Genetics Testcross and Linkage
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Questions and Answers

What phenotype corresponds to the genotype b+ vg+ pr+?

  • Wild type body, vestigial wings, purple eyes
  • Wild type body, wild type wings, wild type eyes (correct)
  • Black body, vestigial wings, purple eyes
  • Wild type body, wild type wings, purple eyes

    • Which of the following phenotypes represents the genotype b vg+ pr?

  • Black body, wild type wings, purple eyes (correct)
  • Wild type body, vestigial wings, purple eyes
  • Wild type body, wild type wings, purple eyes
  • Black body, vestigial wings, wild type eyes

    • What effect do double crossovers have on genetic mapping?

  • They help in completely eliminating recombinant offspring.
  • They are less likely to occur than single crossovers.
  • They increase the accuracy of distance measurements.
  • They revert the loci to the parental arrangement. (correct)

    • What is the total number of testcross progeny observed?

    <p>1005</p> Signup and view all the answers

    Which phenotype has the highest count in the testcross progeny?

    <p>Wild type body, wild type wings, wild type eyes</p> Signup and view all the answers

    When is it necessary to perform a c2 goodness of fit test?

    <p>When the recombination frequency is greater than 25%.</p> Signup and view all the answers

    Why are full genetic maps difficult to construct in humans?

    <p>There is a small number of progeny available.</p> Signup and view all the answers

    What is the effect of the third gene on the progeny phenotype determination?

    <p>It has no effect on phenotype expression.</p> Signup and view all the answers

    Which statement highlights the reality of crossover events?

    <p>Crossover events can occur between multiple chromatid strands.</p> Signup and view all the answers

    What is a primary challenge in assessing recombination frequencies for genes that are far apart?

    <p>Inaccurate measurements with increased distances.</p> Signup and view all the answers

    What is the primary purpose of a testcross in the context of linked genes?

    <p>To identify the genotype of an individual with a dominant phenotype</p> Signup and view all the answers

    During the testcross, which phenotypes were produced in the F1 generation?

    <p>pr+ vg+ and pr vg+</p> Signup and view all the answers

    Which of the following phenotypic proportions was recorded from the testcross outcomes?

    <p>1339 wildtype, 154 vestigial, 151 purple, 1195 purple and vestigial</p> Signup and view all the answers

    How does the discovery of linkage influence our understanding of genetic inheritance?

    <p>It demonstrates that genes can be inherited together more often than expected</p> Signup and view all the answers

    What combinations represent linked genes in the given testcross?

    <p>pr+ vg+ with pr vg</p> Signup and view all the answers

    What is the percentage of recombinants identified for the first trait combination?

    <p>6.1%</p> Signup and view all the answers

    Which trait combination contributes the highest number of parental types?

    <p>Wild type body and wild type wings</p> Signup and view all the answers

    What is the total number of offspring observed in the experiment?

    <p>1,005</p> Signup and view all the answers

    How many total recombinants were calculated for the black body and vestigial wing combination?

    <p>179</p> Signup and view all the answers

    Which trait combination shows a total of 60 offspring with black body and wild type eyes?

    <p>Black body and purple eyes</p> Signup and view all the answers

    What percentage corresponds to the total recombinants seen for the combination of purple eye and vestigial wing?

    <p>17.8%</p> Signup and view all the answers

    Which of the following combinations had the least number of recorded offspring?

    <p>Black body, wild type wings, and wild type eyes</p> Signup and view all the answers

    What is the correct recombinant calculation for the trait combination of purple eye and vestigial wing?

    <p>61</p> Signup and view all the answers

    What is the recombination frequency (RF) between loci b and vg?

    <p>18.4%</p> Signup and view all the answers

    What does a coefficient of coincidence (C) of 0.4 indicate in this context?

    <p>There is interference affecting double crossover occurrences.</p> Signup and view all the answers

    How do you calculate the expected number of double crossovers (DCO) among 1005 offspring?

