1. You are mapping genes on the second chromosome of Drosophila mel...

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1. You are mapping genes on the second chromosome of Drosophila melanogaster using three different true‐breeding mutant strains: strain 1: flies have black bodies (wild type body colour is tan) and bright red eyes (wild type eye colour is dull or brick red); strain 2: flies have tan bodies and brown eyes; strain 3: flies have black bodies and white eyes. You cross males from strain 1 to females of strain 2 and the resulting F1 flies are completely wild type (tan bodies and dull red eyes). You then cross F1 females to males from strain 3 and obtain the following phenotypes amongst 1000 F2 flies: Number of flies Black bodies; bright red eyes 285 Tan bodies; brown eyes 300 Black bodies; brown eyes 25 Tan bodies; bright red eyes 31 Black bodies; wildtype eyes 10 Tan bodies; white eyes 14 Black bodies; white eyes 175 Tan bodies; wildtype eyes 160 TOTAL 1000 Note: There are two separate eye pigment pathways in Drosophila: one produces red pigments and the other produces brown pigments; together, these give the wild type eye colour (if both pathways are non‐functional, no eye pigment is produced). a. State the number of linked genes that account for the phenotypes observed and explain the basis of the bright red, brown, and white eye phenotypes. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Using genetic symbols appropriate to your explanation in part a, give the genotypes of the strains 1, 2, and 3 flies and the F1 flies. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ c. Draw a linkage map showing the order of the genes involved and distances between them in map units. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 2. A corn plant with three heterozygous linked loci is crossed to the triple homozygous recessive. Unfortunately, the page with the progeny data was partially damaged before you were able to read it: Phenotype Number + + + 359 a + + 92 + b + 3 + + c 32 4 41 86 383 1000 a. Fill in the rest of the table and draw the gene map, indicating order of genes and map distance in centimorgans. Show your work. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Calculate the value of interference for this interval. Show your work. Express your answer to two decimal places. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 3. If a female animal with genotype A/A . b/b (the back‐slash indicates the chromosome) is crossed with a male that is genotypically a/a . B/B and the F1 is testcrossed, determine the percentage of the progeny from the testcross that will be a/a . b/b, if the two genes are: a. Unlinked (Show your work). ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Completely linked, i.e., no crossing over at all (Show your work). ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ c. 10 map units apart (Show your work). ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ d. 24 map units apart (Show your work). ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 4. Consider the imaginary linked genes A and B in Neurospora. 35% of meioses display no crossover. 35% have one crossover in the region between the genes. 30% have two crossovers in the region between the genes. a. What is the predicted recombination frequency? Show your work, including the formula used to calculate the recombination frequency. Hint: For the correct formula, please see the relevant section in your module book. You should also consult Figure 4.25 of your text for relevant information. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Which tetrad type numbers in the progeny should be compared to determine if genes in Neurospora are linked? Explain why. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 5. Drosophila has four pairs of chromosomes: the sex chromosomes (XX in the females; XY in the males) and three pairs of autosomes. You are studying a new mutant strain with blue eyes. You wish to know if blue eye colour is a dominant mutation and on which chromosome the blue eye locus can be found. When you cross a pure‐breeding female blue‐eyed fly with vestigial wings and spineless bristles to a male fly with wild type eye colour, wings, and bristles, all of the F1 progeny are completely wild type. An F1 male is backcrossed to a triple mutant female, producing the following progeny (remember that there is no crossing over in Drosophila males): Wild type eyes, wings, and bristles: 72 females, 69 males Wild type eyes and bristles, vestigial: 83 females, 77 males Wild type wings, blue eyes, spineless: 60 females, 65 males Blue eyes, spineless, vestigial: 62 females, 58 males a. Is the mutation dominant or recessive? Explain. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Since you know that the locus for vestigial wings is on chromosome 2 and the locus for spineless bristles is on chromosome 3, what chromosome carries the locus for blue eyes? Show your work, including the step‐by‐step reasoning you used to arrive at your answer. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ c. When an F1 female is backcrossed to a triple mutant male, the following 1,000 progeny are produced: Wild type eyes, wings, and bristles: 90 females, 89 males Wild type eyes and bristles, vestigial: 83 females, 98 males Wild type eyes and wings, spineless: 37 females, 35 males Wild type wings and bristles, blue eyes: 36 females, 34 males Wild type wings, blue eyes, spineless: 92 females, 88 males Wild type eyes, spineless, vestigial: 34 females, 39 males Wild type bristles, vestigial, blue eyes: 33 females, 37 males Blue eyes, spineless, vestigial: 90 females, 85 males Calculate the map distance between blue eye and any linked genes. Show your work. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 6. A phenotypically normal woman has a child with Down syndrome. The woman is found to have 45 chromosomes. a. What kind of chromosome abnormality can account for these observations? Justify your answer. