Genetic Linkage, Recombination, and Mapping Study Notes

Genetic Linkage, Recombination, and Mapping Study Notes

Course Information

  • Course Topic: Genetic Linkage, Recombination, and Mapping

  • Week: 8

  • Exam Details:

    • Exam #2 scheduled for Thursday.

    • Extra Credit Exam will cover material from Exams #1 - 3 and will replace the lowest exam score among these.

    • Extra Credit Exam is scheduled for Thursday, December 4, during class, replacing lecture and Quiz #7 originally scheduled for that day.

I. Gene Linkage

  • Concept: Gene Linkage refers to the association between genes located on the same chromosome, which results in alleles being inherited together instead of independently assorting.

  • Mendel's Law of Independent Assortment: States that segregating pairs of unit factors assort independently during gamete formation. This law only applies when genes are located on separate chromosomes.

  • Implication: When genes are found on the same chromosome, they do NOT assort independently.

A. Dihybrid Phenotype Ratios

  • In traditional Mendelian dihybrid crosses where the two genes are on separate chromosomes, the expected phenotype ratio is 9:3:3:1.

  • When two genes are linked (on the same chromosome), the dihybrid phenotypic ratio changes, requiring analysis of linkage ratios.

B. Linkage Ratio

  • Linkage Ratio: A unique F2 phenotype ratio resulting from the F1 mating of individuals who are each heterozygous for two linked genes without crossing over. For example, in Drosophila, considering the traits of eye color (Brown vs Red) and vein thickness (Thin vs Heavy):

    • Wild Type = +

    • Brown Eyes (bw)

    • Heavy Veins (hv)

  • Example Phenotype Ratio for Linked Genes: 1:2:1.

II. Crossing Over

  • Definition: Crossing over is the exchange of parts of chromosomes between non-sister chromatids from homologous chromosomes during meiosis.

  • Consequences: This can lead to recombinant (non-parental) gametes.

  • Mechanism:

    • Crossing over involves one set of non-sister chromatids; the outer chromatids do not cross over.

    • Distance between genes influences the frequency of crossing over; closer genes have a lower likelihood of crossover events.

A. Single and Multiple Crossovers

  • Single Crossovers: Occur randomly along non-sister chromatids; the distance between two genes determines their crossover frequency. As distance increases, the frequency of recombinant gametes rises but does not exceed 50%.

  • Multiple Crossovers: More than one crossover event can occur, leading to a change in gametes. At least three genes need to be analyzed to observe double crossovers.

III. Testing for Gene Linkage and Crossing Over

A. Standard Test Cross for Linkage

  • Method: A dihybrid test cross involves crossing a heterozygous individual (AaBb) with a tester individual who is homozygous recessive for both genes (aabb).

  • Purpose: To determine whether the alleles assort independently or are linked.

B. Outcomes of Gene Linkage

  1. Complete Linkage:

    • When two genes are linked without crossing over, produces parental allele combinations in equal frequency (50% each).

  2. Incomplete Linkage:

    • If crossing over occurs, non-crossover gametes (AB and ab) occur at higher frequencies than crossover gametes (Ab and aB).

    • Non-crossover gametes are both over 25%; crossover gametes are under this frequency.

C. Summary of Diagnostic Cross Results

  • For unlinked genes, all four F2 phenotype combinations appear with equal proportions (25% each).

  • Complete linkage results in only two phenotypes appearing at equal proportions (50% each).

  • Incomplete linkage reflects unequal proportions, typically with two parental types being more frequent than two non-parental types.

D. Notation for Linked Genes

  • Genotypes are indicated with two lines representing homologous chromosomes, where alleles are written above and below the lines to represent their positions. Example:

    • Heterozygote: A B

    • a b

IV. Gene Mapping

A. Recombination Frequency (RF)

  • Formula for calculating RF:
    RF = \frac{\text{# recombinant progeny}}{\text{total # of progeny}} \times 100

  • Example calculation of RF:

    • Given 40 recombinants out of 200 total progeny:
      RF = \frac{40}{200} \times 100 = 20\%

  • The RF percentage indicates the distance between two genes and is also used to express map units (1 map unit = 1% RF).

B. Crossover Frequency and Distance

  • Crossover likelihood varies based on gene positions:

    • Genes that are extremely close exhibit zero likelihood of crossover (complete linkage).

    • Farther genes have a higher likelihood of crossover, leading to incomplete linkage and observable recombinant frequencies.