HETEROZYGOTE CEIAEACTEES. 



57 



Cases like those cited seem to be due to the presence of two charac- 

 ters m different A'arieties wliich are not manifest in tliose varieties: 

 but when brought together by hybridization they react on each other 

 in some miknown manner so as to give rise to a new character. At 

 the same time they form a Mendehan pair and separate again on the 

 formation of gametes. Perhaps in such cases two clu-omosomes, 

 wliich meet to form a bivalent in the reduction division, each tlu-ow 

 off mto the cell a different chemical substance, and these two sub- 

 stances, by reacting on each other j give rise to the new character. 

 Wlien these two clu"omosomes are not together in the same cell the 

 character does not appear. 



Most of such characters are probably reversions to lost characters. 

 The fact that there are mottled races of beans like those produced by 

 Shull and others, and Avliich reproduce true to seed, is in favor of this 

 suggestion. In these mottled beans which reproduce true to type 

 we may suppose that each of the chromosomes in question produces 

 both of the chemical substances wliich we have supposed give rise to 

 the character. If this is the case, then nonmottled varieties of three 

 types could arise from a mottled variety by the loss on the part of 

 the clu'omosonies producing the two necessary substances of the 

 power of producmg one or the other or both of these substances. If 

 in one variety of beans one of these substances and in another variety 

 the other substance is missing, crossing the two varieties woidd cause 

 the lost character to reappear. Tliis matter will be considered more 

 fuUy when vre are considering the subject of latency of hereditary 

 characters. 



Thus far we have considered only those new characters arising in 

 crosses and which appear only m heterozygote form. There is a sec- 

 ond class of characters arising in crosses that may be fixed by proper 

 procedure. In worldng with gillyflowers Bateson and his coworkers 

 foimd an interesting case of tliis Idnd, wliich ^\dll be considered more 

 fully under the head of latency and need only be referred to briefly 

 here. In crossing a cert am wliite variety vritli a cream-colored 

 variety the progeny produced red flowers, and in the second genera- 

 tion some individuals were obtained having red flowers and reproduc- 

 ing true to seed. Evidently tliis result was due to the fact that two 

 characters neither of which produced any effect when alone but when 

 brought together gave rise to a visible character did not form a 

 Mendelian pau and could consequently both be transmitted together. 



The reason why a character of this type can be fixed is seen in the 

 following. We have assumed that the new character arises by the 

 bringing together of two other characters that are not alleloniorphic 

 to each other ; that is, do not form a pan' which must separate on the 

 formation of gametes. If we call one of these characters A, its 



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