Linkage and Chromosome Mapping Practice Test (page 2)

By — McGraw-Hill Professional
Updated on Aug 22, 2011

Recombination Among Linked Genes Questions

  1. There is 21% crossing over between the locus of p and that of c in the rat. Suppose that 150 primary oocytes could be scored for chiasmata within this region of the chromosome. How many of these oocytes would be expected to have a chiasma between these two genes?
  2. The longest chromosome in the sweet pea has a minimum uncorrected map length (based on known genetic markers) of 118 units. Cytological observations of the longest chromosome in meiotic cells revealed an average chiasmata frequency of 2.96 per tetrad. Calculate the maximum number of crossover units remaining in this chromosome for the mapping of new genes outside the range already known.

Genetic Mapping Questions

  1. The distances between eight loci in the second chromosome of Drosophila are presented in the following table. Construct a genetic map to include these eight loci. The table is symmetrical above and below the diagonal.
  2. The recessive gene sh produces shrunken endosperm in corn kernels and its dominant allele sh+ produces full, plump kernels. The recessive gene c produces colorless endosperm and its dominant allele c+ produces colored endosperm. Two homozygous plants are crossed, producing an F1 all phenotypically plump and colored. The F1 plants are testcrossed and produce 149 shrunken, colored : 4035 shrunken, colorless : 152 plump, colorless : 4032 plump, colored.   (a) What were the phenotypes and genotypes of the original parents?   (b) How are the genes linked in the F1?   (c) Estimate the map distance between sh and c.
  3. The presence of one of the Rh antigens on the surface of the red blood cells (Rh-positive) in humans is produced by a dominant gene R; Rh-negative cells are produced by the recessive genotype rr. Oval-shaped erythrocytes (elliptocytosis or ovalocytosis) are caused by a dominant gene E; its recessive allele e produces normal red blood cells. Both of these genes are linked approximately 20 map units apart on one of the autosomes. A man with elliptocytosis, whose mother had normally shaped erythrocytes and a homozygous Rh-positive genotype and whose father was Rh-negative and heterozygous for elliptocytosis, marries a normal Rh-negative woman.   (a) What is the probability of their first child being Rh-negative and elliptocytotic?   (b) If their first child is Rh-positive, what is the chance that it will also be elliptocytotic?
  4. The Rh genotypes, as discussed in Problem 6.15, are given for each individual in the pedigree shown below. Solid symbols represent elliptocytotic individuals.   (a) List the E locus genotypes for each individual in the pedigree.   (b) List the gametic contribution (for both loci) of the elliptocytotic individuals (of genotype Rr) beside each of their offspring in which it can be detected.   (c) How often in part (b) did R segregate with E, and r with e?   (d) On the basis of random assortment, in how many of the offspring in part (b) would we expect to find R segregating with e, or r with E?   (e) If these genes assort independently, calculate the probability of R segregating with E, and r with e in all 10 cases.   (f) Is the solution to part (c) suggestive of linkage between these two loci?   (g) Calculate part (e) if the siblings II1 and II2 were identical twins (developed from a single egg).   (h) How are these genes probably linked in I1?
  5. Two recessive genes in Drosophila (b and vg) produce black body and vestigial wings, respectively. When wild-type flies are testcrossed, the F1 are all dihybrid in coupling phase. Testcrossing the female F1 produced 1930 wild type : 1888 black and vestigial : 412 black : 370 vestigial. (a) Calculate the distance between b and vg. (b) Another recessive gene cn lies between the loci of b and vg, producing cinnabar eye color. When wild-type flies are testcrossed, the F1 are all trihybrid. Testcrossing the F1 females produced 664 wild type : 652 black, cinnabar, vestigial : 72 black, cinnabar : 68 vestigial : 70 black : 61 cinnabar, vestigial : 4 black, vestigial : 8 cinnabar. Calculate the map distances. (c) Do the b–vg distances calculated in parts   (a) and   (b) coincide? Explain.   (d) What is the coefficient of coincidence?
  6. In corn, a dominant gene C produces colored aleurone; its recessive allele c produces colorless. Another dominant gene Sh produces full, plump kernels; its recessive allele sh produces shrunken kernels due to collapsing of the endosperm. A third dominant gene Wx produces normal starchy endosperm and its recessive allele produces waxy starch. A homozygous plant from a seed with colorless, plump, and waxy endosperm is crossed to a homozygous plant from a seed with colored, shrunken, and starchy endosperm. The F1 is testcrossed to a colorless, shrunken, waxy strain. The progeny seed exhibit the following phenotypes: 113 colorless, shrunken, starchy : 4 colored, plump, starchy : 2708 colorless, plump, waxy : 626 colorless, plump, starchy : 2 colorless, shrunken, waxy : 116 colored, plump, waxy : 2538 colored, shrunken, starchy : 601 colored, shrunken, waxy.   (a) Construct a genetic map for this region of the chromosome. Round all calculations to the nearest tenth of a percent.   (b) Calculate the interference in this region.
  7. A gene called "forked" (f) produces shortened, bent, or split bristles and hairs in Drosophila. Another gene called "outstretched" (od) results in wings being carried at right angles to the body. A third gene called "garnet" (g) produces pinkish eye color in young flies. Wild-type females heterozygous at all three loci were crossed to wild-type males. The F1 data appear below.
  8. (a) Which gene is in the middle?   (b) What was the linkage relationship between alleles at the forked and outstretched loci in the maternal parent?   (c) What was the linkage relationship between alleles at the forked and garnet loci in the maternal parent?   (d) On what chromosome do these three genes reside?   (e) Calculate the map distances.   (f) How much interference is operative?

