364 EVOLUTION AND GENETICS 



is frequently the added member. Brachydactyly is extreme 

 shortening of the fingers, which sometimes lack one of the three 

 phalanges and sometimes have an extremely short terminal pha- 

 lanx. Symphalangism is fusion of the phalanges of toes or fingers 

 so that one of the usual joints is stiff. 



Hefner has traced the last condition through six generations 

 of a family in which it behaved as a Mendelian dominant and 

 appeared in both males and females (Fig. 200). In his report on 

 this case he cites another remarkable record: "John Talbot, first 

 Earl of Shrewsbury, was supposed to have had fingers with stiff 

 joints. He was killed in battle near Bordeaux in 1453, by a blow 

 on the head, received after his thigh had been broken. He was 

 buried in Shrewsbury Cathedral. Recent alterations made it 

 necessary to disturb his grave, when tradition was confirmed and 

 his bones identified by the fused finger-joints, the cleft skull, and 

 the broken thigh-bone. By a strange coincidence this work was 

 under the direction of one of Talbot's direct descendants in the 

 fourteenth generation, the joints of whose fingers were fused like 

 those of his remote ancestor. ..." 



Sex- Linkage. Color blindness, a sex-linked character, is 

 inherited in man in the same way that other sex-linked characters 

 are inherited in Drosophila. It is recessive to normal vision. A 

 color-blind man and a woman with normal vision cannot produce 

 color-blind children but one-half of their daughters are carriers 

 and can produce color-blind sons even if mated with normal men. 

 Their daughters would be one-half normal and one-half carriers. 

 Color-blind females can be produced only when both parents 

 supply factors for this condition, since one x chromosome comes 

 from each parent. The following diagrams show how the char- 

 acter is transmitted in the four possible crosses (Fig. 201). 



These diagrams make evident a number of interesting phe- 

 nomena. A shows that a color-blind parent may have children 

 with normal vision. Even a color-blind female, as shown in B, 

 may have some children with normal vision, and if she had only 

 daughters all would be apparently normal although able to 

 transmit the defect. Diagram D shows how parents with normal 

 vision may produce color-blind sons. A combination of such cases 

 as these shows how a sex-linked defect may be transmitted genera- 

 tion after generation through a female line, to crop out in an 

 occasional male. Because of the small size of human families 



