PS5_Key - Reinagel BICD100 Genetics Fall 2005 Problem Set 5...

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Reinagel BICD100 Genetics Fall 2005 Problem Set 5: Linkage and Maps Answer Key 1. In a plant, leaf color and leaf shape are controlled by two linked genes. Leaves of the wild-type plant are red. A recessive mutation in this gene causes white leaves. Wild-type leaves are pointed, and a recessive mutation in this gene causes them to be smooth. The following crosses were performed: Cross 1: pure breeding white, smooth X pure breeding wild type gives F1: all red, pointed Cross 2: red, pointed F1 X pure breeding white, smooth (test cross) gives F2: 40 smooth 36 red, pointed 10 white, pointed 14 red, smooth What is the recombination frequency between the gene for color and for shape? The parental classes are (white, smooth) and (red, pointed). You know this for two reasons: these are the types of the parents in cross 1, and also, these are the types that occur disproportionately often in the F2. The recombination frequency is: # nonparental x 100 = (10+14) x100 = 24% r.f. = 24 m.u. = 24 cM # progeny 10+14+40+36
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Reinagel BICD100 Genetics Fall 2005 2. In pea plants, the peas can be either rough or smooth, and either soft or hard. You do some crosses and obtain the following results. cross 1: purebred rough, soft pea X purebred smooth, hard pea gives F1: all rough, hard cross 2: rough, hard F1 X smooth, soft ( note: test cross, NOT self cross) gives F2: 115 rough soft 110 smooth hard 8 rough hard 12 smooth soft a) using letters of your own choosing, state the genotypes of the parents, the F1, and each phenotypic class in the F2. Let R = rough (dominant), r = smooth (recessive) Let H = hard (dominant), h = soft (recessive) Parents are RRhh x rrHH, I recommended that you write this way to help you visualize the two chromosomes: Rh/Rh x rH/rH F1 are dihybrids: Rh/rH which are crossed to the tester strain rh/rh F2 are therefore RH/rh rough hard (the gamete from the F1 is RH) nonparental rH/rh smooth hard (the gamete from the F1 is rH) parental Rh/rh rough soft (the gamete from the F1 is Rh) parental Rh/rh smooth soft (the gamete from the F1 is rh) nonparental b) which F2 progeny inherited parental type gametes from the F1 parent, and which nonparental type? (See labels in a). The parental types of gametes are Rh and rH, because these allele combinations match the chromosomes of the parents of the F1. c) are the genes for roughness and softness linked? If so, what is the recombination frequency (distance in centimorgans) between the genes? Parental types > nonparental (recombinant) types, therefore, YES they are linked r.f. = # recombinants x100 = 8+12 x100 = 8.2% rf = 8.2 cM total 8+12+112+110
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Reinagel BICD100 Genetics Fall 2005 3. Three genes in Drosophila with the recessive alleles l, m, and n are known to be linked. You wish to determine the map distances between them and perform the cross shown below. ( Note: The genes are arbitrarily shown in alphabetical order - you need to determine the order of these genes on the chromosome) female male + m + x l m n l + n l m n Progeny with the following phenotypes are obtained: # progeny l m n l,m m,n l,n 99 l m n non non par 0 l m + non par non 901 l + n par par par 0 l + + par non non 0 + m n par non non 899 + m + par par par 0 + + n non par non 101
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