Multiple Alleles 27 



organism has been produced which has only one genome, but 

 such organisms are usually very weak and delicate. With a few 

 exceptions, every locus must be represented at least once for the 

 organism to survive. If a piece of a chromosome is missing, as 

 may result from subjecting germ cells to X-rays, a deficiency 

 results. If the deficiency is present in the same region of two 

 homologous chromosomes, it is homozygous, and if the deficiency 

 is in one chromosome but not in the homologue, it is heterozygous. 

 Heterozygous deficiencies frequently have marked phenotypic 

 effects which are sometimes mistaken for the results of gene 

 action. Frequently organisms with heterozygous deficiencies are 

 less viable than those with two complete genomes, and this is 

 especially marked when the missing segment is a long one. AVhen 

 the deficiency is homozygous, the organism usually fails to sur- 

 vive past the egg stage. In a few cases, however, where the 

 deficiency is very short, as in the yellow deficiency of Drosophila, 

 the organism may occasionally reach the adult stage. From 

 these deficiencies we can conclude that, with a very few excep- 

 tions, at least one member of every pair of genes must be pres- 

 ent for an organism to develop normally and that in most organ- 

 isms two complete genomes provide the best background for 

 normal development. Plants with more than two genomes are 

 discussed in later sections of this book. 



Multiple Alleles 



In normal diploid organisms, there are two genomes and every 

 locus is represented by two genes. It has been shown, however, 

 that the two genes at a given locus are not necessarily alike, for 

 in heterozygotes one is an allele of the other ; but in every hetero- 

 zygote, there can be only two different alleles at any one locus. 



Although any individual diploid plant or animal may have 

 only two genes at any given locus, in many species three or more 

 different alleles may be found at the same locus of a given 

 chromosome distributed among the different individuals, with no 

 individual having more than two. For example, in the common 

 bean, some plants may be homozygous for G, a gene that de- 

 termines yellow pods and green foliage, and others may be homo- 

 zygous for the allele g, when they will have yellow pods and 

 yellow foliage. Other plants may be Gg and will look like GG 

 plants because G is completely dominant to g. However, in some 



