lecture 19 & 20 Genetic Linkage and Mapping Notes

Genetic Linkage and Mapping

  • Thomas Hunt Morgan's Contribution
    • Awarded the Nobel Prize for establishing the chromosome theory of inheritance.
    • Identified and explained genetic linkage and recombination.
    • Applied linkage and recombination to genetic mapping.

Key Concepts in Genetic Linkage and Recombination

  • Genetic Linkage

    • Refers to the tendency of genes located close to each other on a chromosome to be inherited together.
    • Linked genes do not assort independently during meiosis, leading to certain allele combination patterns in offspring.

  • Recombination

    • The process that generates new combinations of alleles through crossing over during meiosis.

Mendel's Principles Summarized

  • The Law of Segregation

    • Each trait is defined by a pair of alleles which segregate into separate sex cells.
    • Offspring inherit one allele from each parent.

  • The Law of Independent Assortment

    • Alleles of two (or more) different genes assort independently of one another.

  • The Law of Dominance

    • If two alleles are present, one (dominant) can mask the expression of the other (recessive).

Linkage and Genetic Mapping

  • Concept of Linkage

    • Genes that are far apart (≥ 50cM) on a chromosome assort independently (unlinked).
    • Genes that are closer together are likely to be inherited together (linked).

  • Types of Linkage

    • Complete linkage: Genes are very close and no recombination occurs, leading to only parental-type gametes.
    • Partial linkage: Occasional crossing over occurs, resulting in a mixture of parental and recombinant gametes.

Mechanisms of Genetic Linkage

  • Independent assortment

    • Occurs when genes are on different chromosomes or are far apart on the same chromosome, leading to a 1:1:1:1 ratio of offspring phenotypes.

  • Crossover Events

    • Occurs during prophase I of meiosis, producing recombinant chromosomes.
    • The probability of recombinant gametes correlates with the physical distance between genes.

Calculating Recombination Frequencies

  • Recombination Frequency Formula:
    r=number of recombinantstotal offspring×100r = \frac{\text{number of recombinants}}{\text{total offspring}} \times 100

    • Recombination frequency is used to estimate the distance between genes on a genetic map, measured in map units (centiMorgan; cM).

  • Mapping Distance Metrics

    • 1% recombination = 1 map unit (cM).
    • Closer genes have lower recombination frequency; farther genes have higher.

Experimental Analysis of Linkage

  • Testcross:

    • A cross between a dihybrid and a homozygous recessive to identify linkage between two genes.
    • Examine progeny proportions to determine deviation from expected ratios, indicating linkage.

  • Chi-square Analysis:

    • Used to compare observed offspring proportions against expected proportions of independently assorting genes.

Three-Point Test-Cross Analysis

  • More efficient than two-point tests for mapping as it helps determine the order of three linked genes simultaneously.
  • The analysis involves identifying parental, single crossover, and double crossover offspring.

Gene Mapping Tools

  • Genetic Markers:
    • Use of variable number tandem repeats (VNTRs), single nucleotide polymorphisms (SNPs), and restriction fragment length polymorphisms (RFLPs).
    • Genetic markers assist in tracking the inheritance of genes through generations.

Application of Genetic Mapping

  • Genome-Wide Association Studies (GWAS):

    • Identify genetic variants associated with specific diseases by associating certain traits with clustered SNPs.

  • Linkage Equilibrium and Disequilibrium:

    • Linkage equilibrium suggests random assortment of alleles, while linkage disequilibrium indicates non-random associations of alleles.

  • Importance of Genetic Mapping:

    • Essential for identifying genes related to diseases and understanding genetic contributions to phenotypes.