Explanation:
We have three genes with 2 alleles each:
- an+ wildtype dominant, an anther recessive
- br+ wildtype dominant, br brachytic recessive
- f+ wildtype dominant, f fine recessive
The heterozygous F1 is testcrossed and the following offspring is obtained:
- 355 an br+ f+
- 339 an+ br f
- 88 an+ br+ f+
- 55 an br f
- 21 an+ br+ f
- 17 an br f+
- 2 an+ br f+
- 2 an br+ f
Total: 879
The expected 8 types of gametes were generated, but the frequency in which they appear differs greatly so the genes are most probably linked.
Recombination is a rare event, so the most abundant gametes are the parentals (P). The least abundant gametes are the double crossovers (DCO).
<h3><u /></h3><h3><u>1) Determine the gene in the middle</u></h3>
To determine the gene in the middle we have to compare those types of gametes, because the flipped allele is the gene in the middle. when comparing <em>an br+ f+</em> with <em>an br+ f </em>and <em>an+ br f</em> with<em> an+ br f+</em> we notice that <em>f</em> is different, so <em>f </em>is the gene in the middle of the other two.
<h3><u>2) Identify the single crossover gametes</u></h3>
We know the parental gametes, so the F1 individual that generated all 8 types of gametes had the genotype an f+ br+/an+ f br.
- The single crossover (SCO) gametes resulting from recombination between genes an and f are an+ f+ br+ and an f br.
- The single crossover (SCO) gametes resulting from recombination between genes f and br are an+ f br+ and an f+ br.
<h3><u>3) Calculate the recombination frequencies between genes</u></h3>
Recombination frequency (RF) = #Recombinants/Total progeny
- RF [an-f]= (88+55+2+2)/879=0.167
- RF [f-br]= (21+17+2+2)/879=0.048
<h3><u /></h3><h3><u>4) Calculate the distance in map units</u></h3>
Distance (mu) = RF x 100
Distance [an-f]= 0.167 × 100 = 16.7 mu
Distance [f-b]= 0.048 × 100 = 4.8 mu
<u>The gene map therefore looks like:</u>
an-------------16.7 mu-----------------------f---------4.8 mu-----------br
<h3>
<u>5) Calculate the interference value</u></h3>
If there is interference (I), then the occurrence of a CO between two genes prevents the occurrence of another CO between the other two genes.
It can be calculated as:
I = 1 - coefficient of coincidence = 1 - (observed DCO/expected DCO)
We expect the two CO to be independent events, so the expected DCO are calculated as RF[an-f] × RF [f-br] x N = 0.167 × 0.048 × 879 = 7.
I = 1 - (4/7)= 0.43.