Genes Are Arranged on Chromosomes in A Linear Fashion

The data of W. BATESON and colleagues on Lathyrus odoratus provided an experimental approach to the localization of genes on chromosomes. In Lathyrus odoratus , two characteristics are coupled that are nevertheless sometimes separated from each other. BATESON's results were not sufficient to draw further conclusions. It was data gained on Drosophila that allowed more insight into the way genes are organized on chromosomes.

The chromosome theory claims that the number of linkage groups corresponds to the number of chromosomes in a haploid set (n= 1). But actually, at first in Pisum sativum later on also in other species, more linkage groups than n = 1 chromosomes were detected. H. de VRIES assumed therefore in 1903 that the surplus in the number of linkage groups was founded on the exchange of fragments between chromosomes. The necessary structural preconditions are given during the prophase of meiosis when the homologous chromosomes pair. A. F. JANSSENS observed the cross of two chromosomal arms during the diplotene of meiosis in Batrachoseps attenuatus, an amphibian, and called this phenomenon chiasma. He, too, saw the possibility that the chromatids might break in such a position and that the broken parts might join the 'wrong' chromatid, thus causing an exchange of fragments.

It was T. H. MORGAN who could finally show that an exchange of the hereditary factors between two X-chromosomal characteristics occurs and that it is due to a crossing-over. In other words: genetic analyses lead to the assumption of an exchange of chromosomal fragments.

Relations between Crossing over and Chiasma
(Redrawn from C. BRESCH, 1964)

The relative probability of crossing-overs is decisive for the drawing-up of a genetic map. If two loci of one chromosome are in vicinity, then the probability that they are separated by a crossing over is low. It increases with growing distance. A. H. STURTEVANT analyzed the exchange frequency of several X-chromosomal genes this way. He could prove that it is indeed correlated to the distances between the genes and that these distances are additive. That means that the distance between gene A and gene C is the same as the sum of the distances between gene A and gene B and between gene B and gene C. This again indicates that the gene loci A, B and C are arranged on the chromosome one behind the other. The method has its weaknesses, since the actual values for large gene distances are often smaller than calculated. This is caused by a phenomenon called interference that happens, because the preceding crossing-over has an influence on the probability of the following ones.

The first, at the beginning still very simple gene map of a linkage group was that of the X-chromosome of Drosophila. It was soon followed by additions and further maps. The first gene map of a plant chromosome that consisted of only three loci was drawn up as soon as 1926 (L. J. STADLER, University of Missouri, Columbia, 1926).

After the mapping of numerous new gene loci, the distances originally determined by STADLER had to be stated more precisely, since they were falsified by double crossing-overs. It was the precise analysis done with the help of numerous gene loci, so called marker genes that led to an approach to the real distances.

Apart from meiotic crossing-overs, a mitotic crossing-over with sister chromosome exchange is known, too.