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DNA molecular testing and DNA Typing


Cloning the CF between markers ... Used to jump between chromosomal locations without cloning the intervening DNA. ... positional cloning, four candidate ... – PowerPoint PPT presentation

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Title: DNA molecular testing and DNA Typing

DNA molecular testing and DNA Typing
Genetic testing
  • An individual has symptoms or
  • An individual is at risk of developing a disease
    with a family history.
  • DNA molecular testing
  • A type of testing that focuses on the molecular
    nature of mutations associated with the disease.

  • Many different mutations can cause symptoms of a
    single disease.
  • BRCA1 and BRCA2 are implicated in the development
    of breast and ovarian cancer.
  • Hundreds of mutations can be found in these
    genes the risk of cancer varies among the
  • General screening and genetic testing are
    different (mammograms vs. testing for specific
    mutations in the gene).

Genetic testing
  • Prenatal diagnosis is the fetus at risk?
    (amniocentesis or chorionic villus samples
  • Newborns can be tested for PKU, sickle cell
    anemia, Tay-Sachs.

Methods of Genetic Testing
  • Restriction Fragment Length Polymorphism
  • Loss or addition of a RE site is analyzed.
  • RFLP is a DNA marker.
  • RFLPs are useful for
  • Mapping the chromosomes.
  • Finding out different forms of genes/sequences.

  • RFLPs may be changes in the gene of interest
    (such as with sickle cell).
  • Often, RFLPs are associated with, but not in,
    the gene of interest. A RFLP which is very near
    the allele of interest will usually indicate the
    presence of the allele of interest.
  • RFLPs can be used to follow a genetic lineage
    (in essence, a specific chromosome) in a
    population, and is a useful tool in population

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Different alleles of Hb
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Microsatellites and VNTRs as DNA Markers
  • Analysis of microsatellites ( short tandem
    repeats or STRs, 2-4 bases repeat), and VNTRs
    (Variable number of tandem repeats, 5- 10s of
    bases repeat) sequences is used in many genetic
  • Repeated sequences are often more variable (due
    to replication errors and errors in crossing
    over) than non repeating sequences, therefore
    lots of alleles are generally present in a
  • In other words, two individuals have a higher
    chance of genetic differences at STRs and VNTRs
    than at most sequences in the DNA.

Microsatellites and VNTRs as DNA Markers
Analysis of Microsatellites and VNTRs
  • One way of thinking about these analyses is that
    this is a specialized RFLP analysis, the power is
    that there are lots of alleles in a population,
    so there is a high chance that two individuals
    will be different in their genotypes
  • Two techniques are common in these analyses
  • Southern blot followed by hybridization with a
    probe that will detect the sequence (as in RFLP
  • PCR with a pair of primers which flank the
    variable sequence.

  • Population studies finding differences in allele
    frequencies can identify separate populations
    (not interbreeding).
  • Locating specific genes associating a specific
    VNTR allele with a genetic disease can help
    localize the gene to a region of the chromosome,
    or trace the allele through a pedigree.
  • DNA typing paternity testing (also useful in
    population studies, in animal breeding etc.) and
    in forensic analysis.

DNA Typing in Paternity Testing
  • Comparison of VNTRs can definitely exclude an
    individual from being the parent of a child
    (neither allele the child has is present in the
    alleged father).

DNA Typing in Paternity Testing
  • Proving paternity is more difficult, and relies
    on statistical arguments of the probability that
    the child and the alleged father are related.
    Multiple loci (different VNTRs) must be examined
    to provide convincing evidence that the alleged
    father is the true father. The same statements
    (exclusion versus proof of identity) are true for
    forensic arguments. Ethnicity of the accused is a
    factor allele frequencies for VNTRs are
    different in different population, be they elk or
    human., and often the frequencies (which are the
    basis of the statistical arguments) are not known
    for a specific group.

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Finding a Gene Chromosome Walking
  • Identifying the gene associated with a specific
    disease requires years of work.
  • The first step is to identify the region of the
    chromosome the gene is in (pedigree analysis,
    identifying breaks in chromosomes which cause the
    disease, etc.)
  • Once the gene has been localized to a region of a
    chromosome, is to walk along the chromosome.
  • The walk starts at a sequence known to be nearby,
    and continues until the gene of interest is

Isolation of Human Genes
  • Positional cloning Isolation of a gene
    associated with a genetic disease on the basis of
    its approximate chromosomal position.

Cystic Fibrosis Gene
  • Cystic fibrosis disease is a common lethal
    disease inherited as an autosomal recessive
  • Identify RFLP markers linked to the CF gene.
  • Identify the chromosome on which the CF gene is
  • Identify the chromosome region on which the CF
    gene is located (finer mapping).
  • Clone the CF gene between the flanking markers.
  • Identify the CF gene in the cloned DNA.
  • Identify the defects in the CF gene.

