General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

    323/384 (page 305)
    §6.4 Genetic chromosome maps 305 was used as the pollen parent in a testcross with yy ++. Since y cannot be transmitted through the pollen as long as it is located in the same chromosome as the deficiency, non-recombinants give yyY triploid endosperm and all yyy (white) seeds must result from recombination between y and the deficiency. There were 101 white seeds in a total of 635, which amounts to 15.9% recombina¬ tion. The normal genetic distance between y and pi is 28%, so the deficiency break-point must be half way between y and pi. McClintock designed another method for locating genes in chro¬ mosomes with the use of deficiencies. She hybridized strains with dominant and recessive markers and immediately upon fertiliza¬ tion irradiated the heterozygous zygotes. When recessive mutants occurred for the genes involved, they could be recognized imme¬ diately and then were analysed cytologically. When they were the result of a recognizable deficiency, which was checked in a test- generation, the location in the chromosome could be determined. A similar method was later (1968) used in the tomato by Khush and Rick, who irradiated normal pollen that was used to pollinate homozygous récessives. There are several more specialized meth¬ ods for locating genes on chromosomes. 6.4.3. Genetic chromosome maps from cytological data The example of Creighton and McClintock (sections 2.6.1, 6.4.2) demonstrates that the genetic distance between cytological markers can be determined even without using genetic markers. A somewhat different way of estimating genetic distance between cytological markers is counting the number of AI and All bridges in paracentric inversion heterozygotes. It gives information on the genetic length of the inversion segment and on the distance be¬ tween the centromere and the proximal break-point, but the rela¬ tion between bridge frequency and crossing-over is somewhat com¬ plicated (section 4.4.4). One can go further and determine the genetic length of chromosome arms or segments directly from chiasma frequencies (compare Brown and Zohari, section 2.6.1 ; Henderson, section 2.8.1). Even when chiasmata cannot be counted reliably and when only can be determined whether or not a segment has any chiasmata, an