125 



detected crossover with some cytologically 

 detectable event involving a particular chro- 

 mosome region. Such a connection cannot be 

 made if both members of a pair of homologous 

 chromosomes are identical in cytological ap- 

 pearance. This is true because a crossover 

 strand, having exchanged a cytologically 

 identical segment with its homolog, would 

 appear the same as a noncrossover strand 

 (cf. p. 120). But it is possible to construct a 

 dihybrid for linked genes in which one homo- 

 log differs cytologically from its partner on 

 both sides of the loci being tested. Such a 

 genetic dihybrid is, simultaneously, cytologi- 



cally dihybrid in the way specified (Figure 

 16-9). In this case, it is possible to collect 

 noncrossover progeny and show cytologically 

 that they invariably retain the original chro- 

 mosomal arrangement, while crossovers al- 

 ways show cytologically a new chromosome 

 arrangement that is explained by a mutual 

 exchange between the homologs of specific 

 chromosome regions.^ 



^ In such a way, genetic crossovers were correlated 

 exactly with cytological crossovers by Stern (1931) 

 using Diosophila, and by Creighton and McClintock 

 (1931) using maize. 



TETRAD 

 WITH ONE 

 CHIASMA 



=^31 



X 



FIGURE 16-9. Correlation between 

 genet iced and cytological crossovers. 



tiZ 



MEIOTIC 

 PRODUCTS 



Noncrossover 

 Crossover 



Crossover 

 Noncrossover 



SUMMARY AND CONCLUSIONS 



A crossover chromosome carried by a gamete is derived from a tetrad in which a chiasma, 

 involving only two of the four strands, has occurred between the linked genes showing 

 recombination. For closely linked genes, crossover frequency is one-half the frequency with 

 which a chiasma occurs between their loci. 



It is hypothesized that crossover frequency is directly related to distances between genes 

 on a chromosome. One unit of crossover distance between genes is defined as one crossover 

 per one hundred postmeiotic cells (spores or gametes). Since different genes linked to the 

 same gene show different percentages of crossing over with this gene, they are presumably 

 different distances from it. 



