28 GENETICS 



tuting one chromosome for another. Examine the two kinds 

 of sons in figure lo. Half of them have one kind of X-chro- 

 mosome (black, that came from the original mother). The 

 other half have another X-chromosome substituted for it, 

 namely, the one that came from the original father (white). 

 And this substitution changes the eyes from normal to bar- 

 eyes. The same substitution has been made in the case of 

 the two kinds of daughters, with the same effect. Wherever 

 an X-chromosome from the original father (white) is 

 substituted for one from the original mother (black), the 

 animal has bar-eyes. Wherever an X-chromosome from the 

 mother (black) is substituted for one from the father 

 (white), the individual has normal eyes instead of bar-eyes. 



It is certain therefore that bar-eye is an abnormality 

 resulting from a defect in certain X-chromosomes. It is 

 clear that X-chromosomes can become defective, and that 

 this change in the X-chromosome causes the individual to 

 develop in an abnormal way, so that it possesses a bodily 

 abnormality — in this case, an abnormality of the eye. This 

 abnormality appears, as above seen, in any individual that 

 has the defective X-chromosome, whether it also has a 

 normal X-chromosome or not. 



Dominant and Recessive Characters Resulting from Sub- 

 stitution of Chromosomes. — The daughters produced in 

 the experiment of figure 9 contain X-chromosomes from 

 two different sources — one from the normal-eyed mother, 

 one from the bar-eyed father. The one from the mother 

 gives when by itself a normal eye, the one from the father 

 a bar-eye. As the daughters have bar-eyes, it is evident that 

 the effect of the chromosome from the father prevails over 

 the effect of that from the mother. It is the custom to call 

 the character that thus prevails and is manifested, when 

 two chromosomes of different tendency are present, a 

 dominant character. Thus bar-eye is dominant, as compared 

 with normal eye. 



