A Drosophila melanogaster with tan body and red eyes, line A, is crosses with a

Question

A Drosophila melanogaster with tan body and red eyes, line A, is crosses with a D. melanogaster with a black body and cinnabar eyes, Line B. Both lines are true breeding. The resulting offspring are all tan bodied with red eyes. One of the F1 females is crossed with a male from line B. The resulting F2 offspring are as follows: Tan body, red eyes 44 Tan body, cinnabar eyes 6 Black body, red eyes 8 Black body, cinnabar eyes 42

a) Are the two loci linked? What hypothesis do you implicitly test to find out? Do a chisquare test, cite your value and explain what it means.

b) If they are linked, how many map units apart are they?

c) What proportion of the gametes from the Line B males have the allele for black body and cinnabar eyes? What proportion of the gametes from the F1 males have the alleles for black body and cinnabar eyes?

Explanation / Answer

a). Let us consider the genotype of true breeding for tan body and red eyes is TTRR and the genotypr of true breeding for black body and cinnabar eyes is ttrr. Cross between these will have the offspring with the following genotype and phenotype.

TTRR* ttrr = all TtRr -----------> F1(100% tan bodied with red eyes) (Means Tan is dominant to black and red eyes is dominant ot cinnabar eyes).

Cross between F1 female and line B males is,

TtRr* ttrr = TtRr (1/4), Ttrr (1/4), ttRr (1/4), ttrr (1/4). Means all offspring are produced in 1:1:1:1 proportion. The given data (Tan body, red eyes 44 Tan body, cinnabar eyes 6 Black body, red eyes 8 Black body, cinnabar eyes 42) is deviated from this, means the genes are linked.

C2 = (Observed freq- Expected freq)2 / Expected freq

= (44-25)2/25 + (6-25)2/25 + (8-25)2/25 + (42-25)2/25 = 14.4 + 14.4 + 11.56 + 11.56 = 51.92

b). The distance between T and R genes = (8+6/ 100) *100 = 14 m.u

(c) The gametes produced by Line B males (ttrr) are all tr. (Means 100% gametes from the Line B males have the allele for black body and cinnabar eyes). The gametes produced by F1 male (TtRr) are TR, tr, Tr, rT. Means, only 1/4th of gametes from the F1 males have the alleles for black body and cinnabar eyes.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.