II.17. Genetic linkage map of barley chromosome 5. I. A procedure to estimate linkage maps applied to literature data.
Jens Jensen, Agricultural Research Department, Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark.
The lack of a well-defined procedure for estimating linkage maps for the loci on the barley chromosomes and the conflict between the different published maps have led me to study the possibility of establishing an unambiguous mapping procedure. The suggested mapping procedure (Jensen and Jørgensen, 1974b) utilizes jointly the information from all available crosses and consists of: 1) transformation of the recombination percentages to additive map distances, centimorgans (cM), by the Kosambi mapping function (Jensen, 1973), 2) estimation of the interloci distances from all available data by a maximum likelihood method (for each likely order of the loci), and 3) omission of inconsistent recombination data, and finally finding the most likely order of the loci.
The procedure is applied to chromosome 5 linkage data obtained from the literature up to and including 1972. Thirteen loci fulfill the requirements for being included in the mapping procedure. Between these 13 loci, 43 recombination percentages were available. However, owing to inconsistencies in the data, 14 of the recombination percentages had to be omitted and as a consequence only 11 loci are on the map. The estimated map (distances in centimorgans) is as follows:
The map appears in two separate sections not orientated to each other, but with the Ml-a locus in common. The two loci omitted because of inconsistencies are f7 and JMl-nn. Material with the aim of incorporating f7 in the map is being treated and results not yet published will incorporate JMl-nn in the map (Jensen and Jørgensen, 1974a). In the present relation locus JMl-nz is considered the same as locus Ml-k on account of the very short distance between them.
The order of the loci in the map above is the most likely one based on the available literature data. From the relatively large standard deviation of the distance between fs2 and ert-b it is obvious that the reverse order of these two loci is also likely. Further, the positions of Ml-p and ml-d may not be definite,and eak (-3) and trd may be on the other side of B. Additional recombination studies are required to provide data for establishing the true order of all the loci.
As the distances on the map are given in centimorgans, they can be added without restrictions. Standard deviations of added distances cannot be calculated directly from the data on the map, because covariance between interloci distances is not given (see Jensen, 1974).
Jensen, S. 1973. Proposal for a new procedure for the preparation of gene maps in barley. BGN 3:24-25*
Jensen, J. 1974. Co-ordinators report: Chromosome 5. BGN 4:102-106.
Jensen, J., J. Helms Jørgensen. 1974a. Genetic linkage map of barley chromosome 5. II. An experiment with nine widely spaced loci. BGN 4:42-43.
Jensen, J. and J. HeLas Jørgensen. 1974b. The barley chromosome 5 linkage map. I. Literature survey and map estimation procedure.- In preparation.
