University of Missouri

Diagnosing map order of genes with RFLP markers
--Ed Coe

Following are two simple reference tables we use to simplify diagnosis of order for dominant/recessive loci with codominants in F2 progenies. The first table considers a gene locus between two codominants, showing the scoring classes (A, homozygous for the A allele; B, homozygous for the B allele; D, dominant; H, heterozygous; R, recessive), possible events (p, parental; d, double; 1, single in region 1; 2, single in region 2) , the minimum number of crossovers that will yield that class, and alternative order diagnoses suggested by occurrence of classes that are expected to be rare. The F1 from which these F2 products were derived is ADA/BRB.
Events Min. co Order Diagnosis
ADA p,d 0
ADH 1,2,2/d 1
ADB 2/2,1/2 2
ARA d/d 4 >AAR or RAA
ARH 1/d 3 >AHR
ARB 1/1 2
HDA 1,2,1/d 1
HDH p,1/2,2/2,1 /1,d,d/d 0
HDB 2,2/d,1/d 1
HRA 2/d 3 >RHA
HRH 1/2,d 2
HRB 1 1
BDA 1/1,1/2 2
BDH 1,1/d,2/d 1
BDB d,d/d 2 >BBD or DBB
BRA 2/2 2
BRH 2 1
BRB p 0

In the accompanying note on interval mapping, arrays of data are displayed with adjacent loci comparable to the above triples vertically in columns rather than horizontally.

The second table is for the alternative order in which the gene locus is adjacent to two codominants, for an F1 of AAD/BBR.
Class Events Min. co Order Diagnosis
AAD p,2 0
AHD 1,d,2/d 1
ABD 1/d,d/d 3 >ADH
AAR 2/2 2
AHR 2/1 2
ABR 1/1 2
HAD 1,2/1,d 1
HHD p,2,2/2,1 /1,1/d,d/d 0
HBD 1/2,d,2/d 2
HAR 2/d 3 >AHR
HHR 2,1/d 1
HBR 1 1
BAD 1/1,1/d 2
BHD 1,1/2,2/d 1
BBD 2,2/2 1
BAR d/d 4 >BRA
BHR d 2 >BRH
BBR p 0

The logical extension of these considerations is to peruse the scores down a longer series of loci, anticipating that, for fairly short intervals, crossovers will result in extended chains of non-crossover constitution (e.g., AAAA, BBBB, BBRB, BBBR, HHHH, etc.) or presumed non-crossovers (e.g., AADA, AAAD, HHDH, HHHD, etc.), frequent one-crossover (e.g., AAAH, BBBH, BRBH, HHHR, etc.), some two-crossover (e.g., AAHR, AAAR, etc.), few double-crossover (e.g., BBHR, BBDB, AAHA, etc.), and rare double-double crossover (e.g., BBAR, etc.) events. These evaluations simply extend to codominance the long-familiar (i.e., Sturtevant) approaches to diagnosis of map order in testcrosses. 

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors

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