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Describe the affected child’s karyotype. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ c. Explain how the karyotype of a child with the more commonly occurring type of Down syndrome would differ from the karyotype described in your answer to b? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ d. How does the more commonly occurring type of Down syndrome usually arise? ____________________________________________________________________ ____________________________________________________________________ 7. a. Explain why the typical calico cat is a female with individual patches of orange and black fur. (Be sure to account for the mosaic phenotype.) ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. What is the underlying process an example of and what does it accomplish in terms of gene expression (i.e., why is it important to the cell/organism)? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 8. Assume that you are working with Drosophila and are studying genes, a, b, c, d, e, and f that are all linked in a region on a chromosome, but their order is unknown. In your work, you find that five deletions in the same region uncover recessive alleles of the genes as follows: Deletion 1 uncovers a, b, and c. Deletion 2 uncovers a and c. Deletion 3 uncovers a, c, and e. Deletion 4 uncovers c, d, and e. Deletion 5 uncovers d, e, and f. a. What is the order of the genes? ____________________________________________________________________ ____________________________________________________________________ b. Suppose you systematically attempt to generate a chromosomal deletion (using X‐rays) that lacks all five genes, but you are unsuccessful. Suggest a plausible explanation for this. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 9. The primrose, Primula kewensis, has 36 chromosomes (i.e., 2n = 36) that are similar in appearance to the chromosomes in two related species, Primula floribunda (2n = 18) and Primula verticillata (2n = 18). a. How could P. kewensis arise from these species? ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. How would you describe P. kewensis in genetics terms? ____________________________________________________________________ c. What is the basic or monoploid chromosome set of this group of species? ____________________________________________________________________ 10. In a certain plant, a dominant allele for one gene, R, produces round fruit and the recessive allele, r, produces oblong fruit. A dominant allele of a second gene, Y, produces yellow flowers and the recessive allele, y, produces white flowers. The two genes are linked on chromosome IV. A plant (r y/r y) with normal chromosomes is crossed to a plant (R Y/R Y) homozygous for a reciprocal translocation between chromosomes IV and VIII. The presence of the translocation causes semi‐sterility. The F1 progeny are phenotypically yellow flowered with round fruit and are semi‐sterile. A backcross to the parent with normal chromosomes yields the 750 progeny shown in the table below. a. From these data, draw a map showing all relevant map distances (in centimorgans) for the genes and the translocation breakpoint. Show your work. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ Number of Progeny Fertile Semi‐sterile Yellow, round 3 283 White, oblong 295 2 Yellow, oblong 73 12 White, round 10 72 b. Briefly explain the basis of the semi‐sterility of translocation‐bearing plants. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 11. Challenge question A cross was made between a Neurospora strain that has the genotype (a b +) and a strain that has the genotype (+ + c) and 2500 ordered tetrads were obtained (see below). Determine which genes are linked, the distance between the linked genes. Draw any relevant map(s). Hint: See the Module for the appropriate formula. Note that to determine linkage, you must assign tetrad types for each pairwise combination of the genes and the distance between each gene and its respective centromere. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ 12. Challenge question The following pedigree is of a family with an extremely rare genetic disorder. Assume that no new mutations have arisen in the pedigree and that the trait displays full penetrance. a. What is the mode of inheritance of this trait (i.e., dominant versus recessive/X‐linked versus autosomal)? Justify your answer. ____________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________ b. Assign genotypes to every member in the pedigree using “D” for the dominant allele and “d” for the recessive. c. If the two cousins IV‐4 and IV‐5 marry, what is the probability that their first child will express the trait. Show your calculation. ___________________________________________________________________ ____________________________________________________________________ ____________________________________________________________________

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There are three genes, one for body color and then two genes accounting for eye color; all with two alleles – one dominant over the other. Drosophila eye color is a cumulative effect (two different pigment pathways) of a red gene product and a brown gene product; white is the effect of all recessive alleles, wildtype is one working allele at least for each gene. The red gene is very close to the body gene, and thus the two are linked; the red gene is also somewhat close to the brown gene and the two are linked.

Strain1: black body, bright red eyes       - e/e    .    bw/bw    .   st+/st+
Strain2: tan body, brown eyes                – e+/e+ . bw+/bw+ . st/st
Strain3: black body, white eyes             – e/e    .    bw/bw    . st/st
F1:S1 X S2 : tan body, wildtype eyes    - e/e+ .   bw/bw+ .   st/st+...

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