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# Linkage and Chromosome Mapping Practice Test (page 3)

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By — McGraw-Hill Professional
Updated on Apr 25, 2014

#### Linkage Estimates From F2 Data Questions

(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.

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. 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.
3. 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.
4. 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.

#### 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?

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