No. 519] THE IMPERFECTION OF DOMINANCE 133 



when a character regularly fails t<> dominate it will never 

 be practicable to distinguish the dominant and recessive 

 conditions. But the situation is not so desperate as (hat 

 —one clear difference between dominance and rocessive- 

 ness remains, namely, while the dominant character, even 

 when of duplex origin, sometimes develops fully, some- 

 times imperfectly, sometimes fails altogether; the homo- 

 zygous recessive condition yields, on the other hand, 

 offspring with entire absence of the quality. Since we can 

 not know our homozygous recessives in advance the prac- 

 tical method of determining them is as follows: mate 

 similars; some will yield families that show a great range 

 of variation in the given characteristic from high develop- 

 ment to complete absence, such are dominants or heter- 

 ozygotes; others will yield families that show a limited 

 range of conditions and will all be like the parents— such 

 are homozygous recessives. Their progeny are uniform 

 because absence of a character can not show a variability 

 in ontogeny. 



The foregoing principle may be^ illustrated by an ex- 



viduals) from two syndactyl parents fails to produce some 

 normal children, but several families from two non- 

 syndactyl parents produce in a total of 119 offspring no 

 syndactyl individual. The invariable families are cer- 

 tainly the product of two recessive parents. 



The fact of imperfect dominance bears upon the con- 

 troversy of alternative versus blending inheritance. 

 When a booted fowl is mated to a clean-shanked one the 

 offspring show grades of boot ranging from to 9; — 10 

 being the heaviest grade recognized. For example, when 

 a Cochin is crossed with a White Leghorn the average 

 grade of booting is about 4 on a scale of ten. Booting 

 would seem to be a typical blending character. Yet the 

 evidence of segregation is excellent. For', two extracted 



DR X RR gives 50 per cent, of the offspring of grade 5 

 or over, DR X DR yields 25 per cent, of grade 5 or over, 



