Linkage, Recombination, and Eukaryotic Gene Mapping

7.1 Linked Genes Do Not Assort Independently

  • Linked genes are defined as genes that are located close to each other on the same chromosome.

  • They assort from each other during meiosis only if crossing over occurs between them.

Recall:

  • Principle of Segregation: Alleles separate during meiosis.

  • Independent Assortment: Alleles at one locus sort independently from those at another locus.

  • Recombination: The process by which alleles sort into new combinations.

7.2 Recombination is the Sorting of Alleles

  • F1 gametes can be categorized into:

    • Non-Recombinant Gametes: Same as the parent.

    • Recombinant Gametes: New combinations of alleles.

Example from Bateson, Saunders, and Punnett (1905)

  • Study on Sweet Peas showed non-independent assortment of flower color and pollen shape.

  • Expected 9:3:3:1 ratio was

    • Expected Progeny Ratio: 214:71:71:24

    • Observed: 339 non-recombinant progeny and 42 recombinant progeny.

7.3 Linked Genes Segregate Together and Crossing Over Produces Recombination

  • Complete Linkage leads to non-recombinant gametes and progeny.

  • Crossing Over between linked genes results in recombinant gametes and progeny.

  • If crossing over occurs between normally linked loci, they will sort independently.

  • Crossing over occurs between non-sister chromatids on homologous pairs.

    • Chromatids involved in crossing over are classified as recombinant (representing 50% of total gametes).

    • Others are non-recombinant (remaining 50% of gametes).

7.4 Observed Results with Test Crosses

  • Test crosses can show differences in offspring distribution related to linkages.

  • Standard Notation for Independent Assortment:

    • Example: AA BB x aa bb leads to F1: Aa Bb and F2 with expected ratios.

  • If genes are linked, allele combinations must be indicated:

    • Example Cross:

    • A B a b → P Cross produces offspring ratios deviating from expected due to linkage.

  • Coupling: Wild-type alleles linked.

  • Repulsion: Wild-type and mutant alleles exist on the same chromosome.

7.5 Recombination Frequency Calculation

  • Recombination Frequency:

    • Formula: ext{Recombination Frequency} = \frac{\text{Number of Recombinant Progeny}}{\text{Total Number of Progeny}} \times 100\%

    • Example Calculation:

    • For 15 recombinants out of 123 total: \text{Frequency} = \frac{15}{123} \times 100\% = 12.2\%

  • More than 50% non-recombinant gametes indicate close linkage.

7.6 Effects of Gene Configuration (Coupling vs. Repulsion)

  • Effects of coupling and repulsion on test cross results can be significant.

    • AB/ab (coupling) vs. Ab/aB (repulsion).

7.7 Understanding Testcross Results

  • Examples of offspring distribution under various assignments:

    • For independent assortment:

    • Progeny: 25% Aa Bb (non-recombinant), 25% aa Bb, …

    • For complete linkage:

    • 50% non-recombinant.

    • For linkage with some crossing over:

    • More than 50% non-recombinant and less than 50% recombinant.

7.8 Concept Check: Single Crossovers vs. Double Crossovers

  • Assertion: The frequency of recombinant gametes is half that of crossover occurrences.

  • Scenario: Testcross of AaBb x aabb showing progeny ratios used to conclude the configuration of genes.

7.9 Evidence for Physical Basis of Recombination

  • Walter Sutton’s chromosome theory of inheritance posits that genes reside on chromosomes.

  • Sexual determination linked to specific chromosomes studied by Nettie Stevens and Edmund Wilson.

  • Nondisjunction linked with traits in Drosophila by Calvin Bridges.

  • Harriet Creighton & Barbara McClintock demonstrated physical exchanges between chromosomes led to recombination observed in experiments.

7.10 Predictive Value: Expected Outcomes from Known Recombination Frequencies

  • Predict offspring ratios based on recombination frequency assessed during crosses with linked genes.

  • Genetic maps quantified using “map units” or “centiMorgans (cM)” to measure distances.

    • Genetic distances calculated and reported similarly to 5% recombination being equivalent to 5 cM.

7.11 Utilizing a Three-Point Testcross for Gene Mapping

  • A three-point testcross allows mapping of three linked genes in a single progeny set, enhancing efficiency over two-point crosses.

    • Identify non-recombinants (most numbers) and double-crossover progeny (least numbers) for order determination.

    • The characteristic differing between non-recombinant and double-crossover progeny identifies the middle gene.

7.12 Mapping Distance Determination from Testcross Results

  • Recombination frequency helps derive physical mapping distances evidenced by specific calculations for accuracy.

7.13 Genetic Mapping Limitations & Insights

  • Variability in recombination rates, leading to discrepancies between genetic and physical maps.

  • Frequency variations noted across species, sexes, and chromosome regions ultimately complicate estimates for nucleic distances.

7.14 Physical Mapping Techniques for Gene Location

  • Methods such as somatic-cell hybridization, deletion mapping, and molecular analysis techniques like FISH assist in accurately locating genes on chromosomes.

7.15 Somatic-Cell Hybridization

  • A method for determining which chromosome houses a targeted gene.

  • Processes through cell fusion and analyzing dropout rates from various hybrid cells.

7.16 Deletion Mapping

  • Effectively reveals chromosomal placements of recessive genes through hemizygous expression and characteristics.

7.17 Chromosome Mapping with Molecular Analysis

  • Utilizes single-stranded complementary DNA probes in FISH to visualize gene placements in chromosomes.

7.18 Practice Problems and Application Questions

  • Problems listed for practical understanding and application, including exercises for conceptual reinforcement.

    • Refer to pages numbered for specific problem statements and solutions.