Linkage and Crossing over


Genetic mapping of genes in eukaryotes is based on the mechanisms leading to new combinations of genes: random assortment of chromosomes and crossing‐over.

Keywords: ectopic recombination; genetic mapping; interference; random segregation; sister chromatid exchange; synteny; test cross

Figure 1.

Mitosis, meiosis and parasexuality. The segregation of two chromosomes (I and II) is shown during mitosis of a diploid cell (a), in meiosis (b), during haploidization, and in a human–rodent cell hybrid (c). The same colour is used for homologous chromosomes, the discs indicate centromeres, and the black bar the whole rodent genome. The chromosomes are marked at four genes with A, B, C, D, indicating dominant wild‐type and a, b, c, d recessive mutant alleles. (a) The chromosome transitions in the stages of the mitotic cell cycle are shown: G1 phase (2C DNA content), S phase (DNA replication), G2 phase (4C DNA content), M phase (mitosis). (b) In prophase I of meiosis the homologous chromosomes pair (bivalent formation) and form crossovers. Independent assortment in (MI) leads to one of the two possible configurations (left and right). (MII) then yields the four haploid products. (c) On the left the four possible haploidization products deriving from the heterozygous diploid presented in (a) are shown. On the right the four possible configurations resulting from retention or loss of human chromosomes in a human–rodent hybrid are drawn.

Figure 2.

Tetrad analysis, interference, and mitotic crossing‐over. The effect of crossovers on the segregation of chromosome markers in meiosis (a, b) and mitosis (c) is presented. (a) The chromatids of a bivalent of homologous chromosomes are numbered with I, II, III, IV. Possible crossovers in the intervals AB and BC are numbered 1, 2, 3, 4, 5. On the right the products of a single meiosis is shown as they are present in an ordered ascus (MI: meiosis I, MII: meiosis II). (b) The genotypes of tetrad spores resulting from different combinations of crossovers are given together with the resulting tetrad types (PD, parental ditype; T, tetratype; NPD, non‐parental ditype), and marker segregation types with respect to the centromeres (FDS, first division segregation; SDS, second division segregation). (c) The mitotic segregation after occurrence of crossover 1 is presented, leading either to formation of partially homozygous daughter cells (mode 1), or to fully heterozygous siblings (mode 2, not detectable phenotypically).

Figure 3.

Genetic mapping: three‐point test cross. The genotypes of single gametes (black chromosome) originating from the heterozygous parent (left) are scored by determination of the phenotypes of the progeny of a cross with a tester (right). The recombinant frequences (RF) for the three gene intervals are derived from the counts of the eight types of progeny, as well as the coefficient of coincidence (coc) and the value for interference (I). The determination of the order of the three genes based on the RF values is shown at the bottom and the genetic distances given in map units (mu).

Figure 4.

Ectopic and sister chromatid recombination. The patterns of recombination between repeated DNA sequences, and consequences for chromosome structure, are demonstrated. (a). The repeats (boxes) are marked differentially: heterozygosity (A/a and B/b) in the top repeats, and homozygosity (c and d) in the middle and bottom repeats, respectively. At the sites of c and d in the top repeats the wild‐type alleles C and D have not been indicated. The connections between sister chromatids are the undivided centromeres in meiotic prophase. The types of homologous interactions in this configuration are: (1) classical recombination, (2) sister chromatid recombination, (3) unequal sister chromatid recombination, (4) intrachromatid recombination, (5) heterochromosomal recombination, (6) unequal recombination. The patterns 2, 3 and 4 are termed intrachromosomal recombination, while 3, 4, 5 and 6 are summarized as ectopic recombination. (b) Left: unequal sister chromatid exchange (3) results in deletion and duplication of a chromosome segment, when the involved repeats are identically oriented with respect to the centromeres (arrows). A dicentric chromosome and an acentric fragment form with nonidentical repeat orientation. Right: the consequences of a heterochromosomal crossover (5). A reciprocal translocation, exchange of the chromosome arms distal to the crossover points, results from identical orientation of the repeats. Again, formation of a dicentric chromosome and an acentric fragment occurs with different orientation of the repeats.


Further Reading

Bascom‐Slack CA, Ross LO and Dawson DS (1997) Chiasmata, crossovers, and meiotic chromosome segregation. Advances in Genetics 35: 253–284.

Corcos AF and Monaghan FV (1993) Gregor Mendel's ‘Experiments on Plant Hybrids’: a Guided Study. New York: Rutgers University Press.

Kaback DB, Barber D, Mahon J, Lamb J and You J (1999) Chromosome size‐dependent control of meiotic recombination in Saccharomyces cerevisiae: the role of crossover interference. Genetics 152: 1475–1486.

Novitski E and Blixt S (1978) Mendel, linkage, and synteny. BioScience 28: 34–35.

Petes TD and Hill CW (1988) Recombination between repeated genes in microorganisms. Annual Review of Genetics 22: 147–168.

Petes TD and Pukkila PJ (1995) Meiotic sister chromatid recombination. Advances in Genetics 33: 41–62.

Schmidt R (2000) Synteny: recent advances and future prospects. Current Opinion in Plant Biology 3: 97–102.

Sia EA, Jinks‐Robertson S and Petes TD (1997) Genetic control of microsatellite stability. Mutation Research 383: 61–70.

Tucker JD and Preston RJ (1996) Chromosome aberrations, micronuclei, aneuploidy, sister chromatid exchanges, and cancer risk assessment. Mutation Research 365: 147–159.

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How to Cite close
Kohli, Jürg(Mar 2002) Linkage and Crossing over. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0000815]