Study Guide for Chapter 7: Linkage, Recombination, and Eukaryotic Gene Mapping

7.1 Linked Genes Do Not Assort Independently

• Recall:

  • Principle of Segregation: Alleles separate during meiosis.

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

  • Recombination: Alleles sort into new combinations.

• Definition of Linked Genes: Linked genes are located close together on the same chromosome; they assort together only if crossing over occurs between them.

7.2 Independent Assortment and Dihybrid Crosses

• Recall Independent Assortment: Mendel's Dihybrid Crosses

  • Recombinant Gametes: New combinations of alleles formed during meiosis.

  • Non-recombinant Gametes: Same as the parent alleles.

  • F2 Generation: New combinations in the F2 generation yield recombinant progeny.

7.3 Recombination Description

• Recombination Definition: Sorting of alleles into new combinations.

• F1 Gametes Types:

  • Non-recombinant Gametes: Identical to the parent alleles.

  • Recombinant Gametes: New combinations not found in parents.

7.4 Historical Evidence of Non-independent Assortment

• Bateson, Saunders, and Punnett (1905): Reported non-independent assortment in sweet peas:

  • Expected Ratio: 9:3:3:1 not observed; instead, 339 non-recombinant progeny and 42 recombinant progeny were recorded from 381 total progeny.

7.5 Linkage and Crossing Over

• Linked Genes and Segregation: Linked genes segregate together due to their physical proximity on the same chromosome.

• Crossing Over Definition: Produces recombination between linked genes, leading to the formation of recombinant gametes.

  • Complete Linkage Characteristics:

  • Leads to non-recombinant gametes and progeny.

  • If crossing over occurs, genes sort independently.

7.6 Mechanism of Crossing Over

• Crossing Over Occurrence:

  • Takes place between non-sister chromatids on homologous chromosomes.

  • Resulting Gametes: 50% recombinant and 50% non-recombinant gametes.

7.7 Analysis of Complete Linkage

• Testcrosses for Linkage Analysis:

• Figure 7.4 Observations: Appearance of non-recombinant and recombinant progeny can be determined.

7.8 Standard Notation for Genetic Crosses

• Independent Assortment Example Notation:

  • Parental Cross: AA BB x aa bb

  • F1 Generation: Aa Bb

  • F2 Generation: 9 AB : 3 Abb : 3 aaB : 1 aabb

• Linked Genes Notation Example: (AB/ab Cross)

  • Parental Generation: A B x a b

  • F1 Generation Results: 3:1 ratio.

• Coupling vs. Repulsion Configurations:

  • Coupling (cis configuration): WT genes linked on the same chromosome.

  • Repulsion (trans configuration): WT and mutant genes linked on the same chromosome, affecting testcross outcomes.

7.9 Recombination Frequencies

• Recombination Frequency Calculation:

  • Formula:
    $Recombination requency = \frac{Number ext{ of}<br>ecombinant ext{ progeny}}{Total ext{ progeny}} \times 100\%$

  • Example: 15 recombinants from 123 total progeny gives a recombination frequency of 12.2%.

7.10 Coupling and Repulsion Configurations

• Coupling Configuration:

  • Both wild-type alleles on one chromosome (AB) and both mutant alleles on another (ab).

• Repulsion Configuration:

  • Wild-type and mutant alleles found on the same chromosome in a heterozygous individual.

7.11 Testcross Results Interpretation

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• Different Situations and Progeny Results for Testcrosses:
  

  Situation	Progeny Type	Percentage
  Independent Assortment	AaBb (nonrecombinant)	25%
  Independent Assortment	aa bb (nonrecombinant)	25%
  Independent Assortment	Aa bb (recombinant)	25%
  Independent Assortment	aa Bb (recombinant)	25%
  Complete Linkage	AB/ab (nonrecombinant)	50%
  Complete Linkage	ab/ab (nonrecombinant)	50%
  Linkage with some crossing over (AB/ab)	More than 50%
  Linkage with some crossing over (Ab/ab)	Less than 50%
  7.12 Frequency of Recombinant Gametes

  • Single Crossovers: Frequency of recombinant gametes is half the frequency of crossing over due to the nature of testcross results.

