of two or more genes. There is also the fact of linkage, which holds together 

 through the generations two or more characters, of which one may suit us 

 while the others are quite undesirable. In mammals, for example, genes for 

 coat-colors and genes for ear-defects are curiously linked and so place a limit 

 on carrying out what we intend in animal-breeding. Nevertheless breeders 

 systematically apply the principles of genetics to produce new varieties of 

 animals, that is, combinations of qualities that have never appeared before. 

 The Orpington fowl is an artificial breed of this kind, and new breeds are con- 

 stantly being brought out. And as in the case of plants, breeders supply young 

 hybrid poultry having desired characteristics, but not capable of passing on 

 these qualities. 



Heredity in Man^ So far as reliable facts are available, heredity seems 

 to follow the same course among human beings as among other organisms 

 (see table below). 



Human beings take a comparatively long time to mature. To get com- 

 plete records for many generations it would be necessary to go back several 



Heredity in Man 



DOMINANT CHARACTER 



Curly hair 



Dark hair 



Beaded hair 



Hairlessness, associated with lack of teeth 



White forelock 



Brown eyes 



Normal sight 



Hereditary cataract 



Normal hearing 



Normal ear 



Normal pigmentation 



Hapsburg lip 



Normal muscular tone 



Nervous temperament 



Fused fingers or toes 



Supernumerary digits 



Broad fingers (lacking one joint) 



Fused joints of digits 



Double- j ointedness 



Normal growth 



Limb dwarfing 



Immunity to poison ivy 



SEX-LINKED CHARACTERS 



Normal blood 

 Normal hair 

 Normal vision 



RECESSIVE CHARACTER 



Straight hair 



Light; red 



Even hair 



Normal condition 



Normal, even coloring 



Blue eyes 



Night blindness 



Normal eye 



Deaf-mutism 



Otosclerosis 



Albinism 



Normal lip 



Low muscular tone 



Phlegmatic temperament 



Normal digits 



Normal number 



Normal length 



Normal joints 



Normal condition 



General dwarfing 



Normal proportion 



Susceptibility to poison ivy 



Hemophilia 



Baldness 



Color-blindness 



^See No. 7, p. 505. 

 500 



