GENETICS AND EUGENICS 



837 



the application of Mendel's law to inheritable traits in this, the most 

 complex of all living forms, is correspondingly complicated. Although 

 there are eases in which the 1 :2 :1 ratio occurs in as simple a form as 

 in Mendel's peas, there are many more cases in which the Mendelian 

 principle is manifested as multiple allelomorphs, multiple genes, modi- 

 fying factors, complementary factors, incomplete dominance, change- 

 able dominance, irregular dominance, etc. There is a tendency 

 among some to depreciate our knowledge of human heredity on the 

 ground that there is so much that we do not know. It is right to 

 admit the extent of our lack of knowledge, but it is wise to give 

 proper credit to our present store of information and to take cog- 

 nizance of the rapidity with which the gaps in our knowledge are 

 being eliminated. One by one the complicated problems of human 

 heredity are being solved by patient investigators, and by putting 

 what we already know to use, we stimulate the accession of more data. 

 Some Cases of Human Heredity. — To give a complete summary 

 of our knowledge of human heredity would be beyond the scope of 

 this chapter. The accompanying table gives some of the data that 

 have been accumulated: 



Data on Human Heredity 



TRAIT 



General Physique and Skeleton 



Stature 



Body build 



Polydactyly (extra digits) 

 Brachydactyly (short digits and 



limbs) 

 Symphalangism (fused fingers and 



toes) 

 Zygodactyly (webbed fingers and 



toes) 

 Club foot 



Lobster claw (split hands and feet) 

 Exostoses (outgrowths of long bones) 

 Abnormal fragility of bones 

 Amputation (entire absence of hands 



and feet) 

 Shortened arms 



Skin, Hair and Countenance 



Ichthyosis (scaly skin) 



HOW INHERITED 



DOMINANCE 



A composite 



character 

 Multiple genes, 



two or more 



pairs 

 Simple 

 Simple 



Simple 



Simple 



Simple 

 Simple 

 Simple 

 Simple 

 Simple 



Irregular dom- 

 inance 



Simple. Lethal 

 when homozy 

 gous, 



Many genes for shortness 



are dominant 

 Factors for stoutness are 



dominant 



Abnormality dominant 

 Abnormality dominant 



Abnormality dominant 



Abnormality dominant 



Abnormality recessive 

 Abnormality dominant 

 Abnormality dominant 

 Abnormality dominant 

 Abnormality dominant 



Abnormality dominant 

 Abnormality dominant 



