CONTINUITY OF THE RACE 



613 



influence on intelligence as it is measured 

 with I.Q. tests, although this may not be a 

 valid measure of true intelligence. On the 

 other hand, in regard to such physical traits 

 as height and weight, the inherited genes 

 are strikingly more important. 



Fortunately, because of the stability of 

 genes, the environment has little or no 

 effect upon them. Excellent genes may be 

 carried along for many generations in 

 starved, poorly developed bodies, but as 

 soon as the environment becomes com- 

 pletely adequate they express their full 

 capacity by producing a vigorous, healthy 

 body. In a world that has always suffered 

 periods of famine intermingled with times 

 of abundance only stable genes could have 

 made possible a race that has been able to 

 survive up to the present. 



HUMAN HEREDITY 



Human beings, like all species of animals, 

 show wide variations in height, weight, eye 

 color, hair color, skin color, facial configura- 

 tion, mental ability, and many other traits. 

 Within limits, these variations are highly 

 desirable because they allow evolution to 

 take place. However, at the extremes of 

 each one there are usually either mildly or 

 highly undesirable conditions which do not 

 permit the individual possessing them an 

 equal chance with others to become a self- 

 sustaining member of his own socic^l group. 



Some defects such as albinism render the 

 individual only slightly more handicapped 

 than his normal fellows. Most of us are 

 pretty apt to agree, however, that a com- 

 bination of genes that produces idiots, 

 blindness, or deafness is undesirable. Al- 

 though we are concerned with the inherit- 

 ance of defects in order to breed them out 

 of our people, we should be even more con- 

 cerned with the positive approach, that is, 

 retaining the quality of our present stock 

 and improving it where possible. Let us 

 now consider what is known about some 

 of the common human defects and later 

 some of the problems involved in improv- 

 ing the human race. 



Some congenital defects 



We have previously discussed the slight 



defects of color blindness (p. 604) and al- 



.binism (p. 592), as well as tlie more serious 



anomaly, hemophilia (p. 602). Some others 



are listed in Table 4 (see also Fig. 24-20). 



Ichthyosis, sometimes called fish-skin dis- 

 ease, is a congenital disease. Those suffering 

 from this malady have dark skin, %-inch 

 thick, which is cracked in such a way that 

 it resembles scales, hence the name, fish 

 skin. It occurs over the entire body except 

 the palms of the hands and the soles of the 

 feet. It is fatal, the child dying shortly after 

 birth. The disease was seen for the first time 

 in England in 1731 where it appeared in the 



Disease 



Ichthyosis (one type) 



Congenital cataract 



High-grade myopia 



Astigmatism 



Brachydactylism 



Polydactylism 



Lobster-claw 



Achondroplasia 



Diabetes mellitus 



Ataxia 



Microcephaly 



Amaurotic idiocy 



Deafness 



TABLE 4 



Condition 



Thick skin 



Opaque growths in eye 



Extreme nearsightedness 



Irregularity of cornea 



Short digits 



Extra digits 



Split hand and foot 



Short appendages 



Improper sugar balance 



Paralysis 



Idiots with small heads 



Degeneration of nervous system 



Atrophy of auditory nerve 



Type of Gene 



Sex-linked 



Dominant 



Recessive 



Dominant 



Dominant 



Dominant 



Dominant 



Dominant 



Dominant 



Recessive 



Recessive 



Recessive 



Dominant 



