DIFFERENCES AMONG OFFSPRING 



315 



or poor nutrition or toxic materials of one sort or another. Only 

 by affecting germ cells can the character of the offspring be con- 

 trolled or changed. Recently, it has been demonstrated that in- 

 ternal secretions circulating in the blood of the parent may affect 

 the germ cells. Thus, if a mother's thyroid is overactive and too 

 much thyroxin circulates in the blood, the germ cell may be 

 affected in such a way that abnormalities in the developing off- 

 spring are frequently brought about. 



Differences among offspring. It has long been known that 

 certain characteristics or qualities reappear in successive genera- 

 tions. Why do not all the characteristics of the parents reappear ? 

 If each new individual were merely a portion of an older individual, 

 it could readily be seen how the characters would continue from one 

 generation to the next. When plants are propagated by cuttings 

 or by grafts, the plants thus produced remain rather uniform. 

 Where reproduction is sexual, two germ cells, the gametes, come 

 from different sources. Each germ cell carries many qual- 

 ities that are characteristic of the protoplasm of each parent. 

 These qualities reappear with a certain degree of probability and in 

 relationship after the two germ cells have united. Each individual 

 resembles both parents, but, necessarily, -also differs from both 



When two coins are tossed a great number of times, two tails turn up 25% of the times, two heads 

 turn up 25% of the times and one head and one tail turn up 50% of the time. This occurrence 

 is called the law of chance. In the few matchings shown in the diagram, the result would prob- 

 ably not be as near the 25, 50, 25 ratio as is pictured. This illustrates the law of chance (page 317) . 



parents. A child may have brown eyes like the mother's and curly 

 hair like the father's. Offspring of the same parents also differ 



