L26 



CHAPTER 9 



crossing over, half oi the spores were of 

 the parental types and half were crossovers. 

 So. equating the chiasma with the crossing 

 over, a chiasma Frequency of 109? resulted 

 in 5 r 't of all spores having crossovers. We 

 can express the distance between the loci 

 of a and /' as being five crossover units long. 

 a crossover unit representing that distance 

 between linked nonalleles which results in 

 one crossover per hundred postmciotic prod- 

 ucts (spores, in the present case). Gener- 

 ally, when the genes are sufficiently close 

 together (as in the present example), cross- 

 over frequency (crossover distance) is just 

 one half the chiasma frequency, supporting 

 our expectation of one chiasma for each 

 preceding crossing over. 



Crossover frequency can be measured in 

 several ways in Neurospora: 



1. Spores are tested from each sac (two 

 to five per sac are sufficient) to determine 

 whether or not the sac carries a crossover 

 in the region under investigation. In the 

 a-b example above, 10% of the sacs would 

 have crossovers, 90% would not. Since 

 each sac in the 10% group contains four 

 spores that are crossovers and four that are 

 not, crossover frequency would be 5%. 



TETRAD 

 WITH ONE 

 CHIASMA 





S 



MEIOTIC 

 PRODUCTS 



Noncrossover 

 Crossover 

 Crossover 

 Noncrossover 



figurl 9—11. Correlation between genetical 

 and cytological crossovers. 



2. All the spores from many sacs are 

 mixed, then a random sample of spores is 

 taken and tested. This method would also 

 give 595 recombination with a-b and is sim- 

 ilar to the sampling procedure involved in 

 determining crossover frequency in animal 

 sperm. 



3. One randomly chosen spore from 

 each sac is tested; the others are discarded. 

 Again. 5% crossovers are obtained. This 

 procedure resembles the situation in many 

 females (including Drosophila and human 

 beings) in which one random product of 

 meiosis normally enters the egg and the 

 others are lost. 



In the discussion above, no direct correla- 

 tion was made between a genetically de- 

 tected crossover and a cytologically detect- 

 able event involving a particular chromo- 

 some region. Such a connection cannot be 

 made if both members of a pair of homol- 

 ogous chromosomes are identical in cyto- 

 logical appearance (as is assumed in Figure 

 9-6) because a crossover strand, having ex- 

 changed a cytologically identical segment 

 with its homolog, appears the same as a non- 

 crossover strand. A dihybrid for linked 

 genes can be constructed, however, in which 

 one homolog differs cytologically from its 

 partner on both sides of the loci being tested. 

 Such a genetic dihybrid is also cytologically 

 dihybrid as specified in Figure 9-11. In 

 this case it is possible to collect noncross- 

 over progeny and show cytologically that 

 they invariably retain the original chromo- 

 somal arrangement; crossovers on the other 

 hand always show cytologically a new chro- 

 mosome arrangement explained by a mutual 

 exchange of specific chromosome regions 

 between the homologs. :{ 



:! Using this method, genetic crossovers were cor- 

 related exactly with cytological crossovers by C. 

 Stern (1931) using Drosophila and by H. S. 

 Creighton and H. McClintock (1931) using maize. 



