Three point linkage analysis Genetic analysis has shown that the recessive genes
ID: 218171 • Letter: T
Question
Three point linkage analysis
Genetic analysis has shown that the recessive genes an (Anther ear), br (brachytic) and f (fine stripe) are all found on chromosome #1 of maize. When a plant that is heterozygous for each of these markers is testcrossed with a homozygous recessive plant, the following results are obtained:
Testcross Progeny Numbers:
wild type- 3 (+++)
fine- 48 (++f)
brachytic- 400 (+br+)
brachytic fine – 42 (+brf) Total Offsprings = 1000
anther -45 (an++)
anther fine -402 (an+f)
anther brachytic -56 (anbr+)
anther brachytic fine- 4 (anbrf)
Calculate recombination frequencies between each of these three pairs of genes.
Draw a genetic map for the location of these 3 genes on chromosome #1 of maize. Be sure to show the map distances between each loci.
Calculate the interference.
Explanation / Answer
Answer:
Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.
Hence, the parental (non-recombinant) genotypes is +br+ / an+f.
1).
If single crossover occurs between + & br.
Normal combination: +br/an+
After crossover: ++/an br
++ progeny= 3+48=51
an br= 56+4=60
Total this progeny = 111
The recombination frequency between +&br = (number of recombinants/Total progeny) 100
RF = (111/1000)100 = 11.1%
2).
If single crossover occurs between br&+.
Normal combination: br+/+f
After crossover: brf/++
br f progeny= 42+4=46
++ progeny = 3+45=48
Total this progeny = 94
The recombination frequency between br&+ = (number of recombinants/Total progeny) 100
RF = (94/1000)100 = 9.4%
3).
If single crossover occurs between +&+.
Normal combination: ++/an f
After crossover: +f/an +
+f progeny= 48+42=90
an+ progeny = 45+56=101
Total this progeny = 191
The recombination frequency between +&+ = (number of recombinants/Total progeny) 100
RF = (191/1000)100 = 19.1%
Recombination frequency (%) = Distance between the genes (cM)
an----------11.1cM-------br-----9.4cM------f
Expected double crossover frequency = (RF between w & m) * (RF between m & f)
= 0.111 * 0.094 = 0.01
The observed double crossover frequency = 3+4 / 1000 = 0.007
Coefficient of Coincidence (COC) = Observed double crossover frequency / Expected double crossover frequency
= 0.007 / 0.01
= 0.7
Interference = 1-COC
= 1-0.7 = 0.3
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