THE PRINCIPLES OF HEREDITY 517 



in the urine. In addition, the persons with this condition have very 

 light colored hair and are imbeciles. Here we have three widely diver- 

 gent effects from one gene. They are all tied together, however — the 

 defect of metabolism somehow fails to provide the energy necessary for 

 normal brain reactions or the materials for normal pigment production 

 in the hair. No doubt many genes have multiple effects because of such 

 a chain reaction. 



Sex Determination 



When we stop and think of it, is it not remarkable that children of 

 the same parents can show the great differences which are found be- 

 tween members of opposite sexes? The sex organs, of course, are 

 quite divergent, but there are other differences that extend to practically 

 every part of the body. The skin of a woman has a texture and internal 

 structure which differs from that of a man. Her skeleton is modified 

 in the pelvic region in a manner which permits normal child birth. 

 This leads to modifications of the angle of the hip bones and the knee 

 bones and similar differences are found in the shoulder and elbow 

 joints. A woman's facial bones are broader in proportion to skull 

 height than those of a man. Her muscles have a greater deposit of fat 

 among the muscle fibers which makes them softer and helps to give the 

 flowing lines that characterize a woman's figure. There are blood dif- 

 ferences, differences in the growth of face and body hair, and differences 

 in the degree of development of the mammary glands. The differences 

 also extend to physiology — the rate of metabolism, rate of respiration 

 and heart beat, blood pressure, and nervous reactions all vary with sex. 



With even this incomplete list it is readily apparent that sex dif- 

 ferences cannot be explained on the basis of simple monohybrid inheri- 

 tance nor through multiple genes as we have studied them. Genetic 

 evidence indicates that every person has all of the genes for both sexes, 

 but under normal circumstances only those genes for one sex express 

 themselves and the others are suppressed. The bearded lady of the 

 circus and a man with fully developed feminine breasts are examples of 

 those rare cases where certain of the genes seem to break through and 

 show in a sex in which they are supposed to be suppressed. It is quite 

 possible that homosexuality is another case of displaced emphasis on 

 genes influencing sexual attraction. 



We now need to find the mechanism which will suppress the genes 

 for one sex and allow the others to be expressed. We know that the 

 sex hormones play a part in the vertebrate animals. If we remove the 

 ovary from a young female chick and engraft testes from a male chick, 



