Chapter 9 Answers

    • Crossovers can be observed cytologically directly under the microscope as chiasmata.
    • Recombination is defined genetically as the frequency calculated from the observed phenotypic proportions in the progeny.
    • Crossovers lead to recombination when they are detected using genetic marker loci.
    • Not all crossovers result in recombination – some can’t be detected because no visible markers are recombined.
    • Some recombinants involve crossovers, but not all recombinants result from crossovers.
    • Crossovers between non-sister chromatids can result in recombination, while crossovers between sister chromatids, which have identical alleles, will not show any recombination.
    • When there are two crossovers between the loci being scored for recombination, the result will appear to be parental, not recombinant.
    • Recombination can occur without crossovers when marker loci are on different chromosomes, which then assort independently.
  1. The use of pure breeding lines allows the researcher to be sure that he/she is working with homozygous (known) genotypes. If a parent is known to be homozygous, then all its gametes will have the same genotype. This simplifies the definition of parental genotypes and therefore the calculation of recombination frequencies.
  2. This tight linkage would suggest that individuals with the earlobe phenotype would likely carry alleles that increased their risk of cardiovascular disease. These individuals could, therefore, be informed of their increased risk and have an opportunity to seek increased monitoring and reduce other risk factors.
    1. It assumes that the loci are completely unlinked.
    2. The expected ratio would be all parentals and no recombinants. For example, if the parental gametes were AB and ab, then the gametes produced by the dihybrids would also be AB and ab, and the offspring of a cross between the two dihybrids would all be genotype AABB: AaBb: aabb, in a 1:2:1 ratio. If the parental gametes were Ab and aB, then the gametes produced by the dihybrids would also be Ab and aB, and the offspring of a cross between the two dihybrids would all be genotype AAbb: AaBb: aaBB, in a 1:2:1 ratio.
    1. Parental: CcEe and ccee; Recombinant: Ccee and ccEe
    2. Parental: Ccee and ccEe; Recombinant: CcEe and ccee
    1. Let WwYy be the genotype of a purple-flowered (W), green seeded (Y) dihybrid. The cross is WwYy × Half of the progeny will have yellow seeds whether the loci are linked or not. You cannot tell if they are linked or not given only this information.
    2. You need to know the proportion of the seeds that are white or purple flowered, and in what frequencies they appear with the white and purple flowers, e.g., what the frequencies of the four classes are. This would help you to know about the linkage between the two loci – unlinked, or what degree of linkage.
  3. If the progeny of the cross aaBB x AAbb is test crossed, and the following genotypes are observed among the progeny of the test cross, what is the frequency of recombination between these loci?
    AaBb 135               Aabb 430                     aaBb 390                     aabb 120
  4. Syntenic is the term for genes found on the same chromosome. Linked genes are always found on the same chromosome, and so are always syntenic. If the genes are sufficiently far enough away on the same chromosome, crossover events will make the two genes assort independently, so they won’t appear linked. Therefore, in this latter situation, these genes are syntenic, but not linked.


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Introduction to Genetics Copyright © 2023 by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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