Unit 5: Linkage, Recombination, and Genetic Mapping
How genes riding on the same chromosome betray their order and distance through the recombinant offspring crossing over leaves behind.
10 topics · 37 questions
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Linked Genes and the Departure from Independent Assortment
- How it worksHow does physical location on the same chromosome keep two genes from assorting independently?
- Common mistakeWhy doesn't sitting on the same chromosome mean two genes are always inherited together?
- Going furtherWhat happens to the ratio of parental to nonparental offspring as two linked genes sit farther apart on a chromosome?
Your turn1 of 2
Morgan crossed a dihybrid fly for two traits and got far more parental-looking offspring than the 9:3:3:1 dihybrid ratio predicts, with only a small fraction showing new (nonparental) trait combinations. What does this pattern reveal?Crossing Over Between Homologous Chromosomes
- How it worksHow does a chiasma physically exchange segments between two homologous chromatids during prophase I?
- Worked exampleTrace which two of the four chromatids become recombinant after a single crossover happens between genes A and B.
- Common mistakeWhy doesn't crossing over happen between sister chromatids that are already identical?
- Going furtherWhat happens to a chromosome's gene arrangement when two crossovers occur between the same two genes instead of one?
- Worked exampleFind the percentage of recombinant gametes produced when a single crossover occurs between two genes in every meiosis.
Your turn1 of 2
A single crossover occurs between two genes in 0.692 of all meioses (as a fraction of 1). Since each crossover event recombines exactly two of the four resulting chromatids (half the gametes from that meiosis), what percentage of ALL gametes produced end up recombinant?Recombination Frequency and Map Distance
- How it worksHow does the percentage of recombinant offspring get converted into map units along a chromosome?
- Worked exampleFind the map distance between two genes given a testcross produces 180 parental and 20 recombinant offspring.
- Common mistakeWhy can't recombination frequency between two genes ever climb above 50 percent?
- Worked exampleFind the map distance between two genes given a testcross produces 640 parental and 360 recombinant offspring out of 1000.
Your turn1 of 2
A testcross produces 239 parental-type offspring and 55 recombinant offspring. What is the map distance between the two genes, in map units (percent recombination)?Two-Point Testcross Mapping
- How it worksHow does a testcross expose the genotype of every gamete a heterozygous parent produces?
- Worked exampleFind the map distance between genes b and vg given a testcross of 965 parental and 35 recombinant flies out of 1000.
- Worked exampleFind the map distance between two genes given a testcross yields 41 recombinants out of 500 total offspring.
Your turn1 of 2
A testcross of double heterozygous flies for genes b and vg produces 128 recombinant offspring out of 919 total offspring. What is the map distance between the two genes, in map units?The Three-Point Testcross and Determining Gene Order
- How it worksHow do the eight phenotype classes from a three-point testcross sort into parental, single-crossover, and double-crossover groups?
- Worked exampleTrace how sorting eight phenotype classes from a three-point testcross by frequency identifies the two rarest as double crossovers.
- Worked exampleFind which of three genes sits in the middle by comparing the double-crossover class to the parental class.
- Common mistakeWhy doesn't the most common phenotype class always tell you which gene sits in the middle?
- Ties togetherHow does sorting the eight phenotype classes by frequency connect a three-point testcross to the map distances a two-point cross gives?
Your turn1 of 2
A three-point testcross produces eight phenotype classes. The two classes with the LARGEST number of offspring are declared the parental types. A student assumes the most common class always tells you which gene sits in the middle. Why is that wrong?Three-Point Cross: Calculating Map Distances
- Worked exampleFind the map distance between gene A and gene B from a three-point testcross of 1000 flies with 6 percent recombination between them.
- Worked exampleFind the total map distance across three genes given recombination frequencies of 9 percent and 13 percent between adjacent gene pairs.
- Going furtherWhat happens to a three-point cross's map distances once double crossovers get added back into the recombination count?
- Common mistakeWhy doesn't adding the two adjacent map distances directly give the true distance between the two outer genes?
Your turn1 of 2
A three-point testcross of 1514 flies shows 13 percent recombination between gene A and gene B. How many of the 1514 flies are expected to be recombinant for these two genes?Interference and the Coefficient of Coincidence
- How it worksHow does one crossover physically discourage a second crossover from happening nearby?
- Worked exampleFind the coefficient of coincidence given an expected double-crossover frequency of 1.2 percent and an observed frequency of 0.6 percent.
- Worked exampleFind the interference value given a coefficient of coincidence of 0.4.
- Common mistakeWhy doesn't a coefficient of coincidence of 1.0 mean crossovers are happening more often than expected?
Your turn1 of 2
A three-point cross has an expected double-crossover frequency of 2 percent, but the observed double-crossover frequency is only 0.654 percent. What is the coefficient of coincidence?Ordered Tetrad Analysis in Neurospora
- How it worksHow does the linear arrangement of eight ascospores preserve the order of the two meiotic divisions?
- Worked exampleFind the gene-to-centromere distance given a Neurospora cross produces 18 second-division segregation asci out of 100 total asci.
- Common mistakeWhy doesn't a first-division segregation pattern mean a gene is unlinked to its centromere?
Your turn1 of 2
A Neurospora cross produces 33 second-division segregation asci out of 223 total asci examined. What is the gene-to-centromere distance, in map units (half the second-division segregation frequency, as a percent)?Unordered Tetrad Analysis and Gene Linkage
- How it worksHow do parental ditype, nonparental ditype, and tetratype tetrads reveal whether two genes are linked?
- Worked exampleFind whether two genes are linked given a cross produces far more parental ditype tetrads than nonparental ditype tetrads.
- Worked exampleFind the map distance between two linked genes using tetratype and nonparental ditype frequencies from 200 total tetrads.
Your turn1 of 2
A yeast cross produces 85 tetratype tetrads and 14 nonparental ditype tetrads out of 339 total tetrads. Using the Perkins formula (100 times tetratype over 2, plus 3 times nonparental ditype, all divided by total), what is the map distance between the two genes, in map units?Double Crossovers and Why Mapping Underestimates Distance
- How it worksHow does a double crossover between two genes restore the parental combination and hide itself from a two-point cross?
- Going furtherWhat happens to the accuracy of a genetic map when it is built from several closely spaced markers instead of two distant ones?
- Ties togetherHow does correcting for double crossovers connect two-point mapping to the more accurate three-point testcross?
Your turn1 of 2
A two-point testcross between two genes located far apart on the same chromosome gives a map distance noticeably smaller than the genes' true physical separation. Why does this underestimation happen specifically for far-apart genes?