542
Chicken Genome
Chicken
Turkey
Fig. 3
Chromosome painting of turkey chromosomes with chromosome
paints derived from chicken chromosomes 1 to 5, and the Z chromosome
(kindly provided by Professor Johannes Wienberg). (See color plate p. xxi).
5.2
Radiation Hybrid Maps
A major limitation of genetic markers is
the need to be polymorphic so that we
can track their inheritance. About 10 years
ago, the use of radiation hybrid mapping
panels in humans and mice increased the
rate of gene mapping signiFcantly. In this
method, the genome of the species to be
mapped is fragmented by ionizing radia-
tion. These DNA fragments are rescued by
fusion with a host cell line to create pan-
els of cells, each retaining a speciFc set of
donor chromosomal fragments. Much like
genetic mapping, genes close together on a
speciFc chromosome will tend to be found
in the same RH cell line. The frequency
of coretention can be used to establish
the order and distance between genes. In
this method, since only the presence or
absence of gene marker is required, there
is no need to identify polymorphic mark-
ers; only a simple PCR test is required. In
contrast to the success with other species,
thecon
s
t
ru
c
t
iono
fanRHpane
linthe
chicken had met with limited success un-
til recently. Early results for chromosome
7mapped77loci
,whichareco
linearwith
the corresponding genetic map. Currently,
the chicken consortium are mapping over
a thousand PCR-based anonymous and
gene markers on this panel, which should
prove to be an important tool to comple-
ment the BAC-based physical mapping.
5.3
Contig Maps
A contig map is constructed using a
method that starts by fragmenting and
cloning the genome in large segments
of 100 to 1000 kb. Then various methods
are used to identify overlapping genomic
clones, and the data are Fnally assembled
into a set of overlapping clones or contigs.
Ideally, the process would end with a single
contig for each chromosome, but usually
the process deFnes hundreds of smaller
contigs. The Fnal number depends on
the number of genomic clones analyzed
(usually 100 000 clones) and the success
rate for cloning difFcult regions such as
centromeres and telomeres.
Two types of large insert genomic li-
braries
have
been
constructed
in
the
chicken. The Frst was a YAC library that
provided eightfold coverage of the chicken
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