Chapter 24 



POINT MUTANTS — 

 THEIR DETECTION AND 

 EFFECTS IN INDIVIDUALS 



P 



kOiNT mutants comprise the re- 

 mainder of all genetically de- 

 tected mutants for which no 

 association with intergenic changes can be 

 demonstrated. It is not only of historical 

 importance, but also of current and future 

 interest, to understand something about the 

 genetic methods that are used for collect- 

 ing point mutants. We shall consider two 

 elegant procedures ^ using Drosophila melano- 

 gaster, one for the detection of mutants that 

 are recessive lethals, the other for "visible" 

 mutations, at specific loci, which are viable 

 when heterozygous. 



The technique for detecting recessive 

 lethals is called "'Base'' (see Figure 24-1), and 

 is designed to discover such mutants that 

 arise in the germ line of the male, in X chro- 

 mosome loci which are hemizygous, i.e., have 

 no allele in the Y chromosome. The males 

 used are wild-type, having all normal char- 

 acteristics, including round, dull-red com- 

 pound eyes. The females employed have X 

 chromosomes homozygous for Bar eye (B), 

 for apricot eye color (apr), and for a para- 

 centric inversion (In) of almost the entire left 

 arm {In sc'^^ sc^ whose right breakage point 

 is designated sc'^^ and left 5c^) inside of 

 which is a smaller inversion (InS). '"Base"' 

 derives its name from Bar, apricot, scute in- 

 version. Base females (or males) have nar- 



^ These were invented by H. J. Muller. 

 204 



row-Bar eyes of apricot color. The genotype 

 of the Base female is written 



sc'"^^ B InS apr sc^/sc^'^^ B InS apr sc^. 



A wild-type male is mated to a Base female 

 and the Fi daughters are obtained. These 

 daughters are -\-/sc''^^ B InS apr sc^ and ap- 

 pear heterozygous (wide) Bar (being other- 

 wise wild-type). 



Since the very short right arm of the X is 

 entirely heterochromatic, it is of no concern 

 here. Because each Fi female is heterozygous 

 for two paracentric inversions, any crossing 

 over between the left arms of her X's pro- 

 duces dicentric or acentric crossover strands 

 which fail to enter the gametic nucleus (see 

 Chapter 19). Accordingly, Fi females pro- 

 duce eggs having an X that is, for our pur- 

 poses, either completely maternal 



(5C'^^ B InS apr 5C^), 



or completely paternal (+). If this Fi daugh- 

 ter mates with her Base brothers, half of the 

 sons in the next generation (Fo) receive the 

 -f maternal X, and half receive the Base ma- 

 ternal X. So, if the progeny of a single Fi 

 female are examined, it is a simple matter to 

 recognize the presence of both types of sons, 

 since it is usual to obtain more than forty 

 sons per female. Note that each wild-type 

 Fo son carries an identical copy of the X in 

 the sperm that fertilized the egg which de- 

 veloped as his mother (the Fi female). Even 

 when the sperm used to form the Fi female 

 carries an X-linked recessive lethal mutant, 

 the Fi female will survive because she carries 

 the + allele of it in her Base chromosome. 

 However, each + F2 son will carry this mu- 

 tant in hemizygous condition and will usually 

 die before adulthood, so that no + sons ap- 

 pear in Fo! It becomes clear, then, since an 

 Fi female is formed by fertilization with a 

 + X-carrying sperm, that the absence of + 

 sons among her progeny is proof that the 

 particular Pi sperm carried a recessive lethal 

 mutant. 



