yielding adaptable varieties with enhanced iron and zinc in the grain Gregorio Htut 2003.
A previously genotyped rice population was used to tag the genes QTLs for the high-Fe trait in brown rice. A total of 180 polymorphic markers including
146 restriction fragment length polymorphism, RFLP, 8 isozymes, 14 random amplified polymorphic DNA, RAPD, and 12 cloned genes on a linkage map of
doubled-haploid-derived lines from the cross between IR64 and Azucena. hree QTLs were located on chromosome 7, 8, and 9 and explaining 19 to 30
variation for iron content. Three QTLs for aroma were also reported on chromosome 3, 7, and 8 explaining 16 to 38 variation. Thus, these QTLs for
high iron and aroma have been suggested to be linked Gregorio et al. 2000.None of the above studies mapped genesQTLs in polished rice grains. Quantitative
Trait loci for zinc content in germinating seeds was located on chromosome 5 Avendano, 2000 in a study involving 93 F RILs and 31 SSR markers.
8
2.3. Genetic Basis of Mapping GenesQTLs
Stoskopf et al. 1993 explained that a cross between homozygous parents will result in F
1
with the same degree of uniform heterozygosity for each locus but with homogeneous phenotype. Selfing of F results F
1 2
that have the maximum amount of variability among segregating population. The amount of
variability in the F
2
generation is determined by the number of segregating genes the number of genes that are different between the two parents, and gene
linkage. As the number of different genes distinguishing the parents increases, the number of possible genotypes expected in the F
2
generation increases, as could be seen in the formula:
= 3
n
Number of different genotypes in F
2
n= number of heterozygous loci. For example, if there are 10 heterozygos loci, there would be 59,049 different
genotypes. F
2
population provides a huge variability for selection for the desired traits for breeding and it provides also the maximum variation for mapping
purposes. Harushima et al. 1988 reported a genetic map of rice Oryza sativa L. using 186 F
2
plants from a single cross between Nipponbare japonica and Kasalath indica. They used 2275-markers covering mapping length of 1521.6
6
cM. The high-resolution of the genetic map permitted for the characterization of meiotic recombinations in the whole genome. Positive interference of meiotic
recombination was detected both by the distribution of recombination number per each chromosome and by the distribution of double crossover intervals..
F
2
population was used for mapping of heading date and awn length in a cross between Hwaseongbyeo and WH29001. WH29001 was advanced
backcross line having introgression segment from Oryza minuta. The 197 F
2
plants and 197 F
3
families were evaluated and two QTLs were reported by Linh et al. 2006. Li et al. 2007 reported the utilization F
2
population to fine map the mutant of semi-sterility and anther indehiscence that controlled semisterility in
some crosses. plants could be planted to get F
F
2 3
and the subsequent selfing generations to get homozygous and stabile lines to be used for mapping Recombinant Inbred
Lines or breeding advanced elite lines. By selfing in subsequent generations, the homozygocity of the loci increase and after m generation of selfing, the
proportion of homozygous individuals from n segregating loci is given by
m n
m
2 1
2 +
For qualitatively inherited traits such as vertical resistance, maturity, and dwarfing genes, the genetic composition in any inbred generation can be obtained by
expanding the binomial
[ ]
n m
1 2
1 −
+ For example if there were four loci heterozygous, there would be 88.1 of
plant homozygous for all the four loci at F
6
generation. With the increase in the number of heterozygous loci, more number of generations are needed to fix all the
loci Stoskopf et al. 1993. F
1
population could also be crossed to the recurrent parent termed as backcrossing. The proportion of genes from the donor parents is reduced by one
half following each generation of backcrossing. The content of donor parent is given by the relationship 12
n
, where n is equal to the number of backcrosses to the recurrent parents. On the other hand, the proportion of homozygosity in a
particular generation, m, is given by the relationship
7
m m
2 1
2 −
Backcrossing results in effective recombination only in the gametes of the hybrid progeny and not in the gametes coming from recurrent parent. The lack of
effective recombination in the recurrent parent is due to the homozygous nature of that parent in a self-fertilized species. Backcrossing could be used to incorporate
traits from donor parent with good enough recovery of the recurrent parent feature Stoskopf et al. 1993.
Backcross population has been used as a mapping population. Xiao et al. 1998 reported that 300 families of BC
2
population V20AO. rufipogonV20BV20BCe64 had been used to map 68 QTLs for 12
agronomically important traits. A set of 122 RFLP and microsatellite markers was used to identify the QTLs. Mei et al. 2005 reported the utilization of 254
RILs of LemontTeqing and two backcross hybrid BC F
1 1
of that RIL to study the gene action controlling several traits, such as heading date, plant height, flag leaf
length, flag leaf width, panicle length, and spikelet fertility. Backcrossing has also been used to introgress traits assisted by molecular marker termed as
Molecular Assisted Backcrossing Hospital 2005.
2.4. Molecular Markers and QTL Mapping
