See also: Morgan

The centiMorgan is a Measuring unit of distance on a Genetic card. Abbreviation cm . It represents the probability of Crossing-over at a given distance. I.e. probability of recombination, at the time of the meiosis, between two genes to which the locus are close on the same chromosome, and whose alleles present on each chromosomes define for each one of them, a Haplotype.

1 cm = 1% of recombination (1 crossing-over at this distance for 100 Meiosis S).

Equivalence between centimorgan and length of the segment of DNA separating two genetic markers depends on the sex, the species considered, and strongly varies with the size of its genome and the number of its chromosomes. At plant Arabidopsis thaliana, 1 cm = 200 kilobases approximately.

Example of recombination at the man

Let us suppose two genes dependant on the same chromosome, i.e. very close relations one of the other and not separating by crossing-over at the time of the meiosis. For example in the system of blood group Million, without considering the many variable minors of this system, the gene coding a protein, the glycophorine has (GPA), has two alleles M and NR, and the gene coding one second protein, the glycophorine B (GPB), has two alleles S and S. There are thus 4 haplotypes possible in this system: ms , ms , NS , NS . Let us suppose a father having received his parents the haplotypes ms and NS thus of genotype MS/Ns and phenotype Million, married to a woman homozygote MS/MS of phenotype ms. Their children will be necessarily ms from their mother, and will be ms or NS from their father, but could not be ms or NS from their father. With less than one exceptional recombination, whose frequency of which has occurred in the families gives an idea of the distance between genes expressed in centimorgans, the children MM will be necessarily S, and the children MN will be necessarily S.

Thus, in the preceding example, on 4 children, 2 are ms, and two Million. If we know in the beginning the paternal haplotypes, we conclude that there is no recombining. If we observe, in this family a fifth child who would be Million (the whole of the other markers confirming paternity), it would be one recombining, and we would say that the percentage of recombination in this last family is of a child on five, that is to say 20%. If we do not know the parental haplotypes, the probability of calculated recombination is not exactly the same one owing to the fact that the parental haplotypes are given starting from the children. One thus needed a great number of families (many preferably) of at least two children and/or families studied on three generations, to estimate, before molecular biology, a distance expressed in centimorgans.

The method of the Lod score S makes it possible to cumulate the reports/ratios, with the probability under the assumption of independence, of the probabilities of observations calculated for various rates of recombination within each studied family. Indeed, by expressing in Logarithm S decimal the reports/ratios calculated for each rate chosen, one obtains scores which one can progressively add with the new studied families. The highest score obtained for a given rate, corresponds to the Maximum of probability.

This method of family study made it possible, before molecular biology, to determine not only one frequency of recombination between two genes, but also the odre of genes on the chromosome when three genes, or more, were dependant.

See too

• Thomas H. Morgan
• Haplotype