Linkage and recombination are key concepts in genetics. They explain how genes on the same chromosome tend to be inherited together, challenging Mendel's law of independent assortment. This phenomenon affects genetic variation in offspring and shapes inheritance patterns.

Understanding linkage and recombination is crucial for predicting genetic outcomes. These processes influence how traits are passed down, impact genetic diversity, and play a role in evolution. They're also essential for creating genetic maps and studying disease-associated genes.

Linkage and Recombination

Linkage and gene inheritance

• Linkage refers to the tendency of genes located on the same chromosome to be inherited together more often than expected by chance
  • Violates Mendel's law of independent assortment which states that genes on different chromosomes assort independently during gamete formation
  • Results in certain combinations of alleles being inherited together more frequently (linked genes on the same chromosome)
• The closer two genes are on a chromosome, the higher the probability they will be inherited together due to lower chances of recombination occurring between them
  • Recombination frequency between genes used as a measure of their linkage and distance on the chromosome (centiMorgans or map units)
• Linkage reduces the number of possible gamete combinations compared to independent assortment, limiting the genetic variation in offspring
• Linkage can be complete or incomplete depending on the distance between genes on the chromosome
  • Complete linkage occurs when genes are so close together that they always inherit together with no recombination (no genetic variation in offspring)
  • Incomplete linkage occurs when genes are far enough apart that crossing over and recombination can occur between them (some genetic variation in offspring)

Crossing over for genetic variation

• Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis
  • Homologous chromosomes pair up and form a protein structure called the synaptonemal complex
  • Non-sister chromatids of homologous chromosomes can break and rejoin at corresponding points, exchanging genetic material
• Crossing over creates new combinations of alleles on the same chromosome not present in the parents
  • Introduces genetic variation in offspring by shuffling linked alleles on chromosomes
  • Allows for recombination and separation of linked genes that would otherwise always be inherited together
• The farther apart two genes are on a chromosome, the more likely crossing over will occur between them due to more points of potential breakage and rejoining
• Crossing over frequency between genes used to estimate their relative distance and position on the chromosome
  • Measured in centiMorgans (cM) or map units where 1 cM = 1% recombination frequency
  • Higher crossing over frequency indicates greater distance between genes on the chromosome

Recombination frequency calculation

• Recombination frequency (RF) is the percentage of recombinant offspring produced in a genetic cross involving linked genes
  • Calculated by dividing the number of recombinant offspring by the total number of offspring and multiplying by 100
  • $RF = \frac{number \space of \space recombinant \space offspring}{total \space number \space of \space offspring} \times 100$
• Recombination frequency used to estimate the genetic distance and linkage between genes on a chromosome
  • Genes with a higher RF are farther apart and less tightly linked than genes with a lower RF
  • Genes with a 50% RF are considered unlinked and assort independently according to Mendel's law
• Recombination frequencies between multiple genes can be used to construct genetic linkage maps
  • Shows the relative positions and distances of genes on a chromosome based on recombination frequencies
  • Useful for predicting inheritance patterns and mapping disease-associated genes in humans (gene therapy)

Parental vs recombinant genotypes

• In a genetic cross involving linked genes, the offspring can have either parental or recombinant genotypes and phenotypes
• Parental genotypes and phenotypes:
  1. Offspring that inherit the same combination of alleles for the linked genes as one of the parents
  2. Result from gametes that did not undergo crossing over and recombination between the linked genes
  3. Maintain the original linkage and allele combinations present in the parents
• Recombinant genotypes and phenotypes:
  1. Offspring that inherit a new combination of alleles for the linked genes not present in either parent
  2. Result from gametes that underwent crossing over and recombination between the linked genes
  3. Break the original linkage and create novel allele combinations not seen in the parents
• The frequency of recombinant offspring depends on the genetic distance and linkage between the genes
  • Genes that are closer together will have fewer recombinant offspring due to lower chances of crossing over (more parental offspring)
  • Genes that are farther apart will have more recombinant offspring due to higher chances of crossing over (more genetic variation)