Lecture 14

By the end of the lecture, you should:
- Remind yourself about important genetics terms.
- Understand how polymorphisms can be used as genetic markers.
- Recognize how these markers can aid in building a genetic map of linkage.
- Appreciate the differences between a genetic map and a physical map of a chromosome.

During the 1980s and 1990s, scientists aimed to construct maps detailing the arrangement of genes on chromosomes of various species, including humans.
The motivation behind building these maps includes:
- Enabling the mapping and identification of disease genes (previously discussed in Year 3: BM321).
- Supporting the Human Genome Sequence Project (Lectures 3/4).

Meiosis is a fundamental process for shuffling genetic variation across generations.
During meiosis, the following key processes occur:
- Homologous chromosomes pair up, forming chiasmata.
- Crossing-over events result in recombinant chromatids, contributing to genetic diversity.

Linkage analysis reveals:
1. Whether two traits are inherited together.
2. The physical distance between loci on a chromosome.
Linkage can be measured by analyzing numerous meiotic events to determine if two traits or variants are consistently inherited together or separated.
Example from Drosophila (fruit flies):
- Certain mutant traits (e.g., white-eye mutants) do not segregate independently, indicating physical proximity on the chromosome due to linkage.

Linkage is defined as the likelihood of meiotic recombination occurring between two loci.
This can be classified into:
- Strong linkage: Indicates close physical proximity.
- Weak linkage: Reflects greater separation between loci, leading to more frequent recombination.

Traditional methods of genetic mapping, like those used in model organisms, cannot easily apply to humans due to ethical and practical limitations:
- Cannot set up experimental crosses.
- A lack of sufficient visible traits or mutations to warrant mapping efforts.

New molecular biology technologies have emerged, enabling the use of polymorphisms—specifically those without corresponding phenotypes—as genetic markers. Important technologies include:
- Restriction Fragment Length Polymorphisms (RFLPs)
- DNA sequencing
- Gel electrophoresis/Southern blotting
- Polymerase Chain Reaction (PCR)
Initial strategies in the late 1970s involved examining the influence of one polymorphic locus allele over another.
Common markers have evolved from microsatellites (SSR/STRs) to single nucleotide polymorphisms (SNPs).

In studies of familial disease:
- Researchers require two traits/variants: one present in affected individuals (mutated gene) and another to facilitate linkage analysis.
The concept of "coincidence of segregation" is introduced as a method for linking genetic markers to disease phenotypes.

Using identified microsatellite loci, researchers analyze the inheritance of alleles in family groups (trios: mother, father, child) to assess linkage and inheritance patterns.
Potential outcomes include:
- Complete linkage: Alleles are inherited together every time.
- Partial linkage: Some instances of recombination occur.
- No linkage: Alleles assort independently.

A haplotype refers to the order of marker alleles along a chromosome, constructed from raw genotyping data:
- Assembling data from parents and children enables researchers to infer which alleles are inherited together, leading to the construction of a haplotype map.

Mapping methodology includes:
- Utilizing large reference families or pedigrees to study extensive meiotic events.
- Creating linkage maps by establishing the relative positions of genetic markers based on recombination frequencies observed in family generations.
Importance of meiotic recombination as the core process within genetic mapping studies.

A significant achievement in genetic mapping involved a comprehensive genetic linkage map that:
- Consisted of 1416 loci, including 279 genes and expressed sequences.
- Utilized 1676 polymorphic systems genotyped with the CEPH reference pedigree resource.
- Featured 339 microsatellite repeat markers.
- Provided high coverage across the human genome, spanning at least 92% of autosomal length and 95% of the X chromosome, allowing insights for disease gene localization.

Genetic mapping, while informative, is primarily an estimate of marker locations and requires validation through physical DNA examination during the Human Genome Sequencing Project.
Physical mapping necessitates:
- Organizing and distributing vast amounts of DNA data.
- Creating genomic DNA libraries to ease access and manipulation of DNA fragments.

The generation of genomic DNA libraries is crucial due to:
- The complex nature of entire genomes, making them impractical for direct analysis.
- Ability to produce unlimited copies of specific DNA fragments for experimental purposes.
Cloning involves:
- Transformation of bacteria with plasmids carrying genetic material to generate multiple copies of DNA segments.

Common cloning methods include:
- COSMID library construction, involving bacteriophages.
- Bacterial Artificial Chromosome (BAC) and Yeast Artificial Chromosome (YAC) libraries for larger DNA segments (up to 1,000,000 bp).

To create physical maps, researchers need to:
- Utilize BAC/YAC clones as probes for fluorescent in situ hybridization (FISH) to identify chromosomal locations.
- Analyze overlapping clones to assemble comprehensive maps of genetic markers.

A contig is defined as a continuous stretch of genomic DNA derived from overlapping DNA clones, allowing for detailed examination of specific genomic regions linked to genetic disorders.

Polymorphisms serve as invaluable markers for genetic mapping.
Genetic mapping relies on meiotic data to determine linkage.
Physical mapping is established through the integration and alignment of genomic DNA library clones into continuous sequences, or contigs, enabling a clearer understanding of the genome's structure and function.