22 



Chromosomes and Genes 



stems, and red leaves, and therefore as having several different 

 effects. It is probably more correct to think of the gene as 

 having one general effect, the production of pigment, and not 

 several effects, for, after all, the plant is a unit and it is only 

 the botanist who subdivides it into organs. 



A very interesting situation that might readily be classed as 

 pleiotropy if it were not sufficiently analyzed is the effect pro- 

 duced by the frizzle gene in poultry. This gene is an incom- 



FiG. 8. Frizzle fowl. Lejt, a homozygous frizzle male. Center, homozy- 

 gous frizzle female. Right, bare homozygous frizzle female. (Photographs 

 courtesy of Dr. W. Landauer.) 



pletely dominant gene. In fowl homozygous for frizzle (Fig. 

 8), the feathers are very abnormal, being narrow and curled and 

 very fragile. They break off easily, so that after a while such 

 birds are almost featherless. In heterozygotes the feathers are 

 wavy rather than curly, and the whole frizzled effect is less 

 pronounced. 



The frizzled and fragile condition of the feathers is apparently 

 the only direct effect of the frizzle gene, but fowl homozygous 

 for this gene are very different from normal fowl in many other 

 respects. When all or most of the feathers are broken off, their 

 insulating effect is naturally lost, and such naked fowl lose their 

 body heat much more rapidly than normal fowl. To compen- 

 sate for this excess loss of heat, a number of adaptations take 

 place in the bodies of the frizzle individuals. Their basal metabo- 

 lism is strikingly accelerated, thus increasing the supply of hor- 

 mones from the thyroid and adrenal glands. This increased 

 hormone production severely taxes these glands and results in 



