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Exam 6: Linkage, Recombination, and Eukaryotic Gene Mapping

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Question 1

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In a cross, the arrangement of linked alleles on the chromosomes is critical for determining the outcome. When two wild-type alleles are on one homologous chromosome and two mutant alleles are on the other, they are in the coupling configuration; when each chromosome contains one wild-type allele and one mutant allele, the alleles are in repulsion. -The following testcross produces the progeny shown: Aa Bb × aa bb → 10 Aa Bb, 40 Aa bb, 40 aa Bb, 10 aa bb. Were the genes in the Aa Bb parent in coupling or in repulsion?

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Question 2

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A genetic map provides the order of the genes on a chromosome and the approximate distances from one gene to another based on recombination frequencies. In genetic maps, 1% recombination equals 1 map unit, or 1 centiMorgan. Double crossovers between two genes go undetected; so map distances between distant genes tend to underestimate genetic distances. -How does a genetic map differ from a physical map?

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Genetic maps are based on rates of recombination; physical maps are based on physical distances.

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The coefficient of coincidence equals the number of double crossovers observed divided by the number of double crossovers expected on the basis of the single-crossover frequencies. The interference equals 1 - the coefficient of coincidence; it indicates the degree to which one crossover interferes with additional crossovers. -In analyzing the results of a three-point testcross, a student determines that the interference is ?0.23. What does this negative interference value indicate?

To determine the middle locus in a three-point cross, compare the double-crossover progeny with the nonrecombinant progeny. The double crossovers will be the two least-common classes of phenotypes; the nonrecombinants will be the two most-common classes of phenotypes. The double-crossover progeny should have the same alleles as the nonrecombinant types at two loci and different alleles at the locus in the middle. -A three-point test cross is carried out between three linked genes. The resulting nonrecombinant progeny are s⁺ r⁺ c⁺ and s r c and the double-crossover progeny are s r c⁺ and s⁺ r⁺ c. Which is the middle locus?

To map genes, information about the location and number of crossovers in the gametes that produced the progeny of a cross is needed. An efficient way to obtain this information is to use a three-point testcross, in which an individual heterozygous at three linked loci is crossed with an individual that is homozygous recessive at the three loci -Write the genotypes of all recombinant and nonrecombinant progeny expected from the following three-point cross: $\frac{m^+ ~~ p^+ ~~ s^+}{m ~~~~ p ~~~~ s} \times \frac{m ~~~ p ~~~ s}{m ~~~ p ~~~ s}$

Linkage between genes causes them to be inherited together and reduces recombination; crossing over breaks up the associations of such genes. In a testcross for two linked genes, each crossover produces two recombinant gametes and two nonrecombinants. The frequency of recombinant gametes is half the frequency of crossing over, and the maximum frequency of recombinant gametes is 50%. -For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because