  7.13 Evidence for Physical Basis of Recombination

  • Walter Sutton’s Chromosome Theory: Genes are located on chromosomes.

  • Nettie Stevens and Edmund Wilson: Indicated that sex is associated with specific chromosomes in insects.

  • Calvin Bridges’ Findings: Nondisjunction affecting eye color inheritance in Drosophila.

  • Harriet Creighton and Barbara McClintock (1931): Demonstrated intrachromosomal recombination through physical exchange experiments.

  7.14 Predictive Value of Recombination Frequency

  • Recombination Frequency: Allows for predictions regarding offspring in genetic crosses.

    • Formula:
      $Recombination requency = \frac{recombinant ext{ progeny}}{total ext{ progeny}} \times 100\%$

  7.15 Genetic Mapping

  • Gene Mapping:

    • Genetic maps are constructed using recombination frequencies measured in map units or centiMorgans (cM).

    • Example Frequencies:

    • Gene A and B: 5% (5 cM)

    • Gene B and C: 10% (10 cM)

    • Gene A and C: 15% (15 cM)

  7.16 Considerations for Double Crossovers

  • Double Crossovers: Can result in reduced apparent recombination frequency due to their infrequent nature but can also provide clearer insights on gene mapping.

  7.17 Three-Point Testcross for Mapping Linked Genes

  • Three-Point Testcross Benefits:

    • More efficient than two-point crosses.

    • Allows for order determination of three linked genes from a single progeny set.

    • Methods to identify:

    1. Identify non-recombinant progeny (two most common phenotypes).

    2. Identify double-crossover progeny (two least common phenotypes).

    3. Compare characteristics to determine the middle gene by the differing trait.

  7.18 Calculating Probability of Double Crossovers

  • Double Crossover Probability Formula:

    • Probability is the product of individual crossover probabilities.

  • Meiosis Phases: Visual depiction during meiosis for the relevant phases; Meiosis I prophase to Meiosis II.

  7.19 Mapping Procedures in Drosophila

  • Mapping in Drosophila:

    • Genes are named after their mutation examples.

    • Heterozygous females are always used since crossing over does not occur in males.

  7.20 Results of a Three-Point Testcross

  • Procedure Summary:

    • Writing results enables the arrangement of gene order based on abundance and positioning of double-crossover offspring.

  7.21 Mapping Distance Calculation in a 3-Point Test Cross

  • Recombination Frequencies Calculation: Examples demonstrated through abundance of progeny.

    • Calculate pm for different gene distances using gene arrangements.

  7.22 Concept Checks and Definitions in Testcrosses

  • Understanding Mapping Distance Among Genes: Binomial progeny data utilized to ascertain mapping positions in study activities.

  7.23 Double Crossovers and Interference

  • Impact of Interference: Double crossover frequencies lower than expected reflect possible interference inhibiting repeated crossovers.

  7.24 Summarizing Recombination Rate Variability

  • Variation in Recombination Rates: Varies widely based on species, chromosome regions, and sexes.

  • Genetic vs. Physical Maps Correlation: Maps illustrate relationships but can diverge in accuracy and reliability.

  7.25 Physical Mapping Methods

  • Methods Include:

    • Somatic-Cell Hybridization: Used to determine the chromosome containing a gene of interest.

    • Deletion Mapping: Reveals chromosomal locations of recessive genes.

    • Molecular Analysis Techniques:

    • Fluorescence In Situ Hybridization (FISH): Pinpoints gene locations using fluorescent probes.

  7.26 Final Thoughts and Suggested Problems

  • Suggested Problems for Reinforcement: Listed from assigned reading to practice understanding and verification of concepts discussed in the chapter.