538
Chicken Genome
in DNA recombination and repair and in
signal transduction.
Chickens are also one of the main ver-
tebrate models studied by developmental
biologists. A particular advantage is that ex-
perimental manipulations can be carried
out while the embryo is still in the egg.
Classical manipulations consisted of ablat-
ing and/or transplanting cells and tissues
within chick embryos to study cell–cell in-
teractions and to assay cell determination.
A wide range of manipulations is now
possible, including the construction of
chick/quail chimeras to study cell fate, the
grafting of microcarrier beads releasing
deFned chemicals, such as growth factors
and inhibitors, and genetic manipulations.
DNA can be readily introduced into chick
embryos using retroviral methods and/or
electroporation, and these techniques have
been used widely to overexpress genes at
particular times and locations during em-
bryonic development. A recent report sug-
gests that RNAi should also be applicable to
chick embryos opening up the possibility
of using chick embryos for high through-
put screening of vertebrate gene function.
2
Chicken Karyotype
The chicken genome has a haploid content
of 1
.
2
×
10
9
bp of DNA and this is di-
vided among 39 chromosomes including
not only macrochromosomes, which are
cytologically distinct, but also microchro-
mosomes, which are found in all birds and
in some reptiles. Chromosomes 1 to 8 are
usually classiFed as macrochromosomes.
The sex chromosomes are Z and W; in
birds, unlike in mammals, it is the males
that are homogametic (ZZ), while the fe-
males are heterogametic (ZW). A recent
study suggests that the long-standing view
that dosage compensation does not occur
in birds is incorrect and that the majority
of Z-linked genes are dosage compensa-
ted. The 30 chicken microchromosomes
contain about one-third of the genomic
DNA but originally it was not known
whether this was genetically inert. A num-
ber of studies have now shown that the
microchromosomes are in fact gene rich,
with recent estimates suggesting that mi-
crochromosomes are twice as gene rich as
macrochromosomes.
3
Maps of the Chicken Genome
Maps are used in everyday life to Fnd
objects (houses, streets, towns, etc.) in
different countries around the world. In
the same way, gene maps for a particular
species tell us the chromosome location
of a speciFc gene. The resolution of any
map can vary from simple views of whole
counties, highlighting major towns and
cities, to high-resolution maps showing
individual houses and streets. The same
is true of gene maps, from which we
may simply know the approximate physical
location on a chromosome, as observed in
a microscope (a cytogenetic map), or we
may actually have the nucleotide sequence
of the gene.
There are essentially two types of gene
mapping methods: genetic (or genetic
linkage) and physical mapping. In ge-
netic mapping, the order and position of
genes along speciFc chromosomes is de-
duced from the cosegregation of genetic
markers in genetic pedigrees. Genes close
together on the same chromosome tend
to stay together and those further apart
may be separated by genetic recombina-
tion. The result is an abstract map of each
chromosome based on the frequency of
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