Linkage and Chromosome Mapping Practice Test (page 3)

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

Use of Genetic Maps Questions

  1. Two loci are known to be in linkage group IV of the rat. Kinky hairs in the coat and vibrissae (long nose "whiskers") are produced in response to the recessive genotype kk and a short, stubby tail is produced by the recessive genotype st/st. The dominant alleles at these loci produce normal hairs and tails, respectively. Given 30 map units between the loci of k and st, determine the expected F1 phenotypic proportions from heterozygous parents that are   (a) both in coupling phase,   (b) both in repulsion phase,   (c) one in coupling and the other in repulsion phase.
  2. Deep-yellow hemolymph (blood) in silkworm larvae is the result of a dominant gene Y at locus 25.6 (i.e., 25.6 crossover units from the end of the chromosome). Another dominant mutation Rc, 6.2 map units from the Y locus, produces a yellowish-brown cocoon (rusty). Between these two loci is a recessive mutant oa governing mottled translucency in the larval skin, and mapping at locus 26.7. The loci Rc and oa are separated by 5.1 crossover units. An individual that is homozygous for yellow blood, mottled translucent larval skin, and wild-type cocoon color is crossed to an individual of genotype Y+ oa+ Rc/Y + oa+ RC that spins a rusty cocoon. The F1 males are then testcrossed to produce 3000 F2 progeny. Coincidence is assumed to be 10%.   (a) Predict the numbers within each phenotypic class that will appear in the F2 (to the nearest hole numbers).   (b)On the basis of probabilities, how many more F2 progeny would need to be produced in order to recover one each of the DCO phenotypes?
  3. The eyes of certain mutant Drosophila have a rough texture due to abnormal facet structure. Three of the mutants that produce approximately the samephenotype (mimics) are sex-linked recessives: roughest (rst), rugose (rg), and roughex (rux). The loci of these genes in terms of their distances from the end of the X chromosome are 2, 11, and 15 map units, respectively.   (a) From testcrossing wild-type females of genotype rst + rux/ + rg + predict the number of wild-type and rough-eyed flies expected among 20,000 progeny. Assume no interference.   (b) Approximately how many rougheyed progeny flies are expected for every wild-type individual?   (c) If the females of part (a) were of genotype rst rq rux/ + + +, what would be the approximate ratio of wild-type: rough-eyed progeny?
  4. In Asiatic cotton, a pair of factors (R and r) controls the presence or absence, respectively, of anthocyanin pigmentation. Another gene, about 10 map units away from the R locus, controls chlorophyll production. The homozygous recessive genotype at this locus (yy) produces a yellow (chlorophyll-deficient) plant that dies early in the seedling stage. The heterozygote Yy is phenotypically green and indistinguishable from the dominant homozygote YY. Obviously, testcrosses are not possible for the Y locus. When dihybrids are crossed together, calculate the expected phenotypic proportions among the seedlings and among the mature F1 when parents are   (a) both in coupling phase,   (b) both in repulsion phase,   (c) one in coupling and one in repulsion phase.   (d) Which method [in parts (a), (b), or (c)] is expected to produce the greatest mortality?

Crossover Suppression Questions

  1. A black-bodied Drosophila is produced by a recessive gene b and vestigial wings by another recessive gene vg on the same chromosome. These two loci are approximately 20 map units apart. Predict the progeny phenotypic expectations from   (a) the mating of repulsion phase females × coupling-phase males,   (b) the reciprocal cross of part (a), (c) the mating where both parents are in repulsion phase.
  2. Poorly developed mucous glands in the female silkworm Bombyx mori cause eggs to be easily separated from the papers on which they are laid. This is a dominant genetic condition; its wild-type recessive allele Ng+ produces normally "glued" eggs. Another dominant gene C, 14 map units from Ng, produces a golden-yellow color on the outside of the cocoon and nearly white inside. Its recessive wild-type allele C+ produces normally pigmented or wild-type cocoon color. A pure "glueless" strain is crossed to a pure golden strain. The F1 females are then mated to their brothers to produce the F2. Predict the number of individuals of different phenotypes expected to be observed in a total of 500 F2 offspring. (Hint: Crossing over does not occur in female silkworms.)
  3. Two autosomal recessive genes, "dumpy" (dp, a reduction in wing size) and "net" (net, extra veins in the wing), are linked on chromosome 2 of Drosophila. Homozygous wild-type females are crossed to net, dumpy males. Among 800 F2 offspring were found: 574 wild type : 174 net, dumpy : 25 dumpy : 27 net. Estimate the map distance.
  4. Suppose that an abnormal genetic trait (mutation) appeared suddenly in a female of a pure culture of Drosophila melanogaster. We mate the mutant female to a male from a balanced lethal strain [Cy/Pm, D/Sb, where curly (Cy) and plum (Pm) are on chromosome 2 and dichaete (D) and stubble (Sb) are on chromosome 3]. About half of the F1 progeny (both males and females) exhibit the mutant phenotype. The F1 mutant males with curly wings and stubble bristles are then mated to unrelated virgin wild-type females. In the F2 the mutant trait never appears with stubble. Recall that this species of Drosophila has chromosomes X, 2, 3, and 4. Could the mutation be   (a) an autosomal recessive,   (b) a sex-linked recessive,   (c) an autosomal dominant,   (d) a sex-linked dominant?   (e) In which chromosome does the mutant gene reside?   (f) Suppose the mutant trait in the F2 appeared in equal association with curly and stubble. In which chromosome would the mutant gene reside?   (g) Suppose the mutant trait in the F2 appeared only in females. In which chromosome would the mutant gene reside?   (h) Suppose the mutant trait in the F2 never appeared with curly. In which chromosome would the mutant gene reside?

Recombination Mapping With Tetrads Questions

  1. Given the adjoining meiotic metaphase orientation in Neurospora, determine the simplest explanation to account for the following spore patterns.
  2. A certain strain of Neurospora cannot grow unless adenine is in the culture medium. Adenineless is a recessive mutation (ad). Another strain produces yellow conidia (ylo). Below are shown the results from crossing these two strains. Calculate the map distance between these two genes.
  3. A riboflavineless strain (r) of Neurospora is crossed with a tryptophaneless strain (t) to give
  4. Construct a map that includes these two genes.

  5. Two of the genes s, t, and u are linked; the third assorts independently and is very tightly linked to its centromere. Analyze the unordered tetrads produced by the cross (stu) × ( + + + ). (Hint: See Problem 6.10.)



  1. tetrad analysis
  2. chiasma, or X (chi)
  3. crossing over
  4. coupling phase
  5. repulsion phase
  6. centimorgan
  7. coincidence, coincidence coefficient, or coefficient of coincidence
  8. interference
  9. balanced lethals
  10. tetrad analysis
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