DNA Markers and Molecular Mapping Notes

DNA Markers and Molecular Mapping

Introduction

  • This lecture focuses on using DNA markers to map genes and loci on chromosomes.
  • In the previous lecture, the concentration was on genes with visible or measurable phenotypes for mapping recombination.
  • A key area of interest is mapping human traits, especially diseases, to create genetic maps for improved genome sequencing, genetic counseling, and health management.

Challenges in Human Genetic Mapping

  • Mapping in humans is more complex due to:
    • Few visible, single-gene controlled phenotypes.
    • Smaller family sizes compared to organisms like Drosophila.

Overcoming Limitations with DNA Markers

  • DNA markers offer a way to map genes without relying on visible phenotypes.
  • Types of DNA markers:
    • Single Nucleotide Polymorphisms (SNPs)
    • Insertions
    • Deletions
    • Short Tandem Repeats (STRs)

Techniques for Detecting DNA Variation

  • Restriction Fragment Length Polymorphisms (RFLPs)
  • Allele Specific Oligonucleotides and Arrays
  • Microsatellite DNA markers

Applications of DNA Markers

  • Creating high-resolution genetic maps in humans.
  • Mapping disease loci to facilitate gene cloning and genetic counseling.
  • Applications in human health, agriculture, and other fields.

Lecture Objectives

  • Understanding different types of DNA markers.
  • Using DNA markers to create fine high resolution genetic maps in humans.
  • Mapping disease loci with respect to those markers.

What are DNA Markers?

  • Variations within a DNA sequence detectable by molecular biology techniques.
  • Traditional mapping relies on genes or alleles with phenotypic differences (visible traits).
  • These are known as phenotypic markers, such as eye color, wing morphology and body color in Drosophila.

Limitations of Phenotypic Markers

  • Many genes lack multiple alleles with phenotypic differences.
  • Genes comprise only a small fraction of the total DNA (e.g., only 3% of the human genome).

Advantages of DNA Markers

  • Molecular techniques allow mapping of DNA sequences, even non-genic regions (97% of the human genome).
  • Mapping only requires allelic differences in a specific DNA sequence.

Nature of DNA Markers

  • DNA markers are DNA fragments containing variation such as deletions, insertions, nucleotide substitutions, or short tandem repeats.
  • Identified by direct DNA analysis without association to a specific phenotype or gene function.
  • These are neutral markers within the genome.

Examples of DNA Variation

  • Allele 1 (Wild Type): Reference sequence.
  • Allele 2: Contains a Single Nucleotide Polymorphism (SNP) (C to G) and a deletion of a C.
  • Allele 3: Contains an insertion of two base pairs.
  • Alleles 4 and 5: Variations in Short Tandem Repeats (STRs). Allele 4 has two copies and Allele 5 has three copies of ATG compared to one copy in the wild type allele.

Origin and Location of DNA Variation

  • Variants arise through mutation and are transmitted across generations.
  • Can occur in exons (coding sequence), but are more common in non-coding sequences:
    • Introns
    • Untranslated Regions (UTRs)
    • Intergenic Regions (most common, given that 97% of the genome is intergenic).

Application of DNA Variation

  • DNA variation in non-coding sequences can map gene position or other DNA markers via linkage analysis.
  • Enables building high-resolution genetic maps across entire chromosomes.

Fine Resolution Genetic Maps

  • Genes and DNA markers are scattered throughout the chromosome, with some markers closely linked to genes and others located in intergenic regions.

Single Nucleotide Polymorphisms (SNPs)

  • SNPs are the simplest form of DNA sequence variation.
  • A single nucleotide (A, T, G, or C) differs at a specific DNA sequence between individuals or homologous chromosomes.
  • At a single nucleotide position there are maximum four possible different alleles.

Haplotypes

  • The set of SNP alleles observed on a particular chromosome is called a haplotype.
  • Closely linked SNP loci tend to be inherited together.
  • New haplotypes can occur through mutation or recombination (crossing over).

Inheritance of SNPs

  • SNPs are inherited as allelic variants.
  • Example: A pedigree showing a single SNP with two alleles (G and T) gives rise to three possible genotypes: GG, TG, and TT.