THE HEREDITARY FACTORS AND DIFFERENTIATION 411 



different conditions; there is a lower threshold of temperature 

 below which no melanin is produced, and an upper threshold above 

 which no further increase in the rate can be produced by this gene 

 (although an allelomorphic gene causes a far more rapid rate, and, 

 as a matter of fact, has quite a different relation to temperature).^ 

 Further, the precise shade of adult eye-colour produced may also 

 depend upon a relation between the factors controlling melanin- 

 deposition and those controlling rate of eye-growth. When me- 

 lanin-formation is only moderate, the greater the area of the facets, 

 the more dilute the colour will be (fig. 197). 



The way in which genetic factors can exert their characteristic 

 effect only in a particular cytoplasmic environment is also well 

 shown in Gammanis. The so-called "albino" and "colourless" 

 mutants have no melanin in their eyes. This is due to the fact that 

 this pigment can only be deposited in the retinular portion of the 

 eye, and in these types this portion of the eye is absent. The mu- 

 tation has not altered the genes which produce pigment, as in true 

 albinos, but has prevented the appearance of the only regions in 

 which pigment-producing genes can exert their effects.'^ 



§6 



Finally, it is important to note that the cytoplasm of the egg may 

 be modified by specific factors in the maternal hereditary con- 

 stitution. One of the best examples of this is afforded by the 

 asymmetry of the Gastropod Liimia^a peregra? As mentioned in 

 Chap. IV (p. 71) the spiral coil of the body and shell in this species 

 is normally right-handed (dextral), but a left-handed (sinistral) 

 type also exists, and it has been shown that the difference between 

 them is controlled by a pair of allelomorphic genes: a dextral- 

 determiner and a sinistral-determiner. 



A necessary result of the effect being due to genes present in the 

 mother is the fact that the effects of these genes are delayed by a 

 generation, so that the familiar 3 : i ratio is obtained, not in F^ by 

 individuals, but in F3 by families. If a snail has had one dextral 

 parent, it is found (neglecting certain special complications) that, 

 after self- fertilisation, all its own offspring are dextral, but of these 



^ Ford, 1929. ^ Ford, 1929; Huxley and Wolsky, 1932. 



^ Boycott, Diver, Garstang, and Turner, 1930. 



