122 THE BIOLOGY OF INSECTS 



dark banding of sordiata together with the streaky scaling 

 of prunaria ; these offspring are hybrids, and their hybrid 

 character is recognisable in their appearance although on 

 the whole they resemble sordiata rather than prunaria. If 

 now such hybrids breed among themselves in numbers, a 

 count of the families of the next generation will show 

 approximately a quarter of the population typical, speckled, 

 unhanded prtmaria, a quarter unspeckled, markedly banded 

 sordiata, while half will be speckled and banded like their 

 parents of the first hybrid family. There is no doubt that 

 these colour patterns depend on inherited characters, and we 

 have seen that there is much reason for believing that the 

 chromosomes in the germ-cell nuclei are concerned with 

 the inheritance of such characters ; as it is now usually 

 expressed, the germ-cells carry the '* factors " or " deter- 

 minants " for them. Recalling the behaviour of the germ- 

 cells in maturation, we remember that at the reducing 

 division the chromosomes are first paired together (one of 

 each pair being derived from either parent), and then 

 separated so that they pass into different mature nuclei. 

 Now it is conceivable that the two chromosome partners in 

 the pairing (synapsis) may be alike, or they may differ in 

 certain factors which they contain ; just so will the mature 

 germ-cells resulting from the reducing division be alike or 

 different as regards those factors. The results of such 

 breeding experiments as those with the Orange Moth agree 

 exactly with the indications afforded by the maturation 

 processes. Call the germinal factor that induces the 

 banding of the wings (in sordiata) S and the factor for the 

 unhanded wings (typical prunaria) s. In a strain of pure 

 bred sordiata all the germ-cells carry the factor S, and in 

 pure bred prunaria all carry s ; in each case all the germ- 

 cells are alike as regards one of these alternative characters 

 and the individuals of such strains are '' gametically pure " 

 or homozygous. 



When, however, an tgg of sordiata carrying the factor 

 S, is fertilised by a sperm of prunaria carrying s, the zygote 

 nucleus must contain both factors, and it is from such a 



