Section 14 — Animal Genetics and Breeding 



viable orange-red (R) sons as the denominator 

 and the ratio is equalled to 2/57. The recom- 

 bination fraction of the interval r — ( + ) in the 

 induced X r Y B § is assumed to be five times as 

 high as that in the normal X r Y R ^ as that in the 

 interval ( x) — r. The established linkage map of 

 Y R in the normal male is (y) 0.2 R 1.2 (— ) while 

 that in the induced estrogen-induced X r Y B $ is 

 (y) 1 .0 R 6.0 ( — ). A full account will be published 

 in Genetics (U.S.A.) in the near future. 



A strain has been fixed for extra-abdominal 

 legs in some 30 per cent of the flies, but the 

 character does not segregate after outcrossing. 



The coiling direction of the hypopigium and 

 the orientation of the asymmetrical sclerites 

 completing the male terminalia seem to be 

 pleiotropic effects of the same gene and inde- 

 pendent developmental processes. 



Close inbreeding leads quickly to sterility. 



14.15. (D.) Formal Genetics of the Housefly. 



M. G. Rubini- Franco (Pavia, Italy). 



14.16. (D.) The Inheritance of Insecticide-Resistance. 



M. T. Lanna (Pavia, Italy). 



The complement of the housefly (Musca 

 domestica L.) is n = 6; somatic pairing is 

 generally very good in metaphasic plates from 

 supra-aesophageal ganglia; colcemide can easily 

 be supplied with good results both to larvae and 

 to adults by ingestion or to larvae by injection. 



Variability of paleo- and neo-arctic popu- 

 lations shows great similarities in abdominal 

 pigmentation and load of minor wing venation 

 abnormalities, all having incomplete penetrance 

 and variable expressivity. 



Complex mutant phenotypes with lethal 

 effects are common; they are of little value for 

 formal genetics, requiring cumbersome breeding 

 methods; good mutants have allowed identi- 

 fication of the five autosomal linkage groups. 

 The complement includes big X Chr. ; however 

 no diaginic mutant has been found so far. The 

 loci of the II linkage group show occasionally 

 holandric inheritance (flies from Australia and 

 Florida); cytological evidence of two distinct 

 Y-chromosomes has been obtained. 



Dominant mutants are rare and doubtful, 

 apart from some genes for insecticide resistance, 

 giving intermediate hybrids. 



Three not allelic recessive genes causing 

 similar yellow eyes are known mutant larvae 

 developing with normal ones and can result in 

 various pinkeyed flies. 



Genetically determined eyelessness or de- 

 formed eyes are fairly common, but can hardly 

 be fixed in true-breeding strains; narrow Bar-like 

 eyes are recurrent in some strains but their 

 inheritance is still obscure. 



Among mutants affecting appendages differ- 

 entiation special mention is deserved by 

 (1) aristapedia which turns the arista into tarsal 

 segments and generally reduces bristles; (2) 

 antennapedia, which is irregularly dominant and 

 of erratic expressivity and (3) tarsi -fusi (fused 

 tarsi). 



The response of houseflies to DDT can be 

 measured either by the time required for the on- 

 set of well-defined symptoms of intoxication 

 (knockdown) or by the probability of survival 

 to dosages directly applied to the fly (topical 

 application). 



The first approach is adopted when samples are 

 confined into containers having the walls coated 

 or impregnated by the toxicant and the knock 

 down times are recorded; the second approach 

 is followed when dosage/ response regression 

 lines are needed; either methods has its short- 

 comings and its points in favour for genetical 

 research. 



Tolerance levels for DDT generally show 

 great variability within populations, and response 

 to selection is prompt and speedy. 



Three different genes are known, all of the 

 II linkage group, which independently cause 

 DDT-resistance ; all provide clear F2-segregations 

 and in the following generations behave ac- 

 cording to expectation of monofactoriality. 



Genetical research based on the study of 

 knockdown times or on that of dosage/mortality 

 correlations have provided parallel results, but 

 the evaluation of dominance differs somehow. 



Dieldrin resistance is usually measured by 

 topical application and has been found widely 

 distributed among laboratory strains of various 

 origin, some of which can positively be assumed 

 as never having had any experience of dieldrin. 



Hybrids from susceptible and resistant flies 

 are intermediate; the F2S give clear indications of 

 simple inheritance, with free recombination 

 with linkage groups II and V on which genes for 

 DDT- and for OP- resistance respectively are 

 located. 



Free recombination of Dieldrin resistance 

 occurs also with IV Chr. markers. 



254 



