34 Genes and Characters 



Thirty-nine chicks in Hutt and Child's study showed the 

 tremor character, but the extent to which it appeared varied 

 greatly in different individuals. In some chicks this tremor was 

 so pronounced that they could not even stand up, whereas others 

 showed only a barely perceptible tremor. Various intermediate 

 conditions were observed. This difference is known as the 

 expressivity of the character. Penetrance and expressivity are 

 not the same thing. In determining penetrance, every chick was 

 counted that manifested the character in any degree irrespective 

 of the extent of the expression of the character in that individual. 

 In determining the penetrance of the gene for poliomyelitis, Ad- 

 dair and Snyder also took expressivity into account. Although 

 cases with high expressivity were readily recognized, the possi- 

 bility existed that there might have been susceptible individuals 

 in which the expressivity was so low that the infection produced 

 only a fever or other mild illness instead of the usual crippling 

 paralysis. A careful check was made to learn whether any 

 brothers or sisters of paralyzed children had mild cases during 

 the period when the less fortunate members of their family were 

 affected. Since no such cases were found, it seemed clear that 

 the expressivity of the gene was high and that the penetrance 

 was not complete. 



Inherited Characters in Plants and Animals 



Several examples of characters produced by one or more genes 

 have been mentioned in this and the last chapters; many more 

 are discussed later in this book. We might mention here, how- 

 ever, that all organs of plants and animals are under genie con- 

 trol. In plants, a list of inherited characters would include stem 

 height, length of internodes, type of branching, leaf shape, chloro- 

 phyll deficiencies, flower color and color patterns (Fig. 10), shape 

 of flower parts, shape of fruit, color of seed coats and endo- 

 sperm, and even seedlessness. In animals we could list such 

 traits as abnormalities of bone growth in the skull and other 

 bones (Fig. 11), the presence of excess fluid between the brain 

 and the skull, absence or reduction of the jaws, eye color, con- 

 genital cataract, color of the fur or feathers (Fig. 12), albinism, 

 woolly hair, hairlessness, inherited bleeding, size and weight, 

 glandular abnormalities, and many more far too numerous to 

 mention. Dunn and his co-workers described in 1940 a very 



