CYTOGENETIC CORRELATIONS AND CROSSING OVER 99 



IV, in Drosophila, for example, the w^' allele differs from a deficiency 

 for tfie locus only by its ability to back-mutate towards wild-type ex- 

 pression. Such an allele, totally lacking in expression, is called an 

 amorph. Recessive alleles which produce a diminished expression, e.g., 

 a pale pigmentation, are called hypomorphs. 



Studies of gene dosage have also provided evidence of hypomorphism. 

 For example, in two recessive mutants of Drosophila, bobbed and 

 shaven, increasing the number of recessive alleles (possible because 

 these genes are carried on the Y chromosome which can be present three 

 times without lethal effect), leads towards the expression of the wild- 

 type phenotype. Apparently each allele contributes a small positive 

 phenotypic effect, undetectable when present only once, but cumulative 

 at higher doses. 



Small nonlethal deficiencies have been very useful for the cytological 

 localization of particular genes. A heterozygous deficiency appears as 

 an unpaired loop in the paired chromosomes of meiotic prophase of 

 Drosophila salivaries, and, consequently, very small deletions can be 

 detected. When the deletion includes a gene of known function, its 

 location can be determined with considerable precision on the cytological 

 map. 



Duplications of chromosomal segments are less apt to be lethal than 

 are deficiencies. In salivary gland chromosomes of dipteran insects, 

 duplications have been identified as repeats, short segments which 

 precisely repeat the banding pattern found elsewhere, often in an adja- 

 cent location. It has been suggested that repeats provide genetic mate- 

 rial for progressive evolution; since the organism does not need to have 

 the same information present twice, one of the repeats is available for 

 mutation. 



This suggesion is supported by the existence of closely linked genes 

 with similar but not identical functions. In most organisms, such in- 

 stances are found but rarely. In the bacteria, however, the close linkage 

 of genes with related functions in a biosynthetic seqjjence has been 

 found especially in Salmonella (see Chapter 6). An example of a repeat 

 that has been observed and studied is Bar in Drosophila. Sturtevant 

 studied this locus and inferred from genetic data alone that what had 

 been called mutation at this locus was actually the result of oblique 

 pairing and unequal crossing over, leading to the formation of repeats. 

 It only became possible to test this inference by direct cytological 

 examination some ten years later, and at that time, the correlation found 

 between the Bar phenotypes and the presence of cytologically observable 

 repeats fully supported and confirmed Sturtevant's hypothesis. 



