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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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