Please be able to do a 3-point testcross for Exam I.  
If you need further help, re-read pages 154 - 156 in 
your text.

In Drosophila:

y = yellow body color / y+ = gray
v = vermilion eye color / v+ = red
sn = singed bristles / sn+ = straight bristles

test cross performed, with resulting progeny:

yellow, vermilion, singed	53
yellow, vermilion		108
yellow, singed			331
yellow				5
vermilion, singed		3
vermilion			342
singed				95
wildtype			63	
TOTAL				1000

Identify the parental (P) phenotypes:
							P chromosomes
	yellow, singed		331			y	+	sn
	vermilion		342			+	v	+

Identify the double recombinant (2xR) phenotypes:
							2xR chromosomes
	yellow			5			y 	+	+
	vermilion, singed	3			+	v	sn

what is the gene order?

	y	sn	v 	OR	v	sn	y

because if you compare the parental chromosomes to the 
2xR chromosomes, you will see that the gene that has the 
opposite orientation is the sn locus.

How far apart are the genes in map units?

	between y and sn:
		95 + 108 + 5 + 3 = 211

		211/1000 = 0.211 x 100 = 21.1 map units

	between sn and v
		63 + 53 + 5 + 3 = 124
	
		124/1000 = 0.124 x 100 = 12.4 map units

What is the coefficient of coincidence and interference 
for this region of the chromosome?

the observed frequency of double crossovers (from the data 
above) is 5 + 3 = 8/1000, or 0.008

probability of a crossover between y and sn is 0.211
probability of a crossover between sn and v is 0.124
therefore, the expected frequency of 2 crossovers between 
y and v is (0.211) x (0.124) = 0.026

the coefficient of coincidence is the observed frequency 
divided by the expected frequency:  0.008/0.026 = 0.307

the interference is 1 - the coefficient of coincidence:
1 - 0.307 = 0.693