512 



HUMAN INHERITANCE 



seems to hold for weight also. It is even more difficult 

 to ascertain the factors governing weight since it is so 

 greatly influenced by diet and exercise. 



Color-Blindness serves well to illustrate sex-linked 

 inheritance in man. The reader has already learned 

 that there are many sex-linked characters in the vinegar 

 fly, whose genes or factors are carried in the so-called 



Fio, 148. — Dwarfs. At the right is a man of normal stature. 

 The next two figures are those of dwarfs with very short legs but 

 with nearly normal trunk. The next two figures are of dwarfs whose 

 bodies are proportionally reduced throughout. The figure to the 

 left i.s a dwarf (cretin) whose underdevelopment is due to the failure 

 of a ductless gland to produce the normal amount of hormones. 



Plate, Vererbungslehre. Englemann. 



" sex-chromosomes. " The type of color-blindness in 

 which the aff"ected person is unable to distinguish red and 

 green is fairly common (about one person in twenty-five) 

 among men but rare among women. When a color- 

 blind man marries a normal woman he transmits to his 



