1 62 MENDELISM CHAP. 



the unaffected, show a considerable preponderance. 

 The unfortunate nature of the defect, however, forces 

 us to rely for our interpretation almost entirely upon 

 the families produced by the unaffected females who 

 can transmit it. Our knowledge of the offspring 

 of " bleeding " males is as yet far too scanty, and 

 until it is improved, or until we can find some 

 parallel case in animals or plants, the precise 

 scheme of inheritance for haemophilia must remain 

 undecided. 



Though by far the greater part of the human 

 evidence relates to abnormal or diseased conditions, 

 a start has been made in obtaining pedigrees of 

 normal characters. From the ease with which it 

 can be observed, it was natural that eye -colour 

 should be early selected as a subject of investigation, 

 and the work of Hurst and others has clearly demon- 

 strated the existence of one Mendelian factor in 

 operation here. Eyes are of many colours, and the 

 colour depends upon the pigment in the iris. Some 

 eyes have pigment on both sides of the iris on the 

 side that faces the retina as well as on the side that 

 looks out upon the world. Other eyes have pigment 

 on the retinal side only. To this class belong the 

 blues and clear greys ; while the eyes with pigment 

 in front of the iris also are brown, hazel, or green in 

 various shades according to the amount of pigment 

 present. In albino animals the pigment is entirely 

 absent, and as the little blood-vessels are not ob- 

 scured the iris takes on its characteristic pinkish-red 

 appearance. The condition in which pigment is 

 present in front of the iris is dominant to that in 

 which it is absent. Greens, browns, or hazels mated 



