Genie Control of Development 379 



tion they may be directly compared with some already studied in 

 which different alleles or compounds (e.g., in ci or vg) overlapped 

 with normal. If, for example, vg"yvg"' was 100 per cent normal, but 

 vg^yvg"" had 0.2 per cent nicked individuals, this was the same 

 phenomenon as penetrance in a homozygous mutant, though we are 

 entitled just as well to say that here notched is a little dominant. The 

 overlap of the two concepts is most conspicuous when we are dealing 

 with a dominant which is a homozygous lethal and exists, therefore, 

 only as a heterozygote. Let us take Beaded in Drosophila as an 

 example. The heterozygote Bd/+ has 3-5 per cent beaded-wing indi- 

 viduals. We can call this an incomplete dominance, but since no 

 homozygote exists we may also say that Bd is 3-5 per cent penetrant. 

 We may of course arbitrarily set the rule that we speak of dominance 

 when a heterozygote is involved and of penetrance only in the homo- 

 zygote. A compound is a heterozygote and therefore vg°Vvg°° shows 

 a little dominance. But it could also be justified to treat a compound 

 as partly a homozygote and therefore use the term "penetrance" for 

 the partial effect. All these facts tend to show that the two phenomena 

 have the same basis, namely, dosage (or potency) and threshold 

 effects, and therefore supply the same kind of information on genie 

 action and, incidentally, must be studied from the point of view of 

 physiological genetics as opposed to formal genetics. 



Many cases of incomplete penetrance are known and many have 

 been studied quantitatively since Timofeeff's basic work. (Much of 

 this work is reviewed in my book of 1938. ) Recently we have studied 

 the phenomenon extensively ( Goldschmidt, Hannah, and Piternick, 

 1951; Goldschmidt, 1952^ ) . The details will not be analyzed here, be- 

 cause they would repeat the earlier discussions on dosage, potency, and 

 dominance. I shall enumerate simply the main features, taking the 

 examples from our recent work. 



1. Mutant effects of lower penetrance may be based upon single 

 mutant loci which always produce the same incompletely penetrant 

 effect in the same external and genetic environment. However, an 

 incompletely penetrant effect is often based upon a system of multiple 

 factors. It is this type we studied in the so-called podoptera mutants 

 of Drosophila, which make a wing bud change into a leglike or haltere- 

 like organ or, partly, into thorax tissue. There are strains with very 

 low penetrance of a small percentage but, nevertheless, controlled by 

 a number of mutant loci in different chromosomes. We discussed 

 some of these facts (l 2 C d dd) when I presented my opinion that 

 these are heterochromatic mutants (a point which is not relevant to 



