Linkage Analysis

Learning Objectives

• Explain homologous recombination and genetic linkage

• Describe how genetic disease can be classified by the number of genes required to cause the disease

• Describe the difference between genetic maps and physical maps of the genome

• Describe the principles of linkage analysis and why it is useful

• Describe the differences between the two types of genetic marker routinely used in linkage studies

• Briefly describe the genotyping technologies used in linkage analysis

• Describe what a haplotype is and how it is built

• Briefly describe the statistical approaches used in linkage analysis, with specific emphasis on the logarithm of the odds (LOD) score

• Summarise how linkage analysis can be combined with whole exome sequencing to identify a disease-causing variant

Homologous Recombination and Genetic Linkage

Homologous Recombination

• Definition: Homologous recombination occurs during meiosis, enabling the exchange of genetic material between homologous chromosomes (one from each parent).

• Mechanism:

  1. Chromosomes align during prophase I.

  2. DNA strands break and rejoin, swapping segments between homologous chromosomes.

  3. This produces recombinant chromatids, contributing to genetic diversity.

Genetic Linkage

• Definition: Genetic linkage occurs when alleles at different loci on the same chromosome are inherited together more frequently than expected by chance.

• Relevance: Genes close together on a chromosome are less likely to be separated by recombination and are therefore "linked."

Classification of Genetic Diseases by Genes Involved

1. Monogenic (Mendelian)

   • Disease caused by mutations in a single gene.

   • Examples: Cystic fibrosis, sickle cell anemia.

2. Polygenic

   • Disease influenced by mutations in multiple genes, each contributing small effects.

   • Examples: Type 2 diabetes, psoriasis.

3. Multifactorial

   • Involves both genetic predisposition and environmental factors.

   • Examples: Heart disease, asthma.

Genetic Maps vs. Physical Maps

Principles of Linkage Analysis

Definition:

Linkage analysis maps the location of a disease gene by studying its inheritance pattern with genetic markers in families.

Why It’s Useful:

1. Identifies linked markers to narrow down the region containing the disease gene.

2. Useful for diseases with clear inheritance patterns.

Key Principle:

Markers near a disease gene are inherited together in affected individuals more often than expected by chance.

Genetic Markers Used in Linkage Studies

1. Microsatellite Markers:

   • Short, repetitive DNA sequences (e.g., CA repeats).

   • Advantages: Highly variable; good for tracking inheritance.

   • Disadvantages: Labor-intensive genotyping; sparse distribution.

2. Single Nucleotide Polymorphisms (SNPs):

   • Single base-pair differences between individuals.

   • Advantages: High density; automated genotyping; genome-wide coverage.

   • Disadvantages: Lower variability per marker compared to microsatellites.

Genotyping Technologies for Linkage Analysis

1. Microsatellite Genotyping:

   • PCR amplifies repeat regions.

   • Size differences visualized via gel electrophoresis.

2. SNP Genotyping with Microarrays:

   • DNA hybridized to probes specific for SNPs.

   • Fluorescence signals indicate SNP alleles (e.g., homozygous or heterozygous).

Haplotype: Definition and Construction

• Definition: A haplotype is a set of alleles at linked loci on a single chromosome inherited together from one parent.

• Construction:

  1. Identify markers on the same chromosome.

  2. Track inheritance of markers within families.

  3. Recombination events define haplotype boundaries.

Statistical Approaches in Linkage Analysis

Logarithm of the Odds (LOD) Score

• Definition: A statistical measure to evaluate whether two loci (marker and disease gene) are linked.

• Formula: LOD=log⁡10(Probability of linkageProbability of no linkage)\text{LOD} = \log_{10} \left( \frac{\text{Probability of linkage}}{\text{Probability of no linkage}} \right)

• Thresholds:

  • LOD ≥ 3: Significant evidence of linkage (odds 1000:1).

  • LOD ≤ -2: Evidence against linkage.

• Additive Property: LOD scores from multiple families can be combined to strengthen evidence for linkage.

Combining Linkage Analysis with Whole Exome Sequencing (WES)

Process:

1. Perform Linkage Analysis: Identify candidate regions linked to the disease using markers.

2. Refine the Region: Narrow the interval using additional markers or recombination events.

3. Sequence Candidate Genes: Use WES to sequence genes within the refined region.

4. Identify Variants: Compare variants to a reference genome. Focus on rare, functional mutations shared among affected individuals.

5. Validate Findings: Confirm the disease-causing variant using experimental methods or additional family samples.

Example:

For autosomal dominant diseases, linkage analysis may reveal a significant peak on a specific chromosome. WES can then identify variants within this region, such as missense or nonsense mutations, leading to disease characterization.

Feel free to ask if you'd like a more in-depth explanation of any part!