In Drosophila black body color is recessive to gray, and vestigial wings are rec
ID: 5749 • Letter: I
Question
In Drosophilablack body color is recessive to gray, andvestigial wings are recessive to normal wings. Apure breeding gray-bodied, normal-winged fly iscrossed with a pure breeding black-bodied,vestigial-winged fly. An F1 female is crossedwith a male that has a black body andvestigial wings. The following progeny areobtained:63 gray, normal 65 gray, vestigial 35 black, normal 37 black, vestigial
Are the two genes, linked? If so, how many map unitsseparate them? In Drosophilablack body color is recessive to gray, andvestigial wings are recessive to normal wings. Apure breeding gray-bodied, normal-winged fly iscrossed with a pure breeding black-bodied,vestigial-winged fly. An F1 female is crossedwith a male that has a black body andvestigial wings. The following progeny areobtained:
63 gray, normal 65 gray, vestigial 35 black, normal 37 black, vestigial
Are the two genes, linked? If so, how many map unitsseparate them?
Explanation / Answer
If the traits are not linked, then the parental and F1 progenygenotypes will be (G = Gray body allele, N = Normal wingsallele): Parental: Grey, Normal X Black, Vestigial GGNN X ggnn Gametes: GN X gn F1: GgNn Then the next cross is: Parental: GgNn X ggnn Gametes: (GN, Gn, gN, gn) X gn F2: GgNn : Ggnn : ggNn : ggnn Translating these to phenotypes: F2: Gray, Normal : Gray, Vestigial : Black, Normal : Black, Vestigial All of these phenotypes would occur in equal numbers if the geneswere not linked. From the data given, we can tell that they are notequal - the first two phenotypes (Gray, normal and gray, vestigial)occur at almost twice the number of the other two phenotypes. Thissuggests linkage. Since the genes are linked, the correct way to write them wouldbe: Parent: GN/GN X gn/gn Gametes: GN Xgn F1: GN/gn Where the '/' indicates the alleles are linked on the chromosome(e.g. there is a 'GN' linked on one chromosome, and 'gn' linked onits copy chromosome.). The next cross would then be: Parent: GN/gn Xgn/gn Gametes: (GN, gn) X gn F2: GN/gn, gn/gn (Grey,normal and Black, vestigial) The phenotypes that are not listed - Black, normaland Grey, vestigial - are called 'recombinant phenotypes' becausethey are produced when recombination 'crossing over' (think of itas an exchange of alleles) occurs - the recombinant phenotypes aredifferent to the parental ones. The 'recombination frequency' isthen the ratio of the number of flies with recombinant phenotypesto the total number of flies: RF = 35 + 65 / (63 + 65 + 35 + 37) = 100 /200 = 0.5(50%) A 'map unit' (mu) is defined as 1% recombination frequency, or 0.01RF. Therefore RF = 0.5 means the genes are 50 map units apart. (Not sure if this is right - it's been a while since I've donegenetics, but I thought 50% recombination frequency meant the genesare independently assorting? You should check with someone else'sresults if you can...)