*20 Waxy endosperm (wx), shrunken endosperm (sh), and yellow seedlings (v) are e
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Question
*20 Waxy endosperm (wx), shrunken endosperm (sh), and yellow seedlings (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 Fi are then crossed with a plant homozygous for the recessive alleles in a three-point testcross. The progeny of the testcross are wx sh Wx Sh v 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 87 94 10,756 a. Determine the order of these genes on the chromosome. b. Calculate the map distances between the genes. c. Determine the coefficient of coincidence and the interference among these genes.Explanation / Answer
Answer:
a). Order of genes = Wx---V----Sh
b). Wx-------7mu------V-----------30mu--------------Sh
c).
Coefficient of Coincidence (COC) = 0.81
Interference = 0.19
Explanation:
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)
Wx-------7mu------V-----------30mu--------------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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