226 LIFE: ITS NATURE AND ORIGIN 



constriction (strangulation) or by reduction of the lumen of ar- 

 terioles (by arteriosclerosis) or capillaries (by drugs or by hor- 

 monal or nervous imbalance), can result in diminished oxygen 

 supply and delayed removal of metabolic products. The normal 

 oxidative breakdown of glucose may thereupon veer toward an- 

 aerobic fermentation whereby lactic acid is released. The con- 

 sequences in the various cells and organs will no doubt differ 

 considerably, as will also the secondary consequences throughout 

 the body; but in general, lack of oxygen will upset the normal 

 function of the cellular enzyme systems, even if it does not lead 

 to enzyme inhibition or even to cellular death. During violent 

 exertion, for example, in a 100-yard dash, some of the muscular 

 energy comes from anaerobic breakdown of glucose. The "oxy- 

 gen debt" of the tissues is gradually restored by accelerated breath- 

 ing or panting. 



As above mentioned, many diseases have a genetic basis, and 

 involve inherited enzyme abnormalities, or are established by 

 structural abnormalities consequent upon abnormal embryologi- 

 cal development. In his recent book (1941) on "Medical Ge- 

 netics" Prof. Laurence H. Snyder of Ohio State University points 

 out that the familiar 3:1 ratio of Mendel no longer covers the 

 major portion of the field of heredity, for geneticists have demon- 

 strated varied types of heredity transmission — dominant, recessive 

 and blending — as well as varied relationships and behaviors of 

 genes, e.g., autosomal, sex-linked, sex-influenced, lethal, epistatic 

 and combined genes as seen in multiple allels and multiple factors. 

 Epistasis, in Mendelian inheritance, means "the expression of one 

 character to the exclusion of another not of the same allelomor- 

 phic pair." Thus in rabbits and mice the factor for gray, if pres- 

 ent, prevents the development of the factors for black and choco- 

 late. This recalls the fact that some plants lie prone ("lazy" corn) 

 or develop chlorophyll-free areas ("variegated"), probably because 

 of the development by them of catalysts whose activities lead 

 to these results. Many traits are transmitted by heredity, even 

 though we have thus far been unable to unravel the precise mech- 

 anism. Thus strains of Dalmatians ("coach dogs") have been se- 

 lected and bred which take definite positions — some close to the 

 horse, some beneath the wagon, and some behind it. 23 Identical 

 twins generally develop like heritable diseases and show like re- 

 actions on exposure to other diseases. 



