122 GENETICS IN RELATION TO AGRICULTURE 
TaBLE XX V.—C.assiFICATION OF Factor COMBINATIONS TRANSMITTED BY FEMALES 
or DROSOPHILA HAVING THE GENETIC CONSTITUTION 
(ywA'biCwmsrfb'X) (Yw'a'B;oVMSRFB'X) 
No crossing-over 186 | 200 | 386 
| 
Crossing-over between the loci ease aie | peg ae | Totals 
Wellowszandewhibe senshi uiecttasered soe 2 5 | 7 
White and abnormal................0.0005 3 5 8 
Abnormialiandubitidis.sejecces seer ne cre eee 4 11 15 
Bitidsandachibiee cee ce sece os ee heer eae 17 27 44 
Clubsandvermilion’ je.cssecies cee ene 46 51 97 
Vermilion and miniature.................. 7 9 16 
Miniaturesandusablezc a. npc ene 18 19 37 
Sable and rudimentary.................4. 4 28 | 38 66 
Rudimentary and forked................... = | 5 5 
Horkedtandibarsa sensation ee eee fg | 1 1 
Total single cross-overs................ | 296 
Double crossing-over Totals 
Vean cls War Cit ande Vaan nest sioner ens 1 vA 1 
VeondeWex MeandsSieere eer cme en or oF 1 1 
Veranda wWasstan Galt sec seis ene nose ern ae 1 1 2 
Vande es eanduiee.c crim: chicas teen nanan? 1 1 
Waand Ae C pancd! Vicsnacsn ene 1 1 
Weand cates Pian Gi lincccae actrees ween eee 1 * 1 
VASE wh ava ey Spek Chita ol ul An sedate capt uethuswterw oe eee oeec tee ay 1 1 
Avands Bs cuS anal ea names rece none 1 o6 1 
Brandis sande Sivas caer amie nee 1 1 2 
Bean duGyexiSs an de lees et tncacn hath reer tee 3 7 
CrandeVeer Veand eMirncein cee rncctienemie 1 1 
GrxendaVas Scand Glteat reer eer eee apr ef 1 8 
Grande sceh an Guinness neactiwas nena oe eee | 2 | 2 
CrandaVerthvand Abie ate eens Sere 1 | 1 
Total double cross-overs............... | 30 
Interference.—Interference is merely a consequence of the sectional 
mode of chromatin interchange between homologous chromosomes. The 
term is used to designate the observed fact that when crossing-over takes 
place at a particular point in the chromosome the regions for some dis- 
tance on both sides are protected from coincident crossing-over. The 
operation of interference is well illustrated in Muller’s data, although 
the numbers are not sufficient to warrant a quantitative determination of 
its effect. With long distances interference decreases, which is in accord- 
ance with expectation. Even for relatively long distances, however, as 
for the loci W, M and B’ which we have already considered in detail 
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