PROBLEM D To What Extent Can Mankind Be 



Improved? 



Heredity in man. If physical characteris- 

 tics are inherited according to the AJen- 

 dehan laws in plants and in animals other 

 than man, it is sensible to suppose that 

 they must be inherited in the same way 

 in man also. But it is extremely difficult 

 to obtain facts about heredity in man. 

 To begin with, human beings grow up so 

 slowly that it takes about thirty years 

 to get a new generation. Therefore it 

 would take about sixty years to get an F. 

 from any P^ mating that we wanted to 

 study. Secondly, the number of offspring 

 produced is very small — far too small to 

 give reliable ratios. When a pair of fruit 

 flies are mated there are four or five hun- 

 dred offspring. Just a few pairs can pro- 

 vide thousands of offspring. Although 

 human parents sometimes have twelve or 

 more children, five or six is more com- 

 mon and very often there are fewer than 

 that. In the third place there is the diffi- 

 culty that we cannot have experimental 

 matings among human beings. 



You may wonder how biologists can 

 study human heredity at all. One method 

 is to study the inheritance of a certain 

 physical character in hundreds of fami- 

 lies. After obtaining very large numbers 

 of cases scientists use the facts as best 

 they can. Suppose we consider eve color. 

 This is not a simple character to study 

 because eyes are of many shades. Biolo- 



gists must use a complicated machine that 

 enables them to match eye colors. By this 

 means they have discovered that when 

 both parents have one special t^^pe of 

 blue eyes all the offspring have blue eyes. 

 When both parents have a certain type 

 of brown eyes some of the children may 

 be blue-eyed. By gathering figures in 

 many such families scientists found a ra- 

 tio of about three brown to one blue, a 

 typical Mendelian ratio. From this it was 

 concluded that this particular type of 

 blue is recessive to this type of brown. 

 Could you make a diagram to show this 

 inheritance of eye color? See Exercise i. 



Many eye colors seem to be produced 

 by two or more pairs of genes, and the 

 inheritance is much more complicated. 

 The same thing is true of the inheritance 

 of skin color. This also is too compli- 

 cated for us to trace here. 



Recording human inheritance. Besides 

 the method described above biologists 

 attempt to study human inheritance by 

 recording the heredity of a specific char- 

 acter in as many generations of individ- 

 ual families as possible. Such a record of 

 family history is known as a pedigree. A 

 human pedigree can be shown in a chart 

 in which males are represented as squares 

 and females as circles. Mating is shown 

 by a line connecting square and circle; 

 and children are shown below attached to 



I 



