I 



,909] CURRENT LITERATURE 83 



morph in a heterozygous condition, the homozygous giving unmottled seeds. 

 This peculiarity results in a new ratio, 18:18:6:6:16, instead of the anticipated 

 27:9:9:3: 16. Latency is held to mean invisibility and not inactivity or dormancy. 

 Bateson's "presence and absence" hypothesis, in which the presence of any 

 character is said to be dominant to its absence, is believed to be of general validity ; 

 andhis'3 more recent terms "epistatic" and " hypostatic," as applied to the capa- 

 rity of one unit to hide or be hidden by another, are accepted. Thus in Mendel's 

 original case, yellow in cotyledons is not to be considered "dominant" over green, 

 but dominant to the absence of yellow and "epistatic" to green, i.e., according 

 to Shcll, causing its " invisibility " but not its "inactivity." This change of view 

 involves some nice distinctions, but appears to obviate some of the difficulties 

 of the older view of dominance, especially in connection with ontogeny. Inciden- 

 tally all that remains of the Mendelism of Mendel is his hypothesis of gametic 



purity. The superstructure erected upon this has grown in complexity with great 

 rapidity. 



With latency thus clearly defined, four types of latency are discussed: (1) 

 "Latency due to separation, in which an allelomorph when acting alone has no 

 external manifestation, and is only rendered patent by combining it with another 

 allelomorph. " This type of latency is not uncommon, and gives rise to such ratios 

 ** Q: 3 : 4, Q'-7> 27:9:28. (2) "Latency due to combination, in which two dominant 

 allelomorphs, each giving rise to a peculiar character when acting alone, lose their 

 external manifestation when coexisting in the same zygote." This gives the 

 ratio first mentioned above in mottled beans, and may account for certain "mid- 

 races." (3) "Latency due to hypostasis, in which the presence of one allelomorph 

 cannot be detected owing to the presence of another allelomorph, the character 

 produced by the latter being unmodified by the activity of the former." For 

 example, a black bean is shown to hide a distinct-brown allelomorph, and a dark 

 °range bean to carry invisibly a light-yellow allelomorph. This condition may 

 !?ve such a ratio as 1 2 : 3 : 1 . ( 4 ) Latency due to fluctuation. Disappearance of 

 " aracters under unfavorable conditions of nutrition, etc.; a very common phe- 

 nomenon which may cause discrepancies from the expected ratio. Some of the 

 ^es formerly called "incomplete or partial dominance" would probably be 



mo t Ratios may also rarely be modified °y the failure of certain allel °" 



or P tc combinations to form a zygote which will develop— R. R- Gates. 



effect 65111 ^ 017 chrom °g ens -— Palladin 1 * has devised a new, very simple, and 

 method meth ° d ° f detectin g the respiratory chromogens in plants. He uses this 

 I n ^ t0 show to* 5 wide distribution of these chromogens in the plant kingdom. 

 s Pecies, ranging from liverworts to dicotyledons, this method showed these 



660. "L ! TES ° N ' WnxiAM > Facts limiting the theory of heredity. Science 26:640- 



Ber Dem^u T' W " Die V erbreitung der Atmungschromogene bei den Pflanzea 

 eut *h. Bot. Gesells. 26a: 378-389. 1908. 



ch 



