154 Linkage and Crossing Over 



gamete can be produced. There is no crossing over in the 

 Drosophila male. (In the inert regions of the X and Y chromo- 

 somes, near the centromere, two chiasmata have been said to 

 form.) In spite of this lack of chiasma formation in the male, 

 pairing at metaphase appears more or less normal, and the 

 chromosomes separate in a regular manner to the opposite 

 poles. This separation, which is not the result of normal meiotic 

 processes, is discussed further in Chapter 13. Such regular fail- 

 ure of chiasma formation in one sex is of very limited occurrence, 

 but is usual in dipteran males and in females of the silkworm. 

 In all such animals, this failure occurs in the heterogametic sex. 



Crossing Over in Plants. Linkage and crossing over are of 

 universal occurrence, but in most organisms crossing over is 

 approximately the same in both sexes. In the third chromo- 

 some of maize, gene a (anthocyanin-1) produces green plants, 

 colorless aleurone and brown pericarp, and its dominant allele, 

 A, produces anthocyanin pigments in these regions. Gene Rg 

 (ragged leaf), on the same chromosome, produces a plant with 

 torn and split leaves; the homozygous RgRg plant is very weak, 

 but the heterozygote is perfectly healthy. 



A plant which was heterozygous for both pairs of genes and 

 whose genetic constitution was Rg A / rg a was testcrossed to a 

 rg a / rg a plant by Brink and Senn. The offspring were: 



Noncrossovers Crossovers 



Rg A rg a Rg a rg A 



Observed frequencies 160 142 103 115 



Expected if 1 : 1 : 1 : 1 130 130 130 130 



Obviously, the offspring did not fall into a typical testcross 

 ratio as there were too many parental and too few nonparental 

 types. The percentage of crossover types was 41.9. As crossing 

 over in maize is the same in both sexes, it should not matter 

 whether the Fi plant was used as the male or as the female. 



Coupling and Repulsion. In discussing independent assort- 

 ment, it was pointed out that the Fo and testcross ratios are 

 the same whether the original cross is AA BB X cia bb or AA bb 

 X aa BB. Is this also true if the genes are on the same chro- 

 mosome? Let us examine a cross involving the anthocyanin-1 

 and ragged leaf genes of maize made by Brink (reported by 

 Emerson, Beadle, and Fraser) in which one parent was rg A / rg A 



