Answer:
a is the middle gene.
Distance [b-a]= 24.7 mu
Distance [a-c]= 15.8 mu
Distance [b-a} = 40.5 mu
Explanation:
A homozygous wild-type female drosophila (a⁺b⁺c⁺/a⁺b⁺c⁺) is crossed with a homozygous recessive male (abc/abc). <u>The order of the genes here is arbitrary. </u>
The F1 is heterozygous for the three genes (a⁺b⁺c⁺/abc). The F1 females were test crossed (crossed with abc/abc males).
The F2 shows the following phenotypic ratios:
- 320 a⁺b⁺c⁺
- 308 a b c
- 102 a⁺ b c⁺
- 112 a b⁺ c
- 66 a⁺ b⁺ c
- 59 a b c⁺
- 18 a⁺ b c
- 15 a b⁺ c⁺
Total = 1000
The male parent is homozygous recessive for the 3 genes, so the observed phenotypes of the offspring correspond to the gametes received from the mother.
Recombination during meiosis is a rare event, so the most abundant gametes are always the parentals: a⁺b⁺c⁺ and abc.
The least abundant gametes, following the same logic, are the double crossovers (DCO): a⁺bc and ab⁺c⁺.
<h3><u>1st. Determine the gene order</u></h3>
Compare the parental and the DCO gametes. The allele that is switched corresponds to the middle gene. In this case, gene a is in the middle of the other two.
<h3><u>
2nd Determine the single crossover gametes</u></h3>
The F1 mother that generated all 8 types of gametes had the genotype b⁺a⁺c⁺/bac (correct order of genes).
- The single crossover (SCO) gametes resulting from recombination between genes b and a are b⁺ac and ba⁺c⁺.
- The single crossover (SCO) gametes resulting from recombination between genes a and c are b⁺a⁺c and bac⁺.
<h3><u>
3) Calculate the recombination frequencies between genes
</u></h3>
Recombination frequency (RF) = #Recombinants/Total progeny
-
RF [b-a]= (102+112+18+15)/1000= 0.247
- RF [f-br]= (66+59+18+15)/1000= 0.158
<h3>
<u /></h3><h3>
<u>4) Calculate the distance in map units
</u></h3>
Distance (mu) = RF x 100
Distance [b-a]= 0.247 × 100 = 24.7 mu
Distance [a-c]= 0.158 × 100 = 15.8 mu
Distance [b-a} = 24.7 mu + 15.8 mu = 40.5 mu
<h3>
<u>The gene map therefore looks like:
</u></h3>
b------------24.7 mu--------------------------a---------15.8 mu-----------c
