Qtl Mapping

QTL mapping means locating the specific chromosomal regions in which QTL are found. This is achieved by locating the QTL with respect to a standard set of Mendelian loci that have been previously mapped using standard genetic or physical gene-mapping procedures. These reference loci are termed genetic markers, since they identify (mark) specific chromosomal locations.

Daughter type

Sire haplotype

Relative proportion among all daughters

Parental

MQ

0.5(1 - r)

Recombinant

Mq

0.5 r

Recombinant

mQ

0.5 r

Parental

mq

0.5(1 - r)

The progeny of the sire that receive the M marker allele from their sire will be of two types: MQ and Mq.

Thus, they will receive a mixture of Q and q alleles, in relative proportion (1 — r):r. If M is close to Q, r will be small. Consequently, among these progeny, there will be a preponderance of Q alleles having positive effects on trait value. The opposite holds for progeny that receive the m marker allele from the sire; among these daughters, there will be a preponderance of q alleles having negative effects on trait value. The net result is that, on average, the progeny carrying the M marker allele are expected to show a higher trait value than the progeny carrying the m marker allele, the expected difference (D-value) being greater the closer M and Q are to one another, while D=0 when marker and QTL are on different chromosomes or far removed on the same chromosome.

In practice, the location of the markers in linkage to QTL is not known in advance; hence the experiment is carried out as a genome scan, in which D-values are calculated for a complete set of markers spanning the entire genome. A typical scan may involve 70 to 150 markers. In single-marker mapping, a QTL in a given chromosomal region is assigned the location of the marker that shows the greatest D-value. In interval mapping, information on all markers in a chromosomal region is used to obtain the most likely position of the QTL using advanced statistical procedures of maximum likelihood estimation and least squares regression mapping.

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