Worksheet - COMPARATIVE MAPPING
The arrangement of genes and other DNA markers is compared between species in Comparative genome mapping.
As early as 1915, the geneticist J.B.S Haldane reported that genes which are linked, or on the same chromosome, are
inherited together. From these early observations it was soon recognised that chromosomal segments are highly
conserved over long periods of time and any changes in gene order can be used, along with other evidence (such as the
fossil record), to chart the evolutionary history of a species.
As the Human Genome Project has unfolded there have been enormous advances in DNA technologies. The development
of PCR and automated sequencing especially have made it possible to construct genome maps with enough information
to make them useful for comparative studies.
Many mammalian genes have homologues in other animal groups. By comparing the amino acid sequences of the
proteins coded for by these genes, or the DNA sequences themselves, it has become clear that many of the 70,000 –
140,000 genes within the mammalian genome have been conserved over hundreds of millions of years of independent
evolution. Although we see a great variety in karyotypes, comparative mapping makes it clear that the order of genes
as well as the sequences of those genes is highly conserved in mammals.
Comparisons of human maps with those from other species provide clues into the evolutionary origin of regions of the
genome. Although interesting in its own right this may also provide new insights into the function of these regions. As
the initial stage of Human Genome Project nears its completion (with a completed DNA sequence expected by 2001),
this will become increasingly important as scientists attempt to determine the meaning of the DNA sequences generated.
Comparative genome maps made using genes and other DNA markers can be used to:
•
predict the location of genes or markers in other species. If a new gene is mapped between two genes in
one species, its homologue is probably located between the same two genes in another species.
•
identify candidate disease genes. Disease causing genes mapped in model species (such as the mouse)
have been used to locate the genes responsible for diseases such as Waardenburg Syndrome Type 1 and
neurofibromatosis in humans.
•
characterise the genetic basis for complex genetic traits. Although comparative maps are valuable for
mapping single genes and identifying candidates for simple traits, they may be even more important for
analysing complex traits such as those responsible for epilepsy, diabetes and hypertension. It is impossible
to plan genetic crosses and control environmental factors in humans but this is possible in experimental
species such as the laboratory mouse and rat with the added bonus of being able to produce large numbers
of progeny. This approach is used to pinpoint regions of the genome important for complex traits. These
regions can then be thoroughly investigated.
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Worksheet - COMPARATIVE MAPPING
KEY TERMS
linked genes
human genome project
PCR
automated DNA sequencing
genome maps
karyotypes
INSTRUCTIONS
Genetic markers have been assigned to human and chicken chromosomes. By looking carefully at these markers:
Either:
Cut out human chromosomes 12 and 22 from the Chromosome Cut Out Sheet (H12 and H22) and paste the segments
onto the answer sheet to make the chicken chromosomes 1 and 18 (C1 and C18).
Or:
Cut out chicken chromosomes 1 and 18 from the Chromosome Cut Out Sheet (C1 and C18) and paste the segments
onto the answer sheet to make human chromosomes 12 and 22 (H12 and H22).
Remember to leave out any extra centromeres because each chromosome should only have one centromere.
Now answer the following questions:
1.
Name the chromosomal mutations involved in the rearrangements you have just made.
2.
Using the data provided is it possible to decide whether the chicken arrangement or the human arrangement is
closer to that of their common ancestor? If not what other data do you need?
3.
Now use the additional information provided on Chromosome sheet 2 to decide whether the chromosomal
arrangement of the chicken or the human is more like the ancestral arrangement? Explain your answer.
4. You should now be able to complete the phylogenetic tree answer sheet.
2
C1
GAPD
LDHB
ITPR2
TRA
H22
C1 - Chicken chromosome 1
C18 - Chicken chromosome 18
H12 - Human chromosome 12
H22 - Human chromosome 22
TRA
DCN
MYF6
IFNG
IGF1
CRYBB1
IGL
MIF
ITPR2
LDHB
IGF1
C18
GAPD
LGALS4
ADSL
NAGA
DCN
MYF6
IFNG
H12
CHROMOSOME CUT OUT SHEET
LGALS4
ADSL
NAGA
CRYBB1
IGL
MIF
Worksheet - COMPARATIVE MAPPING
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C1
C18
ANSWER SHEET
H12
H22
Worksheet - COMPARATIVE MAPPING
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C1
GAPD
LDHB
ITPR2
TRA
D7 - Dolphin chromosome 7
LGALS4
ADSL
TRA
CRYBB1
LGALS4
ADSL
IGL
DCN
MYF6
IFNG
NAGA
D7
MIF
NAGA
DCN
CRYBB1
LGALS4
H22
IGF1
MYF6
IGL
ADSL
IGF1
IFNG
MIF
C18
NAGA
DCN
MYF6
IFNG
H12
CHROMOSOME SHEET 2
Worksheet - COMPARATIVE MAPPING
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ANCESTOR
Phylogenetic tree - answer sheet
Mya - million years ago
Present day
65 mya
300 mya
Worksheet - COMPARATIVE MAPPING
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