Unit Characters and Gene Interaction 23 



changes in the glands themselves. The extent of these changes 

 depends largely upon the temperature at which the birds are 

 kept. If the temperature is as favorable as possible, the thyroid 

 gland may increase in size but be otherwise normal; but if the 

 conditions under which the fowl are kept are less favorable, the 

 gland may be hypertrophied. If the birds are raised in a low 

 temperature, the loss of heat is so great and there is such a drain 

 upon the thyroid glands that they may become exhausted and 

 atrophied. 



As a consequence of heat loss, the oxygen consumption is 

 increased by an increase in the rate of the heart beat, accom- 

 panied by hypertrophy of the heart and an increase in the vol- 

 ume of the circulating blood. The excess loss of body heat also 

 calls for a greater amount of heat production by the fowl, partly 

 accomplished by an increase in the amount of food they eat. 

 The increased food intake produces an enlargement of the pan- 

 creas, crop, gizzard, and kidneys. Frizzle fowl, therefore, show 

 changes not only in their feathers but also in the nature and size 

 of a number of their organs. All these effects are brought about 

 because of one gene, but the effect on the feathers is the only 

 direct effect that that particular gene has. All the other effects 

 are indirect, not caused by the gene, although they are the result 

 of the presence of that gene; they therefore might easily be 

 wrongly interpreted as an example of pleiotropy. Pleiotropy 

 refers only to the production of more than one direct effect by 

 a gene and does not include such cases of indirect effects. 



Unit Characters and Gene Interaction 



We have pointed out that flies with the gene Vg have normal 

 wings whereas those that are homozygous for vg have vestigial 

 wings. In the earlier days of the science of genetics many similar 

 cases were observed in which one character appeared to be due 

 to one gene only whereas a ^'contrasting character" was due only 

 to the allele of that gene. The notion that a pair of contrasting 

 characters was conditioned only by one pair of alleles led to the 

 suggestion that an individual was made up of a large number 

 of characters and that each one was the result of the action of 

 one gene. Such monogenically conditioned characters were 

 called unit characters, and an individual was thought to be a 



