Chicken Genome
539
recombination. The genetic map has a di-
rect colinear relationship with the physical
map, where genes are mapped directly
by a number of methods (FISH (fluores-
cence
in situ
hybridization), RH (radiation
hybrid) mapping, contig building, and
DNAsequencing)asshowninFig
.1
.The
chicken has a long genome mapping his-
tory, starting almost 100 years ago with
the pioneering work on Mendelian inher-
itance and sex determination. The ±rst
genetic linkage map for any domesticated
animal was also in the chicken.
4
Genetic Linkage Maps
There are two requirements to construct
a genetic linkage map: (1) pedigrees in
which the cosegregation of genes can
be followed and (2) polymorphic genetic
markers. Markers must be polymorphic,
that is, alternative forms must exist so that
their inheritance can be followed among
the different members of the pedigree. In
the past, genetic markers were often based
on differences in morphology (e.g. feather
color) or blood groups, but today most
are based on DNA-sequence variants. A
sequence change may alter the function or
activity of a gene; for example, a sequence
difference may produce an altered protein
product or may alter the expression of the
gene. In many cases, such changes may
also result in a change in phenotype (e.g.
defects in growth hormone receptor gene
result in sex-linked dwar±sm). However,
the most frequent changes used to create
gene maps are in noncoding regions such
PE
1
I
1
I
2
I
3
I
4
E2
E3
E4
E5
Splicing (cell)
mRNA
BACs
YACs
Cytogenetic map
DNA probe
Genetic map
Markers, phenotypes
Physical map
YAC, BACs, PAC, cosmids.
..
Genome sequence
Nucleotide variation
cDNA sequencing
ESTs - Expressed sequence tags
Fig. 1
Diagram showing relationship between genetic and physical maps and ESTs. Different
methods of physical mapping are also illustrated.
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