Solutions to Problem Set 2

Transcription

1 Solutions to Problem Set 2 Question 1 In unicorns, coat color (brown or white) is controlled by a single gene with two alleles, A and a. The brown phenotype is dominant over the white phenotype. Height (tall or short) is controlled by a single gene with two alleles, H and h. The tall phenotype is dominant over the short phenotype. These two loci are on different chromosomes. a) What is the phenotype of an HhAa unicorn? The phenotype of an HhAa unicorn is tall and brown b) What is the phenotype of an hhaa unicorn? The phenotype of an hhaa unicorn is short and brown c) If an HhAa unicorn mates with an hhaa unicorn, what fraction of the progeny will be short and brown? i) Show how you would answer this question using a Punnett square. HA Ha ha ha ha HhAA HhAa hhaa hhaa 3/8 ha HhAa Hhaa tall, white hhaa hhaa short, white ii) Show how you would use the laws of probability to answer the same question. p() = p(hhaa) X p(hhaa) = 1/2 X 1/2 X 1/2 X 1/4 = 3/8 1

2 Question 2 For each of the following pedigrees, give the likely mode of inheritance (autosomal dominant, autosomal recessive, sex-linked dominant, or sex-linked recessive). Assume that individuals marrying into a family do not carry the mutant allele. KEY unaffected male affected male unaffected female affected female a) Mode of inheritance: Autosomal dominant b) Mode of inheritance: X-linked recessive c) Mode of inheritance: Autosomal recessive 2

3 Question 3 A rare and delicious fish lives in the shallows surrounding a remote desert island soon to be famous as the site for a SURVIVOR sequel. This fish has two phenotypes of interest: shininess and speed. The shiny phenotype is dominant over the dull phenotype, and the fast phenotype is dominant over the slow phenotype. The producers of SURVIVOR hired you to breed large numbers of shiny slow fish to increase the chance that the new contestants can catch a meal. Phenotype shininess speed alleles D or d E or e You cross a true breeding shiny slow fish to a true-breeding dull fast fish to get an F1 class of progeny that is entirely shiny and fast. a) What is the genotype of the true-breeding shiny slow fish? DDee b) What is the genotype of the true-breeding dull fast fish? ddee c) What is the genotype of the F1 shiny fast fish? DdEe d) You then perform a test cross with an F1 fish. What are the genotypes of the individuals in this test cross? A test cross is a cross with the double recessive parent, in this case, DdEe X ddee e) If the genes controlling these two traits are unlinked, what phenotypes and in what ratios would you expect as a result of this test cross? You would expect: 1:1:1:1, shiny fast fish : shiny slow fish : dull fast fish : dull slow fish f) If the traits were completely linked, what phenotypes and in what ratios would you expect as a result of this test cross? 1:1, shiny slow fish : dull fast fish 3

4 Question 3, continued f) What you actually see when you complete the test cross is: 48 shiny fast fish 199 shiny slow fish 208 dull fast fish 45 dull slow fish i) Circle the recombinant phenotypes. ii) What is the recombination frequency between the shininess and speed loci? RF = # of recombinants/ total number X 100 RF = ( / ) X 100 RF = 93/500 X 100 = 18.6% g) Interestingly, you find that these same fish can be either sharp-eye or blind, where the sharp-eyed phenotype is dominant over the blind phenotype. Use B or b to designate the alleles of the eyesight locus. You determine that the loci for shininess and for eyesight are linked with a recombination frequency of 4.5%. Draw the two potential maps for the D, B, and E loci. On each map, express the distance between the loci as the recombination frequency. B D 4.5% 18.6% E D 4.5% B 14.1% E 18.6% h) What would you have to do to determine which of these maps is the correct one? You would determine the RF between the speed and the eyesight loci. One way this could be done is to cross a true breeding sharp-eyed fast fish to a true breeding blind slow fish to get an F1 class of progeny that is entirely sharp-eyed and fast. Then set up a test cross between an F1 individual and the double recessive parent, in this case, BbEe X bbee. Count the number of blind fast fish and the number of sharp-eyed slow fish to get the number of recombinant progeny. RF = # of recombinants/ total number X 100 4

5 Question 4 On the same island is a seaweed (a diploid organism) that exists in three different colors (this seaweed will soon be woven into bikini tops and breech cloths and will become THE fashion statement in Paris). This seaweed can be, or. The color is controlled by a pathway of two distinct enzymes encoded by the F and G genes. For questions a) and b) below, use F and G to designate the wild-type alleles. Use f and g to designate the loss-offunction alleles. See the partial pathway below. enzyme F enzyme G You cross two true-breeding plants, one that is and one that is. The F1 plants are all. a) Given only the information above, what are two possible genotypes for the parent? FFgg or ffgg b) Given only the information above, what are two possible genotypes for the parent? ffgg or FFgg c) What is the genotype of the F1 progeny? FfGg You cross two F1 progeny and expect to see a 9:3:3:1 ratio of phenotypes in the F2 generation. Surprisingly, you notice that the F2 progeny have the following ratio of phenotypes: 9 : 4 : 3 d) Given this data, circle the correct pathway. *Note, for parts a) and b) above... The two enzymes are part of the same pathway, and the phenotype is the first step in the pathway. enzyme F enzyme G This means that if the enzyme that carries out the transformation from to is missing, it doesn't matter what the alleles of the second enzyme are. So in this particular case ffgg, ffgg, and ffgg are all possible phenotypes for the true-breeding parent. The true-breeding parent could only be FFgg. CHECK IT OUT FOR YOURSELF! 5