124 THE BIOLOGY OF INSECTS 



offspring are of the latter variety ; they are indistinguishable 

 from their dark-winged parent and display no evidence of 

 their hybrid nature. The dark wing-colour is '' dominant " 

 to the pale ; but when hybrids breed among themselves, 

 the resulting families, if sufficiently numerous in individuals, 

 have three-quarters of their members sooty and a quarter 

 pale. The pale character, hidden in the first generation 

 and reappearing in the second, is known as '* recessive." 

 The dark-winged moths of the second generation look all 

 alike, as all resemble their dominant parent, but if the 

 nature of their germ-cells be tested by a study of their 

 offspring, it will be found that a third of them — that is, a 

 quarter of the whole second generation — are pure (homo- 

 zygous) as to the dominant character, and their inbred 

 descendants will continue to be pure douhledayaria^ while 

 the other tvvo-thirds — that is, half of the whole second 

 generation — are hybrids (heterozygous) and will therefore 

 give, if bred among themselves, the same proportion of a 

 quarter pure dominants, half hybrid dominants, and a 

 quarter pure recessives as before. The characters of the 

 alternative forms, douhledayaria and hetularia may be 

 indicated respectively by D and d. Then the zygote com- 

 position of a pure strain of the former will be DD, and all 

 the gametes must carry the D factor, while in a pure 

 betularia strain the zygote composition will be dd and the 

 gametes will all carry d. The hybrids of the first generation 

 must all have the zygotic composition Dd, and half of their 

 gametes will carry either D or d, just as in the hybrid 

 sordiata-prunaria the zygotic composition is Ss and any 

 individual gamete must carry either S or s. 



In breeding experiments of this kind it is often the 

 practice to make what is called a " back- cross," that is, to 

 mate a hybrid with one of its parent forms. If hybrid 

 douhledayaria be paired with betularia it is found that in a 

 large enough population half the offspring resemble either 

 parent, as a consideration of their germinal constitution 

 would lead one to expect. For since the hybrid has the 

 composition Dd, and the betularia-psirQnt is dd, it is clear 



