Question

please help me!!!

5.) You have discovered mutants in two new gene mutants in Drosophila. One mutant, curl, has small, curly, nonfunctional wings. The other mutant, big, has enormous eyes. You decide to test if these two new genes are on chromosome #2, so you perform a three-gene linkage testcross using a gene that you know is on chromosome #2, trp, a mutant that cannot make its own tryptophan (and so must get it from its food). In all three cases, these mutant alleles are recessive to their wild-type counterparts. After doing the appropriate crosses, you obtain a triple mutant, and you cross it with a triple-hybrid (appearing wild-type for all three genes, but the offspring of a triple-mutant). Here are the results of this cross (showing phenotype only). [N.B. the categories are not necessarily listed/organized by "recombinant type" so check that first!] 537 curl, big, trp 519 curl, big, trp 91 curl, big, trp 82 curl, big, trp 81 curl, big, trp 77 curl, big, trp 4 curl, big, trp 3 curl, big, trp a.) Aro one, both, or none of these two genes on chromosome #2? How do you know? b.) Identify the "category types" and which flies are in which categories. c.) Which gene is in the middle of these three? How do you know? d.) Draw a linkage map, with distances in CM, for these three genes. e.) In the map you drew, there are two possible distances for the two outside genes. Which is more accurate and why? 1.) Calculate the crossover interference that is observed in this experiment.

Answer 1

The genotypes of the two parents are:

Trihybrid parent: Curl+ curl big+ big trp+ trp
Triple mutant parent: Curl curl big big trp trp

a)

All three genes are on Chromosome 2, as these genes are not assorting independently in the F2 progeny. Assuming Independent assortment, the phenotype classes

• curl, big --> 537 + 82 = 619
• curl, big+ --> 91 + 4 = 95
• curl+, big --> 81 + 3 = 84
• curl+, big+ --> 519 + 77 = 596

would be expected in a 1:1:1:1 ratio, however, there are present in a 7.37:1.13:1:7.09 ratio. Similarly, the other two pairs (curl - trp and trp - big) also do not assort independently.

b)

Phenotype	No. of F2 progeny	Category	Notes
curl, big, trp	537	Parental
curl+, big+, trp+	519	Parental
curl, big+, trp	91	Recombinant	Single Crossover (SCO) between curl and big
curl, big, trp+	82	Recombinant	Single Crossover (SCO) between curl and trp
curl+, big, trp+	81	Recombinant	Single Crossover (SCO) between curl and big
curl+, big+, trp	77	Recombinant	Single Crossover (SCO) between curl and trp
curl, big+, trp+	4	Recombinant	Double Crossover Curl is the middle locus
curl+, big, trp	3	Recombinant
Total	1394

c)

The curl gene is in the middle of the three. Since the three loci are on the same chromosome, and the F1 trihybrid is Heterozygous for all three loci, the chromosomes in the F1 trihybrid are big+ curl+ trp+ and big curl trp. Double crossovers exchange the central locus between the parental chromosomes to produce recombinant progeny classes. In this case, the two chromosomes exchanged the curl locus and game big+ curl trp+ and big curl+ trp progeny classes.

Since double recombinants are rare, the least frequent of the 8 F2 progeny classes represent the Double Crossover progeny.

d)

(i)

Recombinant Frequency between Curl and Trp
= Number of Curl-Trp recombinants * 100 / Total No. of progeny

Number of Curl-trp recombinants = No. of curl+ trp and curl trp+ individuals = 82 + 77 + 4 + 3 = 166

=> Recombinant Frequency between Curl and Trp = 166 * 100/1394 = 11.908 % $\approx$ 11.91 %

(ii)

Recombinant Frequency between Curl and Big
= Number of Curl-Big recombinants * 100 / Total No. of progeny

Number of Curl-Big recombinants = No. of curl+ big and curl big+ individuals = 91 + 81 + 4 + 3 = 179

=> Recombinant Frequency between Curl and Big = 179 * 100/1394 = 12.84 %

(iii)

Recombinant Frequency between Trp and Big
= Number of Trp-Big recombinants * 100 / Total No. of progeny

Number of Trp-Big recombinants = No. of trp+ big and trp big+ individuals = 91+ 82+ 81+ 77 = 331

=> Recombinant Frequency between Trp and Big = 331 * 100/1394 = 23.74 %

Curl - Big 11.91cm 12.84cm 23.74CM

e.

The distance between the two outside genes, calculated directly is 23.74 cM, whereas adding up the distances based on the distance between Trp and Curl and Big and Curl, gives a distance of 24.75 cM. The distance of 24.75 cM is more accurate as it also takes into account the double crossover events that occurred between Trp and Big, which were not apparent when directly calculating the distance between these two loci.

f.

Expected Frequency of Double Crossovers = Recombination Freq. (Trp-Curl) * Recombination Freq. (Big-Curl)
    = 0.1191 * 0.1284 = 0.0153

Expected Number of Double Crossovers = Expected Freq. * No. of Progeny = 0.0153 * 1394 = 21.33 $\approx$ 21

Coefficient of Coincidence = Observed Number of Double Crossovers/Expected Number of Double Crossovers
    = (4 + 3)/21 = 7/21 = 1/3 $\approx$ 0.33

Interference = 1 - Coefficient of Coincidence = 1 - 0.33 = 0.67