8_Linkage_and_recombination_Sp2025_abb

Genetic Linkage and Recombination

1. Creation of Genetic Linkage

  • Genetic linkage occurs when two loci are located on the same chromosome.

  • During meiosis, homologous chromosomes exchange genetic material through a process known as crossing over.

  • Parental gametes contain allele combinations that are identical to those present in the parents (e.g., AB and ab).

  • Recombinant gametes arise from crossing over, leading to new combinations of alleles (e.g., aB and Ab).

2. Calculating Recombination Frequency

  • Recombination frequency (RF) can be calculated using the formula:

    [ RF = \frac{\text{Number of Recombinants}}{\text{Total Number of Progeny}} \times 100 ]

  • For example, in a dihybrid cross, count the number of progeny for each phenotype after a test cross to compute RF.

3. Coupling vs. Repulsion Configurations

  • Coupling (cis) configuration: Dominant alleles are located together on the same homolog (e.g., AB/ab).

  • Repulsion (trans) configuration: Dominant alleles are on different homologs (e.g., Ab/aB).

  • Understanding the configuration is essential as it influences the outcomes of gamete production and the expected phenotypic ratios in offspring.

4. Predictions Based on Recombination Frequency

  • Using the RF, predictions can be made about the proportions of offspring phenotypes:

    • If genes are unlinked, the expected phenotypic ratio is 1:1:1:1.

    • If linked, there will be a deviation from this ratio, with >50% in nonrecombinant (parental) phenotypes and <50% in recombinant phenotypes.

5. Inferring Genetic Distance

  • The relative genetic distance between loci can be inferred from offspring phenotypes/genotypes in a test cross.

  • The greater the proportion of recombinant offspring, the greater the physical distance between the loci on the chromosome.

6. Correlation of Recombination Frequency and Crossing Over

  • The frequency of recombinant gametes correlates directly with the frequency of crossing over during meiosis I.

  • The maximum recombination frequency is capped at 50%, indicating loci that are either on different chromosomes or far apart on the same chromosome.

General Aspects of Linked Genes

Crossing Over and Genetic Diversity

  • Crossing over during meiosis I leads to genetic variation as non-sister chromatids recombine, creating new allele combinations.

  • A dihybrid (AB/ab) for linked genes produces four gamete genotypes: two nonrecombinant (parental: AB, ab) and two recombinant (nonparental: aB, Ab).

Test Cross Outcomes

  • Nonrecombinant (NR) gametes will exceed recombinant (R) gametes in linked genes, resulting in a skewed phenotype ratio.

  • The separation of linked genes occurs through recombination, enabling analysis of recombination frequency and genetic distance (measured in centimorgans, cM).

Calculating Genetic Distance: Examples

  • Example 1: A-to-B genetic distance calculated based on progeny:

    • Total progeny = 7419

    • Total recombinants = 1717

    • RF = (1717/7419) x 100 = 23 cM.

Linkage and Testcrosses in Drosophila

  • In Drosophila tests, when assessing linked genes, the less frequent phenotypes from a testcross indicate recombinant offspring.

  • For example, if H and N are linked and a test cross is performed, the phenotypic ratios will reflect the linkage and deviate from 1:1:1:1 ratios.

Final Considerations

  • The upper limit to recombinant gamete production is capped at a maximum of 50%.

  • Even with frequent crossovers, only 50% of the gametes can be recombinants due to random crossover locations along the chromosome.