  9. Five sex-linked recessive genes of Drosophila (ec, sc, v, cv, and ct) produce traits called echinus, scute, vermilion, crossveinless, and cut, respectively. Echinus is a mutant producing rough eyes with large facets. Scute manifests itself by the absence or reduction in the number of bristles on certain parts of the body. Vermilion is a bright orange-red eye color. Crossveinless prevents the development of supporting structures in the wings. Cut produces scalloped and pointed wings with manifold (pleiotropic) effects in other parts of the body. At the beginning of our experiments we do not know the gene order. From the results of the following three experiments, construct a genetic map for this region of the X chromosome. Whenever possible use weighted averages.
  10. Experiment 1. Echinus females crossed to scute, crossveinless males produced all wild-type females and all echinus males in the F1. When the F1 females were testcrossed, the results (including both male and female progeny) were as follows:

    Experiment 2. Crossveinless females crossed to echinus, cut males produced all wild-type females and all crossveinless males in the F1. When the F1 females were testcrossed, the results (including both male and female progeny) were as follows:

    Experiment 3. Cut females crossed to vermilion, crossveinless males produced all wild-type females and cut males in the F1. When the F1 females were testcrossed, the results (including both male and female progeny) were as follows:

Linkage Estimates From F2 Data Questions

  1. Two recessive sex-linked genes are known in chickens (ZW method of sex determination; Chapter 5): rapid feathering (sl) and gold plumage (s). The dominant alleles produce slow feathering (Sl) and silver plumage (S), respectively. Females of the Silver Penciled Rock breed, with slow feathering and silver plumage, are crossed to males of the Brown Leghorn breed, with rapid feathering and gold plumage. The F2 progeny data appear below:
  2. (a) Determine the F1 genotypes and phenotypes.   (b) In what linkage phase are the F1 males?   (c) Calculate the amount of recombination expected to occur between these two loci in males.

  3. Assume the genotype AB/AB is testcrossed and produces an F2 consisting of 37 A-B-, 11 A-bb, 12 aaB-, and 4 aabb. Estimate the percentage recombination between A and B by the square-root method and by the product ratio method.
  4. Two recessive genes in the third linkage group of corn produce crinkly leaves and dwarf plants, respectively. A pure crinkly plant is pollinated by a pure dwarf plant. The F2 progeny consist of 104 normal : 43 dwarf : 51 crinkly : 2 dwarf, crinkly. Using the square-root method, estimate the amount of recombination between these two loci.
  5. The duplicate recessive genes r1 and r2 produce a short, velvet like fur called "rex." Two rex rabbits of different homozygous genotypes were mated and produced an F1 that was then testcrossed to produce 64 rex and 6 normal testcross progeny.   (a) Assuming independent assortment, how many normal and rex phenotypes would be expected among 70 progeny?   (b)Do the data indicate linkage?   (c) What is the genotype and the phenotype of the F1?   (d) What is the genotype and phenotype of the testcross individuals?   (e) Calculate the map distance.
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