The Source of Variability 63 



throughout the protistan world. Each chromosome has a definite 

 structure, consisting of the mechanical center or centromere and 

 the arms, the latter typically having areas considered to contain 

 genie material of specific function {euchromatin) alternating with 

 areas for which only a more general type of genie activity has been 

 demonstrated ( heterochromatin ) . 



The chromosomes have distinctive areas each of which exerts 

 specific genie effects. In the crossing-over of chromosomes, breaks 

 normally occur between these areas and not through them. These 

 seemingly unitary areas of genetic action are called genes, so that 

 each chromosome may be considered a succession of areas of spe- 

 cific genie action, in truth a "gene string." 



ALLELES 



Many genes occur in two or more forms, called alleles. These may 

 form the two complementary units of the paired chromosomes in 

 diploid generations and the separate units in the gametes of the 

 haploid generation. A classical example of alleles occurs in Mendel's 

 peas, one allele being for red flowers, the other for white. If the 

 two alleles in the zygote are both of the red type, the flower is red, 

 if both are of the white type, the flower is white. If the zygote is 

 heterozygous for red and white, the flower is red. This is an ex- 

 ample of complete dominance. As more examples are studied, 

 this phenomenon of complete dominance is proving to be relatively 

 rare, because most heterozygotes produce an effect either inter- 

 mediate between the two homozygote expressions or quite dif- 

 ferent from either. In fowl, for example, black crossed with splashed 

 white produces Andalusian blue. 



The inheritance of many characters once thought to be con- 

 trolled by a single pair of alleles is now known to involve a large 

 complex of alleles (called pseudoalleles) situated so close together 

 on the chromosome that crossing-over seldom occurs between them 

 (Glass, 1955). Eye color in wasps of the genus Mormoniella is 

 controlled by two major genes each having several pseudoalleles. 

 As a result of the large number of resultant distinctive genetic com- 

 binations, a dozen or more distinct shades of eye color can be rec- 

 ognized (Whiting, 1955). 



COMPOUND EFFECT OF GENES 



A single gene may produce two or more quite different phenotypic 

 effects in an organism. The two principal types of such correlation 

 are pleiotropy and allometry. 



