524 GENETICS IN RELATION TO AGRICULTURE 



The Inheritance of Defects. By defects we refer specifically to 

 abnormal structures or processes which are the almost invariable ex- 

 pressions of particular genetic constitutions. Of these a vast number are 

 known, particularly in man where in recent years a considerable amount 

 of attention has been devoted to them. A very familiar example is 

 haemophilia, a bodily condition such that the blood does not possess the 

 ability to clot when blood-vessels are ruptured. The defect in man is 

 determined by a sex-linked recessive factor and exhibits the same type of 



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FIG. 203. Pedigree of a family of "bleeders" the K. family, located in and about Car- 

 roll Co., Maryland. The son, II, '2, was a bleeder but died without issue. The eldest son, 

 III, 1, of the daughter, II, 3, was a bleeder from 18 up to 45 years, "often bled till he 

 fainted." He had 2 unaffected brothers and 3 normal sisters but 1 sister, III, 10, was a 

 " bleeder until 40." He had a son, IV, 1, who was a very bad bleeder from 18 until toward 

 middle life and a daughter, IV, 2, who often "bled until she fainted" and eventually died of 

 dysentery. All 19 children of the 2 normal brothers were normal and 9 children of the 

 normal sister, III, 7. The affected sister, III, 10, had 3 sons and 2 daughters who were 

 affected. IV, 5, is stated to be "a bleeder" and had by an unaffected husband 2 bleeding 

 sons and 1 bleeding daughter besides 4 others who died of scarlatina. Her brother, IV, 8, 

 had a daughter, V, 5, who was a bleeder until 15, and then died of a hemorrhage of the lungs 

 consequent upon tuberculosis. There were other children all of whom died young of 

 scarlatina. The normal brother, IV, 10, had 12 normal children. The next 2 had no 

 offspring. The youngest son, IV, 14, began to bleed while an infant, grew worse until he 

 was 25 and has since improved. He married a cousin who is also a bleeder and they have 

 6 children. Three of the daughters have not bled as yet. V, 9, has been a bleeder since he 

 was 8 months old and bleeds until he faints; V, 10, has been a bleeder since she was 8 

 months old and V, 11, bleeds occasionally but not very severely. (After C. B. Davenport.) 



inheritance as sex-linked factors do in the fruit fly. A family history 

 showing the inheritance of haemophilia is outlined in Fig. 203. There 

 is an analogous defect in the horse. In man a number of other defects 

 have been traced some of which display a Mendelian type of behavior, 

 and a surprisingly large number of them are sex-linked. 



Thus among dominant defects in man Guyer lists achondroplasy, 

 abnormally short limbs along with normal head and body; keratosis, 

 thickening of the epidermis; epidermolysis, excessive formation of 

 blisters; hypotrichosis, a hairless and toothless condition; diabetes 



