Waxy endosperm (wx), shrunken endosperm (sh), and yellow seedling (v) are encoded by three recessive genes in corn that are linked on chromosome 5. A corn plant homozygous for all three recessive alleles is crossed with a plant homozygous for all the dominant alleles. The resulting F1 are then crossed with a plant homozygous for the recessive alleles in a three- point testcross. The progeny of the testcross are:

wx sh V 87
Wx Sh v 94
Wx Sh V 3,479
wx sh v 3,478
Wx sh V 1,515
wx Sh v 1,531
wx Sh V 292
Wx sh v 280
Total 10,756
a. Determine the order of these genes on the chromosome.

b. Calculate the map distance between the genes.

c. Determine the coefficient of coincidece and the interference among these genes.

Please show all work

Explanation:

a). Order of genes = wx-v-sh

b).

Distance between gene wx and v = 7 m.u.

Distance between gene v and sh= 30 m.u.

Distance between gene wx and sh= 33.64 m.u.

c).

Coefficient of Coincidence=0.81

Interference=0.19

Exaplanatiion:

Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.

Hence, the parental (non-recombinant) genotype is Wx Sh V / wx sh v.

1).

If single crossover occurs between Wx & Sh..

Normal combination: Wx Sh / wx sh

After crossover: Wx sh / wx Sh

Wx sh progeny= 1515+280 = 1795

wx Sh progeny = 1531+292 = 1823

Total this progeny = 3618

The recombination frequency between Wx & Sh = (number of recombinants/Total progeny) 100

RF = (3618/10756)100 = 33.64%

2).

If single crossover occurs between Sh & V..

Normal combination: Sh V / sh v

After crossover: Sh v / sh V

Sh v progeny= 94+1531 = 1625

sh V progeny = 87+1515 = 1602

Total this progeny = 3227

The recombination frequency between Sh & V = (number of recombinants/Total progeny) 100

RF = (3227/10756)100 = 30%

3).

If single crossover occurs between Wx & V..

Normal combination: Wx V / wx v

After crossover: Wx v / wx V

Wx v progeny= 94+280= 374

wx V progeny = 87+292 = 379

Total this progeny = 753

The recombination frequency between Wx & V = (number of recombinants/Total progeny) 100

RF = (753 / 10756)100 = 7%

Recombination frequency (%) = Distance between the genes (mu)

Wx7muV30muSh

Locus1(Wx)Locus 2(V)Locus 3 (Sh)

Expected double crossover frequency = (RF between Wx & V) * (RF between V & Sh)

= 7% * 30% = 0.021

The observed double crossover frequency = 87+94 / 10756= 0.017

Coefficient of Coincidence (COC) = Observed double crossover frequency / Expected double crossover frequency

= 0.017 / 0.021

= 0.81

Interference = 1-COC

= 1-0.81 = 0.19

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