RFLP markers linked to the CF gene (linkage
  • Screen many individuals in CF pedigrees with a
    large number of RFLPs.
  • Use Southern blot analysis and hybridize with
    probes to identify different forms.
  • Establish a linkage between the markers and the
    occurrence of the disease.

Which chromosome?
  • Use in situ hybridization, where chromosomes are
    spread on a microscope slide, and hybridized with
    a labeled probe, results are analyzed by
  • A 3H-labeled RFLP probe showed that CF gene is
    located on chromosome 7.

Which chromosomal region?
  • Search other RFLPs located on the chr. 7, to find
    ones that are linked to the CF gene.
  • Again, use the pedigrees and test the DNA for
    associated RFLP markers.
  • Two closely linked flanking markers (one marker
    on each side of the CF gene) were found that are
    0.5 map units apart (500.000 bp).
  • Their locations were 7q31-q32.

Cloning the CF between markers
  • Chromosome walking technique is used to walk
    across the chromosome between the markers.
  • An initial cloned DNA fragment (one of the
    flanking markers) is used to begin the walk.
  • An end piece of this clone is used to screen a
    genomic library for clones hybridize with it.
  • These clones are analyzed by RE mapping to
    determine the extent of the overlap.
  • A new labeled probe is made from right end of the
    clone with minimal overlap, the library is
    screened again.

  • Chromosome walking uses large cloned DNA
    fragments which overlap.
  • Clones are isolated from a library based on
    hybridization to the end of the previous clone.

  • End piece of the clone is repetitive DNA, so that
    many other chromosomal locations will give false
    positive results. So probes should be unique
  • Length of each walk step is limited by the
    library. If a gene spans about 500.000 bp, if
    the library is a cosmid library (50.000 bp), and
    the average overlap between clones is about
    15.000 bp, then 35.000 x15 500.000 bp 15 steps
    in the walk is necessary between flanking markers.

Identifying the CF gene in the cloned DNA
  • Use cloned DNA as probes to hybridize with other
    species DNA.
  • Digest DNA from mouse, hamster or chicken with
    RE, analyze fragments by Southern blotting and
    hybridization with a labeled probed.
  • Select the clone which shows the best
    hybridization with other species.

Identifying the CF gene in the cloned DNA
  • Perform a Northern blot a DNA probe is
    hybridized with mRNAs on the blot.
  • Sequence the selected clone, and look for regions
    that may qualify as promoter regions or exons.
  • Screen a cDNA library and identify the clone.
  • CF gene cDNA is about 6500 bp.

Positional cloning
  • Requires knowledge of the gene product before the
    gene to be cloned.
  • Generates transgenic organisms that express a
    gene only in certain tissues.
  • Is when a cDNA has been cloned into a specific
    orientation in an expression vector.
  • Isolates a disease gene based on its approximate

Chromosome walking
  • Used to obtain a set of overlapping clones from a
    cDNA library.
  • Used to jump between chromosomal locations
    without cloning the intervening DNA.
  • Impossible in eukaryotes because of the amount of
    interpersed repetitive DNA.
  • Used to obtain a set of overlapping clones from a
    genomic library.

What is the difference between a pseudogene and a
  • A pseudogene is a special type of gene that
    contains sequences that hybridize with genes of
    other organisms.
  • A pseudogene is found with CpG islands, while
    genes are found next to them.
  • A pseudogene is stored in heterochromatin, and is
    not a functional copy of the gene.
  • A pseudogene has a sequence resembling a
    functional gene, but lacks appropriate expression

During positional cloning, four candidate genes
are identified.
  • What would be the most convincing evidence?
  • A zoo blot
  • Polymorphisms are present in one of the genes in
    affected individuals.
  • One of them is expressed in the tissue affected
    by the disease.
  • Mutational changes are present in one of the
    genes in affected individuals.

Suppose DNA typing is used in a paternity case.
  • How do exclusion results differ from inclusion
  • Exclusion results are easier to prove-one needs
    to show that the male in question has no alleles
    in common with the baby.
  • Inclusion results require positive identity to be
    established and usually testing for alleles at
    multiple loci.
  • Inclusion results require calculation of the
    relative odds that an allele came from the
    accused or from another person, and requires
    knowing the frequencies of VNTR and STR alleles
    in many ethnic groups.

  • http//www.biology.arizona.edu/human_bio/activitie
  • http//www.biology.arizona.edu/human_bio/activitie
  • Pedigree Collect data Paternity testing
    Missing person RCMP freq. calc.
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