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.
Return to the MNL 67 On-Line Index
Return to the Maize Newsletter Index
Return to the MaizeGDB Homepage