Geoscience Reference
In-Depth Information
breeding methods. In general, the success of a marker-based
breeding system depends on four main factors: (i) a definite
genetic map with an adequate range of uniformly spaced poly-
morphic markers to accurately locate desired QTLs or major
factor(s); (ii) close association between the QTL or a significant
gene of interest and adjacent markers; (iii) Adequate recombi-
nation between the markers and remainder of the genome; and
(iv) a capability to analyse a larger range of plants in a time-
and cost-effective manner. The success of MAS depends on the
association of the markers with various factor of interest. There
are three sorts of relationships between the markers and vari-
ous genes that could be distinguished:
1. Marker is found inside the gene of interest, which is the
most favourable state of affairs for MAS and during
this case, it might be ideally mentioned as gene-assisted
selection. This might be notably helpful for traits that
have laborious or time-consuming phenotypic screening
procedures. Whereas this type of relationship is the most
preferred one, it is also tough to search out this type of
allele-specific markers. For example, SSR markers are
designed using the available nucleotide sequence infor-
mation for the opaque2 allele that confers high lysine
and tryptophan content within the maize kernel. This has
offered an efficient means of tracking the opaque2 allele
in breeding for nutritionally superior maize genotypes,
since the marker is found inside the gene sequence itself
and co-segregates with the target gene.
2. The marker is not in linkage equilibrium (LE) with the
gene of interest throughout the population, called LD.
LD is the tendency of certain combination of alleles to
be inherited together. Once markers and genes of interest
are physically close to each other, population-wide LD
may be found. The selection using these markers might
be referred to as LD-MAS.
3. The marker is in LE with the gene of interest through-
out the population, that is the most tough and difficult
situation for applying MAS. However, in most cases,
particularly for the inheritable traits, the target gene(s)
inside a QTL has not been characterised at the molecular
level. Therefore, the genomic regions to be selected using
MAS are usually chromosome segments carrying QTLs
in case of polygenic traits. It is desirable either to have
two polymorphic DNA markers flanking the target gene
(or a QTL), or a marker inside a QTL (if the chromosome
