Phenotypic Effects of Gene Action 



83 



chemical reactions involved in cellular me- 

 tabolism. 



The Creeper case apparently involves ge- 

 netically determined metabolic changes that 

 take place within certain cells (to produce 

 an abnormal nutritional environment) and 

 affect the functioning of other cells (the 

 differentiation of eye tissue). Let us con- 

 sider two groups of studies with mice to 

 learn more about the genetic control of 

 effects produced external to the cell in which 

 the gene acts. One group of investigations 4 

 involves a comparative study of normal and 

 dwarf mice. The dwarfs have all of their 

 body parts proportionally reduced in size, 

 because of an apparently completely reces- 

 sive gene in homozygous condition. During 

 early development, both dwarf and normal 

 mice grow at equal speeds, but later, the 

 dwarf suddenly stops growing and never 

 reaches sexual maturity. A microscopic 

 study of the anterior pituitary gland shows 

 that it is considerably smaller in the dwarf 

 than in the normal mouse. Moreover, cer- 

 tain large cells, normally present, are absent 

 in dwarf pituitaries; apparently these are the 

 cells that secrete growth hormone. That this 

 is a case of genetically produced pituitary 

 dwarfism is supported by the following type 

 of experiment: using pairs of dwarf litter 

 mates about 30 days old, one mouse of a 

 pair is injected with extracts of pituitary 

 glands from dwarf mice (Figure 6-12, B) 

 each day for 30 days, whereas the other 

 mouse is injected in a comparable way with 

 extracts of pituitary glands from normal 

 mice (Figure 6-12. A). During this period 

 of treatment, the former mouse remains es- 

 sentially dwarf, while the latter grows until 

 it is virtually normal. Here, then, we are 

 dealing with a chemical messenger, pituitary 

 hormone, which regulates growth in general, 

 and whose presence is dependent upon a 

 single pair of genes. 



4 Based upon work of G. D. Snell, of P. E. Smith 

 and E. C. MacDowell. and of T. Francis. 



44 49 54 



AGE IN DAYS 



figure 6-12. Effect of injecting pituitary 

 viand extracts into dwarf mice. (See text for 

 explanation. ) 



The second group of studies is concerned 

 with mouse tails. The normal (+ +) 

 mouse has a long tail; a particular mutant 

 strain has a shortened tail (Brachyury, or 

 Brachy). 5 Brachy crossed with Brachy pro- 

 duces % Brachy: V 3 normal offspring, a ratio 

 which suggests that the gene for Brachy, T, 

 is dominant for short tails and recessive for 

 lethality. Brachy mice should, therefore, be 

 T -f. When the embryology of offspring 

 produced from a mating between Brachys 

 (T + X T +) is studied, about 25% of the 

 embryos are normal (+ +), about 50% 

 show tail degeneration by 1 1 days of de- 

 velopment (7-f-), and about 25% of the 

 embryos (TT) are monsters (Figure 6-13) 

 which have misdirected posterior limb buds, 

 zigzag neural tubes, and no notochord. 

 Since their whole posterior part is undevel- 

 oped, T T individuals cannot make a placen- 

 tal connection and die after about 10 or 11 

 days of development. 



Consider further the T T individual, whose 

 segments, or somites, in the posterior part 



5 Based upon work of L. C. Dunn, P. Chesley, 

 and D. Bennett. 



