Cross and Birley (1986) studied restriction site variation in the region of the

Question

Cross and Birley (1986) studied restriction site variation in the region of the alcohol dehydrogenase (Adh) gene of fruit flies in a population descended from flies trapped at a Dutch fruit market in Groningen. The following data were obtained:

Gene 1

Gene 2

Number

AdhF   M

EcoRI+S

22

AdhF M

EcoRI-s

3

AdhS N

EcoRI+S

4

AdhS N

EcoRI-s

5

Where AdhF and AdhS are the allozyme alleles of Adh and EcoRI+ and EcoRI- indicate the presence of absence of an EcoRI restriction site 3.5kb downstream from Adh.

1. Test for the presence of linkage disequilibrium.

Gene 1

Gene 2

Number

AdhF   M

EcoRI+S

22

AdhF M

EcoRI-s

3

AdhS N

EcoRI+S

4

AdhS N

EcoRI-s

5

Explanation / Answer

Haplotypes

Frequency

Numbers of individual haplotypes/ Total progenies

M +S (A1B1)

X11

=22/ 34 =0.65

M –s (A1B2)

X12

=3/34=0.88

N +S (A2B1)

X21

=4/34=0.12

N –s (A2B2)

X22

=5/34=0.15

Allelic frequency-

A1 =p1= X11 + X12

A1 =p1= 1.53

A2 =p2= X21 + X22

A2 =p2=0.27

B1 =q1= X11 + X21

B1 =q1=0.77

B2 =q2= X12 + X22

B2 =q2=1.03

Linkage disequilibrium for each haplotype

D = X11 –p1q1

D= 0.65 –(1.53 x 0.77)

D = -0.53

Now,

D = p1q2 -x12

D = 0.7

D = p2q1 –x21

D =0.087

D= x22 –p2q2

D= -0.1281

If we add all disequilibrium values, we get 0.1289 that is more than Zero (0). For, the absence of linkage disequilibrium the value should be 0. That is why here we find linkage disequilibrium.

Haplotypes

Frequency

Numbers of individual haplotypes/ Total progenies

M +S (A1B1)

X11

=22/ 34 =0.65

M –s (A1B2)

X12

=3/34=0.88

N +S (A2B1)

X21

=4/34=0.12

N –s (A2B2)

X22

=5/34=0.15

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