Genetic Markers and Pedigree Risk Analysis

Genetic markers are known DNA sequences used to provide information regarding the risk of inheriting specific mutations.
A marker must be polymorphic (different between individuals) to be useful for distinguishing maternal and paternal chromosomes or identifying specific alleles in a population.
Markers are typically located outside of actual genes (in non-coding regions) and usually have no effect on an individual's phenotype.
The primary function of a marker is to serve as a tracker to follow specific chromosomal segments through a pedigree.
Inheritance rules for markers:
- For autosomes, every individual has two alleles for every marker.
- For sex-linked markers (X chromosome), females have two alleles (XX) while males have only one allele (XY).

There are three primary types of genetic markers:
- Restriction Fragment Length Polymorphisms (RFLPs).
- Variable Number Tandem Repeats (VNTRs).
- Single Nucleotide Polymorphisms (SNPs).
While RFLPs and VNTRs are considered somewhat "old school" and labor-intensive, they remain fundamental for interpreting lab results like gel electrophoresis in family studies.
The utility of a marker depends entirely on its variability; if everyone in a population has the same sequence, the marker cannot differentiate between individuals.

RFLPs refer to variations in the length of DNA fragments generated when DNA is cut by restriction enzymes.
Restriction Enzymes: These are enzymes derived from bacteria and lower-order organisms (e.g., $\text{EcoR1}$ , $\text{BamH1}$ ). They function as defense mechanisms against parasites in their original hosts.
- Recognition Sites: Each enzyme recognizes a specific nucleotide sequence. For example, $\text{EcoR1}$ recognizes the sequence $\text{GAA TTC}$ and cuts the DNA at that specific point.
- Polymorphism Mechanism: If a person has a mutation at a recognition site that changes one of the nucleotides, the enzyme will no longer recognize or cut the DNA at that location, resulting in a different fragment length compared to someone whose DNA is cut.

Digestion: Restriction enzymes are added to the DNA sample to cut it at specific sites.
Gel Electrophoresis: The DNA fragments are loaded onto an agarose gel.
- DNA is negatively charged, so a positive charge is applied at the bottom of the gel to drag the DNA through the substance.
- Size separation: Smaller fragments travel faster and further than larger fragments.
Southern Blotting: Gel is delicate, so the DNA is transferred (blotted) onto a stable nitrocellulose filter (a type of shiny paper) using a buffer.
Hybridization and Detection:
- A DNA probe (sequence of interest) is labeled for detection. Historically, radiation ( $^{32}P$ ) was used, requiring X-ray film; modern labs use fluorescent labeling detected by cameras at specific wavelengths.
- The probe binds to its matching sequence on the filter, appearing as a band that indicates the size of the fragment.

Suppose a region of DNA has three potential $\text{BamH1}$ cut sites:
- Site 1 to Site 2 distance = $\text{10,000 base pairs}$ ( $10\,kb$ ).
- Site 2 to Site 3 distance = $\text{6,000 base pairs}$ ( $6\,kb$ ).
If the middle site (Site 2) is polymorphic (absent in some individuals):
- Site 2 Present: The enzyme cuts, and the probe detects a fragment of $6\,kb$ .
- Site 2 Absent: The enzyme skips the site, resulting in a single large fragment of $\text{10,000} + \text{6,000} = \text{16,000 base pairs}$ ( $16\,kb$ ).
Interpretation of Results:
- A "16" band is designated as allele $\text{A}$ .
- A "6" band is designated as allele $\text{B}$ .
- If an individual (e.g., the father) shows only a "B" band on an autosome, they are homozygous ( $BB$ ) rather than monosomic; they simply have two identical alleles that cannot be discriminated.

VNTRs: These markers rely on repeat sequences where the number of repeats varies between individuals.
- Fragment size changes based on the number of repeats present between two fixed points.
- Analyzed via restriction enzymes or Polymerase Chain Reaction (PCR).
- VNTRs are highly polymorphic, meaning they can have many alleles ( $1, 2, 3, 4, …, n$ ).
SNPs (Single Nucleotide Polymorphisms): The most common type of variation, occurring approximately every $\text{300--400 base pairs}$ in the human genome.
- They typically involve a single base swap (e.g., a $G$ vs. a $T$ ).
- While usually having only two alleles, they are extremely frequent, allowing for fine-scale mapping using microarrays and Next-Generation Sequencing (NGS).

The closer a marker is to a disease-causing gene, the more useful it is for tracking.
Linkage Logic: The marker does NOT cause the disease. It is a physical nearby landmark on the same chromosome. If the marker is very close to the gene, it will likely be inherited with the mutation during meiosis.
Recombination Risk: If a marker is far from the gene, a recombination event (crossover) during meiosis may swap the disease mutation onto a different chromosome relative to the marker, leading to an incorrect risk prediction.
Key Distinction: A specific marker allele (e.g., allele $\text{1}$ ) is not universally linked to a disease in the general population. It is only linked within a specific family pedigree relative to who the individual inherited the chromosome from.

Disease Profile: Autosomal dominant, late-onset neurodegenerative disorder caused by a repeat expansion in the Huntington gene on chromosome $4$ .
Marker Utility: Because the expansion itself can be difficult to amplify/sequence, labs often use a nearby polymorphic VNTR marker.
Linkage Example: If an affected parent has markers $A$ and $B$ , and their affected children all inherit allele $A$ , it is deduced that the disease mutation is physically located on the chromosome carrying allele $A$ in that specific family.

Pre-implantation Genetic Diagnosis (PGD): Used to test embryos to ensure they have the lowest possible risk of inheriting a mutation.
Modified Risk Scenarios (Example: Autosomal Dominant Mutation):
- Suppose a daughter has a \text{50%} risk because her father was affected, but her own status is unknown.
- Her baseline risk to pass it to a fetus is $1/2 \times 1/2 = 1/4$ or \text{25%}.
- If marker analysis shows the fetus inherited the chromosome the daughter got from her unaffected mother (grandmaternal allele), the risk drops to \text{0%}.
- If the fetus inherited the chromosome from the affected grandfather (grandpaternal allele), the risk remains at \text{50%} (matching the mother's risk).
- If the parents sharing identical marker alleles (e.g., both are $AB$ ), the markers are non-informative, and the risk remains the baseline \text{25%}.
Best Practices: In clinical practice, a "panel of markers" (upstream, downstream, and internal to the gene) is used to ensure no silent recombination events have occurred.