4. Genetic Mapping and Linkage

Mapping Genes

Open Question
Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.

Draw two possible genetic maps for these three genes, and identify the recombination frequencies predicted for each map.
Multiple Choice
The genetic distances between three genes (ab, nm, kf) were determined using a two-point mapping cross. Determine the sequence of the three genes.
Multiple Choice
True or False:Recombination frequencies are never greater than 50%
Multiple Choice
Using the following data collected from a test cross, calculate the recombination frequency.
Open Question
In a diploid species of plant, the genes for plant height and fruit shape are syntenic and separated by 18 m.u. Allele D produces tall plants and is dominant to d for short plants, and allele R produces round fruit and is dominant to r for oval fruit.

Give the same information for a plant with the genotype Dr/dR.
Open Question
Genes A, B, and C are linked on a chromosome and found in the order A–B–C. Genes A and B recombine with a frequency of 8%, and genes B and C recombine at a frequency of 24%. For the cross a⁺b⁺c/abc⁺×abc/abc, predict the frequency of progeny genotypes. Assume interference is zero.
Open Question
Gene G recombines with gene T at a frequency of 7%, and gene G recombines with gene R at a frequency of 4%.

Assuming that organisms with any desired genotype are available, propose a genetic cross whose result could be used to determine which of the proposed genetic maps is correct.
Open Question
Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

The test cross indicates the genes recombine with a frequency of 28%. If 1000 progeny are produced by this test cross, determine the number of progeny in each outcome class.
Open Question
Genes A, B, C, D, and E are linked on a chromosome and occur in the order given.

Previous genetic linkage crosses have determined that recombination frequencies are 6% for genes A and B, 4% for genes B and C, 10% for genes C and D, and 11% for genes D and E. The sum of these frequencies between genes A and E is 31%. Why does the recombination distance between these genes as determined by adding the intervals between adjacent linked genes differ from the distance determined by the test cross?
Open Question
Colored aleurone in the kernels of corn is due to the dominant allele R. The recessive allele r, when homozygous, produces colorless aleurone. The plant color (not the kernel color) is controlled by another gene with two alleles, Y and y. The dominant Y allele results in green color, whereas the homozygous presence of the recessive y allele causes the plant to appear yellow. In a testcross between a plant of unknown genotype and phenotype and a plant that is homozygous recessive for both traits, the following progeny were obtained:colored, green 88colored, yellow 12colorless, green 8colorless, yellow 92Explain how these results were obtained by determining the exact genotype and phenotype of the unknown plant, including the precise arrangement of the alleles on the homologs.
Open Question
In the cross shown here, involving two linked genes, ebony (e) and claret (ca), in Drosophila, where crossing over does not occur in males, offspring were produced in a 2 + : 1 ca : 1 e phenotypic ratio:These genes are 30 units apart on chromosome III. What did crossing over in the female contribute to these phenotypes?
Open Question
In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed:pr–adp 29%pr–vg 13pr–c 21pr–b 6adp–b 35adp–c 8adp–vg. 16vg–b. 19vg–c 8c–b. 27In another set of experiments, a sixth gene, d, was tested against b and pr:d–b 17%d–pr 23%Predict the results of two-point mapping between d and c, d and vg, and d and adp.
Open Question
In a series of two-point mapping crosses involving five genes located on chromosome II in Drosophila, the following recombinant (single-crossover) frequencies were observed:pr–adp 29%pr–vg 13pr–c 21pr–b 6adp–b 35adp–c 8adp–vg. 16vg–b. 19vg–c 8c–b. 27Given that the adp gene is near the end of chromosome II (locus 83), construct a map of these genes.
Open Question
In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.Phenotype Offspringsc s v 314+ + + 280+ s v 150sc + + 156sc + v 46+ s + 30sc s + 10+ + v 14No determination of sex was made in the data.Determine the sequence of the three genes and the map distances between them.
Open Question
In Drosophila, a cross was made between females—all expressing the three X-linked recessive traits scute bristles (sc), sable body (s), and vermilion eyes (v)—and wild-type males. In the F₁, all females were wild type, while all males expressed all three mutant traits. The cross was carried to the F₂ generation, and 1000 offspring were counted, with the results shown in the following table.Phenotype Offspringsc s v 314+ + + 280+ s v 150sc + + 156sc + v 46+ s + 30sc s + 10+ + v 14No determination of sex was made in the data.Using proper nomenclature, determine the genotypes of the P₁ and F₁ parents.