HUI^IAN INHERITANCE 273 



3. Mendelian and Sex-Linked 



(Appearing in males when simplex, but in females only when duplex.) 

 Dominant Recessive 



Normal. Gower's muscular atrophy. 



Normal. Haemophilia (bleeding). 



Normal. Color blindness (inability to 



distinguish red from green). 

 Normal. Night blindness (inability to 



see in faint light). 



4. Probably Mendelian but Dominance Uncertain or Imperfect 



Defective hair and teeth or teeth alone, extra teeth, a double set of permanent 

 teeth, hare-lip, cryptorchism and hypospadias (imperfectly developed male organs), 

 tendency to produce twins (in some families determined by the father, in others by 

 the mother), left-handedness, otosclerosis (hardness of hearing owing to thickened 

 tympanum). 



5. Subject to Heredity, but to what Extent or how Inherited Uncertain 



General mental ability, memory, temperament, musical ability, literary ability, 

 artistic ability, mathematical ability, mechanical ability, congenital deafness, lia- 

 bility to abdominal hernia, cretinism (due to defective or diseased thyroids), defec- 

 tive heart, some forms of epilepsy and insanity, longevity. 



of Human Inheritance" (1909). The data collected by the 

 Eugenics Record Office have been published in part in a 

 series of bulletins and monographs which is being rapidly 

 extended. 



We may provisionally distinguish inherited human traits 

 as (1) blending (probably involving multiple factors); (2) 

 clearly Mendelian (involvmg a single genetic factor); (3) 

 Mendelian and sex-linked; (4) probably Mendelian but with 

 dominance imperfect or uncertain, and (5) hereditary, but 

 to what extent or how, uncertain. 



The grounds on which a category of blending characters 

 may be based have already been discussed. If they are valid 

 for animals and plants, they are also valid for man. Here 

 belong characters which show intermediate inheritance in Fi 

 and also in F2, but with greater variability in F2 than in Fi. 

 Size and stature are good examples. The greater variability 

 of F2 shows that the blending was not perfect in Fi and that 

 multiple factors are probably involved. Indications of 

 segregation more or less complete were observed by Daven- 



