1) (14 pts) You have discovered a new species of worms for which the haploid num

Question

1) (14 pts) You have discovered a new species of worms for which the haploid number i 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 o you set-up 3 crosses as follows: n chromosome II, marker 3 is on chromosome IIl), Cross 1: marker mutation 1 x dpy-1 Cross 2: marker mutation 2 x dpy-1 Cross 3: marker mutation 3 x dpy- Fl 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 1 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). the 1000 FI progeny from the testcross, the following phenotypes were observed 450 470 30 Dpy-1, eyeless, hairy wild type hairy, dpy-I eyeless eyeless, hairy dpy-I eyeless, dpy-I hairy 36 b) What is the order of genes on the chromosome and the map distances between the genes?

Explanation / Answer

(a) According to the data provided in the question , the dpy-1 gene which gives most abundant phenotypes with marker mutation 1 (located on chromosome I), the gene must also be present on the same chromosome.

(b) Test cross is defined as the crossing of an organism with unknown genotype to homozygous recessive organism to determine the genotype of the former depending upon the proportion of phenotype of the offsprings. This helps in determining the homozygosity or heterozygosity for the dominant phenotype. If the cross results in any offspring with recessive phenotype , the parental genotype for the allele is concluded to be heterozygous. However, if the offsprings possess only dominant phenotype, then the parent is also comsidered to be homozygous dominant for the allele under investigation.

According to the information in the question, the parental genotypes are EeHhDd X eehhdd

X = % Cross-over (E-H) = (c+d+g+h/T) X 100 = (30+40+10)/1000 * 100 = 8%

Y = % Cross-over (H-D) = (e+f+g+h/T) X 100 = (34+36+6+4)/1000 * 100 = 8%

Z = % Cross-over (E-H & H-D) = (c+d+e+f/T) X 100 = (30+40+70)/1000 * 100 = 14%

Thus, depending upon the percentage of progeny obtained after cross over events,the gene order will be Ee-Hh-Dd ; where

E : eyeless

H : hairy

D : dpy-1 (capital denominating dominant and small letter denominates recessive allele).

State of crossing over Genotype Phenotype Supposed frequency Number of progeny Non-crossover EHD/ehd EHD a 470 ehd ehd b 450 Crossover (E-H) Ehd/ehd Ehd c 30 eHD/ehd eHD d 40 Crossover (H-D) EHd/ehd EHd e 34 ehD/ehd ehD f 36 Crossover (E-H & H-D) EhD/ehd EhD g 4 eHd/ehd eHd h 6

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