490 - Heredity and Evolution 



pie, always (when homozygous) yields a 

 shortening and scalloping of the wing. But 

 the degree of this effect varies widely accord- 

 ing to the environmental conditions (espe- 

 cially temperature) that are experienced by 

 the individual fly dining its developmental 

 stages. Variations in both penetrance and ex- 

 pressivity, therefore, make the genetic anal- 

 ysis of certain characteristics more difficult. 

 On the other hand, a study of these charac- 

 teristics is beginning to yield valuable data 

 in regard to the basic manner in which genes 

 operate during embryonic development. 



Complementary and Supplementary Genes. In 

 recent years there have been frequent reports 

 indicating that interaction may occur be- 

 tween two or more different gene pairs, and 

 it now is possible to designate two main types 

 of interacting genes. In the case of comple- 

 mentary genes, two nonallelic dominants are 

 involved, but neither of these dominants can 

 express itself in the absence of the other. 

 With supplementary genes, on the other 

 hand, two dominants are again involved, but 

 in this case the first dominant produces its 

 effect when the other is absent, but the sec- 

 ond dominant remains inactive unless the 

 first is present. Obviously genetic experi- 

 ments dealing with complementary or sup- 

 plementary gene pairs are apt to yield un- 

 usual ratios. Therefore a clarification of these 

 types of genie interaction has helped to solve 

 several rather baffling cases, which previously 

 were regarded as non-Mendelian types of 

 heredity. 



Mendel's Laws. The experiments that have 

 been presented up to this point exemplify 

 the first two laws of genetics. These are called 

 Mendel's laws, although Mendel had little 

 knowledge about chromosomes and phrased 

 the laws in different terms. 



1. Law of segregation: the genes of ex>ery 

 allelic pair are always segregated into 

 separate gametes, each gene remaining 

 completely uncontaminated by the 

 oilier. 



2. Law of independent assortment: pro- 

 vided the gene pairs have loci iti dif- 



ferent pairs of chromosomes, they are 

 assorted among the gametes each inde- 

 pendently of the others. 

 Linkage (Dependent Assortment). The 

 number of groups of allelic genes that assort 

 independently cannot be greater than the 

 number of pairs of homologous chromosomes 

 of the species (Table 26-1), although the total 

 number of genes in any organism is always 

 much greater than the number of chromo- 

 somes. Each chromosome provides loci for 

 many genes, and the genes of each chromo- 

 some tend to be linked together during their 

 hereditary transmission. 



Table 26-1— Chromosome Numbers of 

 Some Familiar Organisms 



Drosophila melanogaster provides excel- 

 lent material for the study of linkage, be- 

 cause this species has only four pairs of chro- 

 mosomes (Fig. 26-14). Thus the several hun- 

 dred genes that have been identified in the 

 fruit fly all fall into four linkage groups, 

 which correspond to the four chromosomes. 



Linkage can be demonstrated by following 

 the transmission of two or more gene pairs 

 having loci in the same pair of chromosomes. 

 In Drosophila this specification is met by 



