GENETICS 233 



gene is present. Sometimes one gene suppresses the action of a gene of 

 some other pair. Sometimes two genes, neither of which produces any- 

 thing detectable by itself, combine to produce a visible result when they 

 occur together in the same animal. When the interactions are between 

 dominant genes, they result in Fo ratios which are some modification 

 of the fundamental ratio 9:3:3:1. This ratio is changed because two 

 or more of the classes of individuals appear alike. To describe details 

 of such interactions would go beyond the scope of a first study. The 

 complexity is considerably increased by interactions among three, four, 

 or five different genes. So many examples of combined actions have 

 been found that it seems probable that they are universal. That is, 

 every gene probably interacts with some — even many, or all — other genes. 

 The phenomena of heredity can be very complicated. 





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c 



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a 







Fig. 202. — The divisions of the male germ cells of the bug Anasa: a, polar view of 

 equatorial plate of first division; all the chroniatic bodies are double except one and there- 

 fore represent 21 chromosomes, the somatic number; b, second division in side view, not all 

 of the chromosomes shown; the single chromosome of a is going undivided to the lower pole; 

 c and d, polar view of the two anaphase groups of the second division; 11 chromosomes go 

 into one spermatid (female-producing), 10 into the other (male-producing). (After Wilson 

 in Journal of Experimental Zoology.) 



Inheritance of Sex. — A special genetic situation exists in the dis- 

 tinction between the sexes, for in a large number of animals and some 

 plants the chromosomes of the male and female are in some respect 

 unequal. Either one sex has one more chromosome than the other, or one 

 of its chromosomes is larger than the corresponding chromosome of the 

 other, or certain corresponding chromosomes are of different shapes. 

 When the number of chromosomes is different, most species of animals 

 have more in the female than in the male. An example of this condition 

 is found in a species of bug whose chromosomes are shown in Fig. 202. 

 The male has 21 chromosomes, the female 22. The figure show\s the 

 reduction division of the spermatocytes of the male. At the left (a) 

 are the pairs of chromosomes, mostly so closely united that their double 

 nature is not revealed. At the bottom of a, outside the circle of other 



