PROBLEM 6. How Mankind Caji Be hn proved 



Fig. 456 This pedigree shows the inher- 

 itance of an ahionnal skin condition in 

 which the slightest friction causes blisters 

 to form. Squares represent Diales; circles 

 represent females. Blackened symbols rep- 

 resent individuals having the abnorfjial 

 skin, (snyder) 



509 



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this line. To show that an individual has 

 the trait being studied the square or cir- 

 cle is filled in. Do Exercise 2 to see 

 whether you understand Figure 457. 



There are institutions in London and 

 elsewhere that obtain and keep records of 

 human pedigrees. They often use the 

 word eugenics, which means "improving 

 human beings by breeding." As you read 

 further you will learn that, with our pres- 

 ent knowledge, little can be done to im- 

 prove human beings by breeding. You 

 must, therefore, read very critically all 

 that is written about eugenics to see 

 whether it rests on opinion or on fact. 

 Sometimes people who call themselves 

 eugenists attempt to defend a belief 

 rather than to advance understandine;-. 



Inheritance of physical traits. Certain 

 types of physical characteristics, such 

 as bodily defects, can be studied easily 

 for two reasons: the environment does 

 not ordinarily affect such characteristics 

 and many of the genes seem to act as sim- 

 ple dominants and recessives. For exam- 

 ple, it has been discovered that six-fin- 

 geredness (Polydactyly) and short-fin- 

 geredness {brachy dactyly in which each 

 finger has only two bones) are dominant 

 to the normal condition. Both of these 

 characters are known to have appeared 

 suddenly; they were mutations. 



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Fig. 457 A pedigree showing the inheritance 

 of one type of extreme shortsightedness. In the 

 F^ are the shortsighted individuals {black sym- 

 bols) pure or hybrid? (snyder) 



Another character, lack of pigment 

 (coloring matter) in the skin, hair and 

 eyes is known to be a recessive. This 

 character is called albwism (al'bin-ism). 

 An albino (al-bine'oh) has pink eyes 

 (just like an albino rabbit or white 

 mouse) because there is no pigment in 

 the eye to hide the color of the blood. 

 See Figure 458, page 510. It is believed 

 that albinism has appeared again and 

 again as a mutation. We know that it is 

 recessive because only albino children 

 are produced if both parents are albinos. 

 You will be interested in doing- Exer- 

 cise 3. Exercise 4 is more difficult; can 

 you do it? 



The inheritance of complex physical 

 traits. Such ordinary characters as size, 

 weight, skin color, shape of head, and 

 features of face are complex characters 

 about whose inheritance we know little. 

 Each character is apparently produced 



