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A Maize Genetic Question

Wayne Parrott wparrott at uga.edu
Tue May 2 17:28:32 EST 2000

Homozygosity has been reported to both increase and decrease recombination.
Rees and Thompson (1956) reported rye homozygotes had less chiasmata than F1
hybrids.  In contrast, Robbins et al (1995) reported that petunia homozygotes
had up to 12 x more recombination than the heterozygote.

Other factors besides zygosity may be involved.  For example, the Rm1 gene of
Petunia (Roberts et al, 1991) can affect recombination rates to a huge extent.
Case in point, in an rm1/rm1 background, the recombination rate between An2 and
Rt is 0.5%; the recombination rate between these two genes increases to 26% in
an Rm1/Rm1 background.  The S-5 gene in rice has been reported to have similar
effects, though of lesser magnitude (Yanahira et al, 1992).

Rees, H. and Thompson, J. B. (1956). Genotypic control of chromosome behaviour
in rye.  III.  Chiasma frequency in homozygotes and heterozygotes. Heredity

Robbins, T. P., Gerats, A. G. M., Fiske, H., and Jorgensen, R. A. (1995).
Suppression of recombination in wide hybrids of Petunia hybrida as revealed by
genetic mapping of marker transgenes. Theor Appl Genet 90:957-968.

Robert, N., Farcy, E., and Cornu, A. (1991). Genetic control of meiotic
recombination in Petunia hybrida: dosage effect of gene Rm1 on segments Hf1-Lg1
and An2-Rt; role of modifiers. Genome 34:515-523.

Yanagira, S., Ikehashi, H., Araki, H., and Maruyama, K. (1992). Difference in
recombination frequency between chromogen (C) and Waxy (wx) genes in japonica
and indica rice (Oryza sativa L.) by using japonica lines with a
wide-compatibility gene (S-5n). Japan J Breed 42:15-21.

Lisa Harper wrote:

>  Here is a genetic mystery (to me at least),
>          I now know of two cases where individual researchers have
>  identified an SSR within 1-2 cM of their gene of interest.  These closely
>  linked SSRs were identified in progeny of a self following an outcross to
>  B73.  Upon repetitive backcrossing to B73, the SSR gets genetically farther
>  and farther away; in one case going from 1 cM to ~25cM in one generation-
>  statistically significant with a chi-squared test.  What is happening here?
>  Have people who use marker-assisted breeding seen this?  At first, I
>  thought it was a mistake, but with two independent cases, and all numbers
>  checked for statistical significance, I think it is real.  Do rates of
>  recombination change with increased homozygosity (a hard thing to measure)?
>          Thank you for your help!
>  Lisa Harper
>  Cande Lab
>  Department of Molecular and Cell Biology
>  345 LSA
>  University of California
>  Berkeley, CA 94720-3200
>  (510)-643-8277
>  ---

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