You have discovered a new species of worms for which the haploid number is three

Question

You have discovered a new species of worms for which the haploid number is three. You isolated a dumpy worm and want to determine the linkage group of the gene dpy-1. The dpy-1 mutation is recessive. Using recessive marker mutations for each chromosome (i.e., marker 1 is on chromosome I, marker 2 is on chromosome II, marker 3 is on chromosome III), you set-up 3 crosses as follows: Cross 1: marker mutation 1 x dpy-1 Cross 2: marker mutation 2 x dpy-1 Cross 3: marker mutation 3 x dpy-1 F1 progeny for the different crosses are allowed to mate, and you select the animals carrying the marker mutation from this cross. You find the following results: Of 300 marker mutation l animals, 200 are dpy-1. Of 300 marker mutation 2 animals, 150 are dpy-1 Of 300 marker mutation 3 animals, 15 are dpy-1 a) On which chromosome is dpy-1? You now wish to determine the position of dpy-1 on the chromosome. You have identified several other recessive mutations on that chromosome. These other mutations include eyeless and hairy. You have a female worm that is heterozygous for eyeless, hairy, and dpy-1 and testcross the animal (i.e., mate the het to an animal homozygous for eyeless, hairy, dpy-1). Among the 1000 F1 progeny from the testcross, the following phenotypes were observed: Dpy-1, eyeless, hairy 450 wild type 470 hairy, dpy-1 30 eyeless 40 eyeless, hairy 34 dpy-1 36 eyeless, dpy-1 4 hairy 6 b) What is the order of genes on the chromosome and the map distances between the genes?

Explanation / Answer

a) According to the qualitative and quantitative data provided, dpy-1 gene must be in mutation 1 gene.

b) The most abundant phenotypes are the parental genotypes.

So, the parental genotypes are Dpy-1, eyeless, hairy and wild type.

Double recombinants are present in lower numbers. Here, eyeless, dpy-1 and hairy are the double recombinants. Double-crossover event moves the middle allele from one sister chromatid to the other. From the genotypes of double recombinants, we can find that 'hairy' is the middle allele.

Therefore, the order of genes are dpy-1,hairy, eyeless.

Distance between dpy-1 and hairy,

= 100 x (30+36+4+6) / 1070

= 100 x 76 / 1070

= 7.10 cM

Ditance between hairy, eyeless

= 100 x (40+34+4+6) / 1070

= 100 x 84 / 1070

= 7.85 cM

Therefore, the gene map is,

dpy-1 hairy eyeless

I----------------------------I----------------------------------I

7.1 cM 7.85 cM

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