l66 AN INTRODUCTION TO MODERN GENETICS 



Standard. An example in Drosophila is abnormal abdomen; the Ab 

 gene has an effect more extensive than a deficiency for the wild-type 

 allelomorph. One might say that the gene was doing the same thing as 



the wild-type but with negative efficiency. Another example is ebony: 



+ + -i- 



e e e e ' 



— is lighter than — plus e while - is darker, i.e. adding ^ to — 



e e e e 



Q 



darkens the fly, while adding ^"^ to - lightens it. Dubinin and Siderov^ 



e 



have described another antimorph in the IVth, cubitus interruptus. 



e. Neomorphs are genes which are doing something quite different to 



anything done by the standard gene; in fact, the standard behaves 



Hw 



towards them like an amorph. Hairy wing is an example, r is less 



Hw 



Hw Hw , Hw 



hairy than — - but — - plus Hw^ is the same as — -. Thus addition 

 Hw Hw Hw 



of Hw^ has no effect. Bar behaves as a neomorph. 



Mangelsdorf and Fraps^ described a gene producing yellow pigment 

 and vitamin A in the triploid endosperm of maize. The recessive allelo- 

 morph apparently produces no vitamin A, but each dose of the neomor- 

 phic dominant produces about two units of vitamin per gram of grain. 



8. Dosage compensation 



Sex-Unked genes are present in only a single dose in the hetero- 

 gametic sex, but in a double dose in the homogenetic sex. We should 

 expect a female with two hypomorphic genes to show a different 

 phenotype from a male with only one. But in Drosophila the males 

 and females of mutant types are usually very alike; the differences in 

 dosage have been compensated. This compensation can only be due to 

 the action of modifiers also lying in the X and thus present in the same 

 dosage as the gene whose effect they are modifying. The important 

 thing for the expression of an X chromosome factor is therefore not its 

 relation to the genotype as a whole but an intra-chromosomal balance 

 between the gene and modifiers within the X chromosome itself.^ 



MuUer^ points out that such a system of dosage compensation could 

 hardly be evolved if it had no function. Its function must be its effect 

 on the wild type rather than on the mutant, since it can be of no par- 

 ticular evolutionary importance if the male and female mutant types, 



^ Dubinin and Siderov 1934. - Mangelsdorf and Fraps 193 1. 



2 Stern 1929. * Muller 19326. 



