HEREDITY 20J 



beings. Pure strains of humans cannot be produced, like flies, by long in- 

 breeding of parents with children, brothers with sisters, etc. And where 

 flies have 300 offspring at a tfme and three generations to a month, human 

 couples do not average more than four offspring to a marriage, and only 

 three or four generations to a century. 



So, genetically, in most respects we humans are unknown quantities. 

 With regard to your own genes, you can only make guesses, but in this 

 you will be helped considerably not merely by the characteristics which 

 you yourself reveal, but by those which appear in your parents, grand- 

 parents, brothers, sisters, and other close relatives. If you are dark-eyed, 

 the chances of your carrying a "hidden" blue-eye gene increase according 

 to the number of your relatives who have blue eyes, and their closeness 

 to you. Going further, if you marry a blue-eyed person and have a blue- 

 eyed child then you know definitely that you carry a blue-eye gene. On 

 the other hand, if two, three, four children in a row are all dark-eyed, the 

 presumption grows that you haven't a blue-eve gene. 



Likewise, where both parents are dark-eyed, the appearance of a blue- 

 eyed baby is proof conclusive that both carry "hidden" blue-eye genes. 

 But if all the children are dark-eyed, it still might mean that only one of 

 the parents has no blue-eye gene. 



These qualifications hold for every case where persons have some char- 

 acteristic due to a dominant gene (dark hair, curly or kinky hair, thick 

 lips, etc.) and wish to know what chance they have of carrying a "hid- 

 den" gene which might produce a different trait in their child. 



But before we try to make any predictions these facts should be clear: 



All forecasts as to the types of children people w'ill have are based on 

 averages determined by the laws of chance. 



Wherever dominant and recessive genes are involved, it is like tossing 

 up coins with heads and tails. Toss up coins long enough, and the number 

 of heads and tails will come out even. So if you are carrying one dominant 

 and one recessive gene for any characteristic, were it possible for you to 

 have an unlimited number of children, you'd find that exactly half would 

 get the dominant and half the recessive gene. 



With two parents involved, the results will be like those obtained in 

 "matching" coins. This, of course, conforms to Mendel's laws. 



When we think in terms of the characteristic produced, the result in 

 "mixed" matings will be that the dominant characteristic (dark eyes, dark 

 hair, etc.) will show up three out of four times, the recessive only one in 

 four, as it requires a matching of the recessive genes. 



Of course, where one parent carries two dominant genes, all the chil- 

 dren will show the dominant trait. Where one parent carries a dominant 

 and a recessive, and the other parent two recessives, half the children will 

 show the dominant trait, half the recessive. 



But here is something else to bear in mind: 



