marker-assisted selection in wheat - ictsd

206Marker-assisted selection – Current status and future perspectives in crops, livestock, forestry and fishFigure 2The daughter designMAmaM?m?A?a?Only a single family is shown, although in practice several families will be analysed jointly. The sireis assumed to be heterozygous for a QTL and a linked genetic marker. The two alleles of the markerlocus are denoted “M” and “m”, and the two alleles of the QTL are denoted “A” and “a”. Alleles ofmaternal origin are denoted by question marks.individual genotyped. As each genotypeis associated with multiple phenotypicrecords, the power per individual genotypedin the granddaughter design can befour-fold the power of the daughter design(Weller, Kashi and Soller, 1990). The disadvantageof this design is that the appropriatedata structure (hundreds of progeny testedbulls, sons of a limited number of sires) isfound only in the largest dairy cattle populations.Both daughter and granddaughterdesigns are less powerful per individualgenotyped than designs based on analysisof inbred lines. Furthermore, the half-sibdesigns have the disadvantage that progenywith the same genotype as the sire are uninformative,because the progeny could havereceived either paternal allele.Additional experimental designs havealso been proposed. Coppieters et al.(1999) proposed the “great-granddaughterdesign”. One of the disadvantages of thegranddaughter design is that the number ofprogeny-tested sons of most sires is too lowto obtain reasonable power to detect QTLof moderate effects. Coppieters et al. (1999)proposed that power can be increased byalso genotyping progeny-tested grandsonsof the grandsire. Inclusion of the grandsonsis complicated by the fact that there isanother generation of meiosis between thegrandsire and his grandson.A significant drawback of all the designsconsidered above is that they give noindication of the number of QTL allelessegregating in the population or their rela-