88 Monohybrids 



females are smaller. If Dorset sheep are crossed with a horn- 

 less breed such as the Suffolk, the Fi females are hornless, but 

 the males have horns, although these horns are considerably 

 smaller than those in the pure Dorset Horn breed. Dominance 

 of hornlessness is complete in the females and incomplete in the 

 males. 



Reversal of Dominance 



If dominance is complete, the dominant character occasionally 

 may develop more slowly in the heterozygote, in which there is 

 only one dominant gene, than in the homozygote, in which two 

 dominant genes are present. Shepherd's-purse, Capsella bursa- 

 pastoris, is a good example of this. A gene, A, produces sharp 

 leaf lobes, whereas its allele, a, produces rounded lobes. If a 

 plant homozygous for A is crossed with a recessive, the Fi 

 plants resemble the recessives when young, but their leaf lobes 

 gradually elongate and become pointed as they mature. In the 

 mature condition the heterozygous Fi plants are unmistakably 

 like the homozygous dominants. In the F2 when young plants 

 are examined, the ratio appears to be 1 pointed lobe : 3 round 

 lobes, but in the adult condition the ratio changes to 3 pointed : 1 

 round, as the dominant gene in the heterozygote comes into 

 expression. The gene which produces the elongation of the lobes 

 acts more slowly when present in only half quantity, but the 

 mature heterozygotes are indistinguishable from the homozygous 

 dominants. 



Reciprocal Crosses 



It is conventional in writing crosses (except in human genetics) 

 to write the female first and the male second. The cross, wild 

 type X curved, means that a normal-winged female is mated 

 with a curved-winged male, and curved X wild type means that 

 a female with curved wings is mated with a normal-winged or 

 wild-type male. The cross curved X wild type is known tech- 

 nically as the reciprocal of the cross wild type X curved. For 

 genes on autosomes the results are generally the same no matter 

 in which direction the cross is made. The Fo of the cross 

 curved X homozygous wild type segregates into 3 wild type and 

 1 curved just as did its reciprocal. In testcrosses, also, the 

 results are the same no matter in which direction the cross is 



