32 AN INTRODUCTION TO MODERN GENETICS 



The convention of indicating dominance by capital letters breaks down here 

 since there is more than one dominance relation to be considered, and anyway 

 the dominance within such series is usually incomplete. Sometimes the most 

 common allelomorph in the series and those dominant to it are given capital 

 letters, those recessive to it small letters. Alternatively, when the species has 

 a well-defmed normal or wild form, the normal or "wild-type" allelomorphs 

 are indicated by a cross, to which, if necessary, the symbol of the locus is added 

 (e.g. -|-w is the wild allelomorph of the white eye locus in Drosophila). The mutant 

 allelomorphs of the locus are written with small letters unless they are dominant 

 over the wild (e.g. we eosin, wco coral are allelomorphs of white, Bd beaded is 

 dominant over H-Bd, the wild allelomorph). The cross of the wild type gene is now 

 often written as an exponent, e.g. v/+. 



Complex heterozygotes may be written AaBBCc, or the genes in one chromo- 

 some may be separated from those in the homologue by a dot (e.g. ABC.aBc) 

 or a line (e.g. +4-+/we -f- f, where the two recessives we and f occur in the 

 same chromosome). 



3. Mender s Second Law: Independent Assortment 



Mendel's original experiments dealt with the inheritance of several 

 characters simultaneously : tall or short plants, round or wrinkled seeds, 

 etc. Each pair of allelomorphs was found to behave quite independently 

 of every other pair. In a double heterozygote AaBh half the germ cells 

 will of course contain A, and of these, it was pure chance which con- 

 tained B and which &, so that the combinations AB and Ah were found 

 in equal numbers. 



P1. Round Yellow RR YY x Wrinkled Green rr yy 



(germ cells RY) I (germ cells ry) 



F1 Round Yellow RYry 



(germ cells IRYMRy : IrY : 1ry) 



1 



F1. 1 RY 



1 Ry 



1 rY 



1 ry 



Total of the F2. 9 with R and Y, showing Round Yellow. 

 3 with R and y, showing Round Green. 

 3 with r and Y, showing Wrinkled Yellow. 

 1 with r and y, showing Wrinkled Green. 



This is known as the "chequerboard" method of finding the progeny of a cross. 

 An alternative is to multiply the two series of gametes together algebraically, 

 e.g. to get the F2 above we have [RY -]- Ry -\- rY + ry) {RY + Ry + rY + ry) 

 = RRYY + 2RRyy + 2Rrry + URrYy + RRyy + 2RrYy + rrYY + 2rrYy + rryy. 



Fig, 2. Independent Assortment of Factors in a cross between round yellow 

 and green wrinkled peas (Mendel). 



