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  Section: Genetics » Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
 
 
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A testcross in maize

 
     
 
Content
Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

An example like the one for sweet pea outlined in the previous section is used in order to work out recombination frequencies. C.B. Hutchinson in 1922 made use of two characters known to be located on chromosome 9 in maize. These two characters were coloured aleurone (C) vs. colourless aleurone (c) and full endosperm (Sh) vs. shrunken endosperm (sh) in seed of maize crop. Seed characters have a special utility in inheritance studies, since in the next generation no crop needs to be raised to study the segregation; the characters are expressed at seed stage itself.

Hutchinson performed the experiment in two sets, once having two seed characters in coupling phase and the other having these characters in repulsion phase. The actual results obtained are presented in Table 10.1. The recombination frequencies can be worked out as per cent total population and would be 301/8,368 = 3.6% in coupling phase and would be 1310/44,595 = 2.94% in repulsion phase. It is obvious that strong linkage force exists between aleurone colour character and endosperm shape character.

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