66 The Source of Variability 



Equus (Fig. 25, line A) the face grows at a much faster rate than 

 the cranium (the remainder of the head) until the cranium attains 

 a length of about 16 cm. After that, the two parts of the head grow 

 at more nearly equal rates. Several extinct horses appear to lit this 

 pattern, and one of these, Plesihippus simplicidens, may follow an 

 extention of the steep part of the heterogonic slope beyond the 

 16 cm. "bend" shown by Equus. Several other genera of extinct 

 horses have head measurements which agree with a pattern of 

 heterogonic growth different from, but roughly parallel with that 

 of Equus (Fig. 25, line B). Thus in a small horse such as Hyra- 

 cotherium, mutations resulting in larger body size would automati- 

 cally cause a change from a short-faced head to a long-faced head 

 such as that found in the larger species Merychippus. 



DOSAGE AND DOMINANCE 



It was formerly considered that pairs of recessive and dominant 

 alleles were different from each other, but this apparently is not 

 always the case. In Drosophila melanogaster a certain bristle on 

 the thorax occurs as the normal long form or as an abbreviated 

 form called "bobbed." A single pair of alleles controls this condi- 

 tion, the allele for the normal type being completely dominant over 

 the "bobbed." Experimenting with replicates of the sex chromosome 

 in this species. Stern (1929) was able to produce a zygote con- 

 taining not the normal single sex chromosome but three or more 

 sex chromosomes each bearing a recessive "bobbed" allele. Zygotes 

 with three or four "bobbed" alleles produced not the recessive 

 phenotype, but a fairly long spine, and zygotes with even more 

 "bobbed" alleles produced the typically dominant long form. It is 

 therefore concluded that in this case the dominant allele represents 

 simply a larger dosage of the same material represented by the 

 recessive allele. The same phenomenon is displayed in many types 

 of sex determinants, especially the XO, XY male, and XX female 

 class (White, 1954). Here only one dosage of X produces males, 

 but a double dosage produce females. Thus many pairs of alleles 

 simply may represent dosage effects of the same unitary genetic 

 attributes. 



Added support for this view comes from cases of unequal cross- 

 ing-over. The sex-linked character Bar (reduced number of eye 

 facets) in Drosophila is caused by a tandem duplication of the 16A 

 region of the X-chromosome, a duplication which is caused by un- 

 equal crossing-over. Occasional further unequal crossing-over re- 



