In-Depth Notes on Linkage, Recombination, and Eukaryotic Gene Mapping

Linked Genes and Recombination

I. Linked Genes and Recombination

A. Linked Genes (no crossing over - XO)

• Linkage Arrangements: Linked genes do not assort independently; they are often inherited together due to their physical proximity on the same chromosome.

• Genotype Presentation:

  • In a double heterozygote (Aa Bb), linked genes can be presented in two configurations:
  • Cis Configuration: AB/ab (both dominant alleles on the same homolog)
  • Trans Configuration: Ab/aB (dominant and recessive alleles on different homologs)

• Gametic Outcomes for Linked Genes:

  • Gametes formed without crossing over will produce non-recombinant gametes.
  • Example for Yy Rr (double heterozygote) consideration:
  • Gametes: YR, Yr, yR, yr
  • Outcomes (assuming independent assortment): 25% for each combination.

• Offspring Outcomes for Linked Genes:

  • Example cross: Aa Bb x Aa Bb can show deviations from expected Mendelian ratios if genes are linked.

B. Linked Genes (with a XO event)

• Gametic Products:
  • If a crossover occurs between linked genes, recombination can produce new combinations of alleles.
  • Example:
    • For genes A and B, initial heterozygote AB/ab leads to possible gametes: AB, ab, Ab, aB.

C. Determining if Genes Are Linked

• Two Methods:
  • Controlled Cross: Look for expected Mendelian ratio for unlinked genes, such as 9:3:3:1 from AaBb x AaBb.
  • Test Cross: Cross a double heterozygote with a completely homozygous recessive individual (AaBb x aabb) should lead to a 1:1:1:1 ratio if unlinked.

II. Mapping Eukaryotic Genes Using Recombination Frequency

A. The Two Point Test Cross

• Definition: A genetic cross used to determine the linkage and map distance between two genes.

  • Involves a double heterozygote crossed with a double homozygous recessive.

• Expected Outcomes:

  • Identify and compare phenotypes of offspring to determine linkage and calculate recombination frequency (RF).

B. Genetics of Linkage

• Mapping Distance:
  • Recombination frequency is directly related to the physical distance between two genes.
  • Formula: 1% RF = 1 Map Unit (m.u.) or 1 centiMorgan (cM).
  • Genes that are tightly linked (highly rearranged) will show lower RF.

C. Conclusion on Genetic Mapping

• Importance of Linkage Mapping:

  • Determines the physical location of genes on chromosomes, provides insights into genetic inheritance patterns, and assists in isolating specific alleles for traits based on linkage patterns.

• Practical Implications:

  • Understanding gene mapping has applications in traits selection in agriculture, breeding, and understanding genetic diseases in humans.