    <p>By multiplying the total offspring by the chances of crossover between b and pr and pr and vg.</p> Signup and view all the answers

    What impact do double crossovers have on the parental arrangement of loci?

    <p>They appear as parental for the outer loci but recombinant for the middle locus.</p> Signup and view all the answers

    When calculating true recombination frequency, how are double recombinants factored in?

    <p>They are multiplied by two and added to original recombinants.</p> Signup and view all the answers

    What is the chance of double crossover occurring between loci b and vg based on their recombination frequencies?

    <p>0.0075</p> Signup and view all the answers

    What does the calculated distance of 18.4 mu represent?

    <p>The distance between loci b and vg, after considering double crossovers.</p> Signup and view all the answers

    What does the presence of fewer double crossovers than expected suggest about the genetic loci?

    <p>The loci are closely linked, causing interference.</p> Signup and view all the answers

    Study Notes

    Linkage and Testcrosses

    • Linkage is examined through testcrosses, which reveal the relationship between genes.
    • Example cross: P1 (pr+ pr+; vg+ vg+) and P2 (pr pr; vg vg) produce wild type and purple, vestigial phenotypes.
    • The testcross of F1 generation pr+ pr; vg+ vg with pr pr; vg vg yields specific gametes.

    Phenotype Proportions

    • Observed phenotypes from testcross:
      • Wild type: 1339
      • Vestigial: 154
      • Purple: 151
      • Purple, vestigial: 1195
    • Total offspring: 2839
    • Recombinant frequencies (RF) indicate linkage between genes, with specific RF values:
      • pr and vg: RF = 37.5%, map distance = 57.6 mu
      • w and m: RF = 45.2%, map distance = 56.6 mu

    Double Crossovers

    • Double crossovers can obscure mapping, reverting genes to parental arrangements and complicating interpretations.
    • The likelihood of double crossovers increases with distance between gene loci.

    Testing Gene Linkage

    • High RF (>25%) may suggest that genes are linked; confirm through chi-square goodness of fit testing.
    • Supporting the null hypothesis indicates chance variation, supporting unlinking of genes.
    • Rejecting the null suggests variation resulting from linkage.

    Genetic Mapping Challenges

    • Genetic maps often utilize intermediate genes due to inaccuracies in mapping far-apart genes.
    • Mapping in humans is difficult due to limited progeny, inability to perform desired crosses, and reliance on pedigree analysis.

    Assumptions in Genetic Mapping

    • Crossovers are typically assumed to occur only between non-sister chromatids for simplicity.
    • Reality includes potential double crossovers among four chromatids.

    Mapping with Trihybrid Test Crosses

    • Possible female gametes illustrate genetic combinations.
    • All testcross male gametes yield the same genotype (b vg pr).
    • Progeny phenotype proportions provide data for mapping distances.

    Calculating Map Distances

    • Analyze pairs of genes to calculate recombination frequencies.
    • Example calculations yield:
      • Black body and purple eye recombinants: 6.1%
      • Black body and vestigial wing: 17.8%
      • Purple eye and vestigial wing: 12.3%

    Influence of Double Crossovers

    • Double crossovers between outer loci appear as parental combinations but modify frequencies for the inner locus.
    • Adjust calculations for true RF by including double recombinants.

    Interference and Coefficient of Coincidence

    • Interference assesses whether one crossover affects the occurrence of another nearby.
    • Expected double crossovers calculated using map distances.
    • Coefficient of coincidence (C) indicates the relationship between observed and expected double crossovers, calculated as:
      • C = Observed DCO / Expected DCO, providing insight into crossover interactions.

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    Description

    This quiz explores the concept of genetic linkage through testcrosses, specifically focusing on the linkage of the pr and vg genes. It examines the results of crossing wild type and mutant phenotypes, providing insights into the inheritance patterns of linked genes.

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