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In wheat × sorghum (Sorghum bicolor L.) or wheat × maize (Zea mays L.) cross,
the sorghum and maize chromosomes are eliminated during the early zygotic cell
division (Laurie and Bennett 1986 ). Hybrid necrosis is another problem occurring
in wide crosses; it has been reported by Gregory ( 1980 ) and explained by Zeven
( 1981 ). The dominance of alleles of Ne1 and Ne2 depending on the combinations
results in various degrees of necrosis (Hermsen 1963 ). Some interspecific crosses
produce seeds with either an abnormal embryo or endosperm or both, which is
caused by the imbalance of species polar nuclei activation value or the paternal/
maternal genome ratio (Ishii et al. 2012 ).
Hybridization usually is a simple task when plants from the same species are
involved, crossing plants from two different species is not so straight forward;
sometimes additional techniques are need to get a successful hybrid. The reason of
failure to hybridize may be due to the different mechanisms of the plants that are
crossed.
10 Molecular Markers, Genome Mapping and Genomics
as an Adjunct to Breeding
There are basically three types of markers; phenotypic markers; biochemical mark-
ers and DNA-based markers. Since the discovery of recombinant DNA technology
in population genetics in the mid-1980s, a big stock of genetic markers is available
for population studies and crop improvement. Plant breeding has changed with the
introduction of these molecular techniques.
Molecular markers allow the extension of traditional breeding methods with one
important difference, it allows transferring genetic information into new germplasm
in a more precise and controlled manner. Marker assisted selection (MAS) for
important but complex traits, which are often difficult to select in the routine breed-
ing programs, have enhanced breeding programs in terms of better focused products
and saving in time and resources. The use of DNA markers increases our under-
standing of gene inheritance and action of traits and allows the use of MAS to
complement conventional selection procedures of plant breeding.
Biotechnology and especially the molecular markers have great potential for
improving plant breeding efficiency and productivity. Plant genetic engineering and
DNA marker technologies have now become valuable tools in crop improvement
for rapid precision breeding for specific purposes (Dhillon et al. 2013 ). Molecular
markers are present in most breeding programs worldwide since they were first
adopted by the Australian Plant Breeding Cooperative Research Centre in 1997. In
plant breeding, molecular markers have been used to confirm parent identity, paren-
tal characterization for some specific agronomical traits, assess genetic diversity in
the crossing block or in specific populations and MAS. Molecular techniques have
been and are still being used to monitor the DNA sequence variation in and among
the species to create new sources of genetic variation by introducing new and favor-
able traits from landraces and related grass species. Several types of molecular
Q. Sohail et al.