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R. A. Emerson crossed two different pure-breeding lines of corn and obtained a phenotypically wild-type F1 that was heterozygous for three alleles that determine recessive phenotypes: an determines anther; br is brachytic; and f is fine. He testcrossed the F1 with a tester that was homozygous recessive for the three genes and obtained these progeny phenotypes: 355 anthers: 339 brachytic, fine; 88 completely wild type; 55 anthers, brachytic. fine; 21 fine; 17 anthers, brachytic; 2 brachytic; 2 anthers, fine. Note that if a phenotype Is not mentioned then it is assumed to be WT. Draw a linkage map for the three genes (include map distances). Calculate the interference value. What were the genotypes of the parental lines? Draw them in chromosome format.

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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).

1) Determine the gene in the middle

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 an br+ f+ with an br+ f and an+ br f with an+ br f+ we notice that f is different, so f is the gene in the middle of the other two.

2) Identify the single crossover gametes

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.

3) Calculate the recombination frequencies between genes

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

4) Calculate the distance in map units

Distance (mu) = RF x 100

Distance [an-f]= 0.167 × 100 = 16.7 mu

Distance [f-b]= 0.048 × 100 = 4.8 mu

The gene map therefore looks like:

an-------------16.7 mu-----------------------f---------4.8 mu-----------br

5) Calculate the interference value

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.

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