Question

Question 3 Two true-breeding Drosophila are crossed: a normal-winged, red-eyed female and a miniature- winged, vermillion-eyed male. The Fl generation all have normal wings and red eyes. The FI offspring are crossed with miniature-winged, vermillion-eyes flies. The following offspring of that cross were counted. 233 normal wing, red eye 247 miniature wing, vermilion eye 7 normal wing, vermilion eye 13 miniature, red eye a) Explain what a testcross is (1 point). b) What is the expected phenotypic ratio from a testcross of this nature? (1 point) c) From these results, are the alleles for miniature wings and Vermillion eyes dominant or recessive? (1 point) d) Calculate the distance between the loci for the genes that code for wing length and eye color. (2 point) e) Using the letters mt for normal wings and m for miniature, and for red and v for vermillion. Place the two loci on chromosomes of the parental chromosomes and the FI generation. (3 points) parental generation F1 generation Draw following the illustration above as a model the chromosomes of the 2 recombinants with the alleles labeled. (2 points) Table 2.1 Probability

Answer 1

a) test cross is a cross between an individual who expresses the dominant trait and an individual who is homozygous recessive to determine the genotype of the parent with dominant traits and the linkage between genes in the dominant parent.

b) the expected phenotypic ratio from a test cross is

1:1:1:1

c) all F1 progenies have normal wings and red eyes.

so the normal wings and the red eyes are the dominant traits

d) let the alleles be m+ - normal wings, m- miniature wings

r+ red eyes, r vermillion eyes.

m+m+ r+r+ * mmrr

m+mr+r

the F1 is crossed with mmrr

m+mr+r * mmrr

	m+r+	m+r	mr+	mr
mr	m+mr+r ( normal wings, red eyes)	m+mrr ( normal wings, vermillion eyes)	mmr+r (miniature wings, red eyes)	mmrr (miniature wings, vermillion eyes)

if the genes are not linked

the expected ratio is

normal wings, red eyes : normal wings, vermillion eyes: miniature wings, red-eye:miniature wings, vermillion eyes=1:1:1:1

here the observed ratio is

normal wings, red eyes: normal wings, vermillion eyes: miniature wings, red-eye: miniature wings, vermillion eyes=233:7:13:247=33.28:1:1.86:35.28 ( to simplify the ratio divide with 7)

the observed ratio varies considerably.

recombination frequency between the genes, if the genes are not linked, is 50%, recombination frequency is the percentage of recombinant progenies if the genes are linked then the number of recombinant progenies is less than the number of parental type progenies here the number of the normal wings, vermillion eyes: miniature wings, red-eyes are less than other 2 phenotypes so these are the recombinant progenies.

recombination frequency= ( number of recombinant progenies/total number of progenies)$\times$100= (20/500)$\times$100= 4%

so the distance between genes is 4 cM ( recombination frequency in percentage is the distance between genes in cM)

e) and f)

f) decom bisant progenies are mtelme & matema 5 88 afar cu o r+ 0 or