some 

 genes affect some parts more strongly than others. In a sense, 

 probably all genes have multiple effects and probably all char- 

 acters are influenced to some degree by a number of genes. 



The Genome 



It has been mentioned previously that Drosophila melanogaster 

 has eight chromosomes, or two sets of four chromosomes in its 

 somatic cells. In fact, this is a much more significant way of 

 stating the chromosome number. Since each chromosome has a 

 mate, each gene must have a mate (either a similar gene or an 

 allele). Therefore, this fly also has two sets of genes. One set 

 is located in one set of four chromosomes, and the other set of 

 genes is located in the other four chromosomes. Since each set 

 of four chromosomes w^ith its set of genes is known as a genome, 

 this particular species of fly has two genomes. 



Similarly, in maize there is one set of ten chromosomes with 

 its genes and another set of ten chromosomes which are morpho- 

 logically identical with the first ten and have genes allelic to 

 the genes of the first set. Like Drosophila melanogaster, this 

 plant also has two genomes. 



Many plants and most animals are similarly composed of two 

 genomes, but in a number of plants and in some animals three, 

 four, or more genomes have been found. Occasionally, also, an 



