The Pedigree Chart Shows the Inheritance of the Allele a in a Flower That Can Be Blue or White
Genetics 371B, Autumn 2000
Exam 1: 20 October
Questions 1 & 2 are both based on the following information:
You are working with three different species of plants. In all three, blue flowers (B) is dominant over white (b), tall plant height (T) is dominant over short (t), and the presence of flower fragrance (F) is dominant over absence of fragrance (f).
| 1. | (16 pts total) In plant species #1, the three genes are linked and located on chromosome 1 the gene order is B-T-F, and each interval is 1 cM. | |
| (a) | In a cross between two plants that are heterozygous for all three genes, what fraction of the progeny will produce blue flowers? What fraction of the progeny will be heterozygous for flower fragrance? Explain BRIEFLY. (8 pts) | |
| (b) | Using some powerful biotech methods, you are able to identify the genotypes of pollen grains (gametes) from these plants. Pollen from one plant of this species were found to consist predominantly of genotypes BtF and bTf . In the cell outline below, diagram chromosome 1 in a vegetative, G2 phase cell of the parent plant. Your diagram should be accurate with respect to the number of homologs and chromatids. Mark the genotype of each chromatid. (8 pts) | |
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| 2. | (16 pts total) | |
| (a) | In species #2, the three genes are located on separate chromosomes. A plant that is heterozygous for all three genes is self-pollinated. What is the most probable phenotype among the progeny? How probable is it? Show your work! (6 pts) | |
| (b) | You observe that a plant of species #3 produces the following pollen genotypes: (10 pts)
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| (i) Can you tell from these data whether loci B and T are linked? If they are linked, what is the map distance between them? BRIEFLY explain your reasoning. | ||
| (ii) Can you tell from these data whether loci T and F are linked? If they are linked, what is the map distance between them? BRIEFLY explain your reasoning. | ||
| 3. | (24 pts total) In humans, glucose-6-phosphate dehydrogenase (G6PD) deficiency and red-green colorblindness are both X-linked recessive traits; the genes are linked at a distance of 6 cM. The following questions are all based on the pedigree shown below. Allele designations: g = G6PD-deficient, G = normal G6PD d = red-green colorblind, D = unaffected (normal vision) | |
| (a) | The genotypes of individuals I-1 and I-2 are marked below their symbols in the pedigree. Based on those genotypes, fill in their phenotypes in the pedigree. Use the fill patterns shown in the key above. (2 pts) | |
| (b) | Now, under each of the following six individuals in the pedigree, mark their genotypes (as shown for I-1 and I-2): II-1, II-2, II-3, II-4, III-1, III-4 (12 pts) The genotypes should be accurate with respect to the alleles present and should also show which alleles are present on which homolog (the way the genotypes are shown for I-1 and I-2). | |
| (c) | Which individual or individuals show clear evidence of being recombinant? Explain in 1-2 sentences how you reached your conclusion. (4 pts) | |
| (d) | What is the probability that individual III-2 is heterozygous for the G6PD gene? Show your work. (6 pts) | |
| 4. | (22 pts total) You are given two strains of the worm C. elegans. One strain is homozygous for a recessive allele that makes the worm short, called dumpy (d). The other strain is homozygous for a recessive allele that makes the worms uncoordinated (unc). The two genes are unlinked. | |
| (a) | What crosses can you perform to obtain a strain that is homozygous recessive for both genes? Show the genotypes of all parents/offspring and the ratios of offspring phenotypes from each cross you propose. (12 pts) | |
| (b) | What fraction of the final progeny would have your desired genotype (homozygous recessive for both genes) if the two genes were linked, with the genes 10 cM apart? (10 pts) | |
| 5. | (22 pts total) The Australian echidna comes in two varieties, one variety with soft spines and one with hard spines. Spine hardness/softness is controlled by a single gene. | |
| (a) | Crosses between any two hard spine echidnas always yield only hard spine progeny, while crosses between any hard spine and any soft spine echidna always give hard spine and soft spine progeny in equal proportions. What do these observations tell you about the genotypes of the two varieties of echidna? (Use H and h for the two alleles of the spine hardness gene.) (6 pts) | |
| (b) | Crosses between any two soft spine echidnas also always give hard spine and soft spine progeny. Use Punnett squares to explain this observation, indicating which progeny genotypes give which phenotype. Your answer must be consistent with the data given in parts (a) and (b). (8 pts) | |
| (c) | Some echidnas collected in the wild were found to have short snouts. A cross between two hard spine short snout echidnas gave progeny that all had hard spines and were 1/4 long snout, 1/2 short snout and 1/4 snout-less. Explain the inheritance of the snout length trait. (8 pts) | |
Source: http://depts.washington.edu/genetics/courses/genet371b-aut00/public_html/problems/371B_exam1.html
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