marker-assisted selection in wheat

208Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 4The modified granddaughter designQ1 Q2 M1M2H1H2Q1M1Q2M2H3H4H1 Q1 H2 M1 H3 M2Q2H4Only alleles for the QTL are shown. Alleles originating in the heterozygous grandsire are termed “Q1” and Q2”. Alleles originatingin the grand-dams are termed “M1” and “M2”. Alleles originating in the sires are termed “H1”, “H2”, “H3” and “H4”.negative grandpaternal QTL allele, and halfshould receive neither grandpaternal allele.In the third case, the granddaughter receivedone of the QTL alleles of her grand-dam,the mate of the heterozygous grandsire.These grand-dams can be considered arandom sample of the general populationwith respect to the allelic distribution of theQTL. All genetic and environmental effectsnot linked to the chromosomal segment inquestion are assumed to be randomly distributedamong the granddaughters, or areincluded in the analysis model. Thus, unlikethe daughter or granddaughter designs, it ispossible to compare the effects of the twograndpaternal alleles with the mean QTLpopulation effect.Assuming that the QTL is “functionallybiallelic” (i.e. there are only two alleleswith differential expression relative to thequantitative trait), and that allele origincan be determined in the granddaughters,the relative frequencies of the two QTLalleles in the population can be determinedby comparing the mean values of thethree groups of granddaughters for thequantitative trait. Using the modifiedgranddaughter design it is also possible toestimate the number of alleles segregatingin the population, and to determine if thesame alleles are segregating in differentcattle populations. Weller et al. (2002)estimated the frequency of the QTL allelethat increases fat and protein concentrationon BTA6 in the Israeli Holstein populationas 0.69 and 0.63, relative to fat and proteinpercent, by the modified granddaughterdesign. This corresponded closely to thefrequency of 0.69 estimated for the Y581allele of the ABCG2 gene for cows bornduring the same time period (Cohen-Zinderet al., 2005).