Question

In tomatoes, a heterozygous plant with normal fruit and purple stems is crossed with a recessive plant having fasciated fruit and green stems. The following distribution of offspring is observed:
normal fruit, purple stems 38.5%  fasciated fruit, green stems 38.5%
normal fruit, green stems 11.5%
  fasciated fruit, purple stems 11.5%
What conclusion can be made regarding the loci for fruit shape and stem color? HINT determine the RF

What the loci and RF here mean? How to answer this question?

Answer 1

Let the genotype of the heterozygous tomato plant with normal fruit and purple stems be represented as FfSs.
Let the genotype of the recessive tomato plant with fasciated fruit and green stems be represented as ffss.
Upon crossing, as per Mendelian Laws, we should observe either FS or fs gamete from the heterozygous plant and only fs gamete from the recessive one.
As such the progeny should be 50% normal fruit, purple stems & 50% fasciated fruit, green stems.
Production of the intermediate varieties, that is normal fruit, green stems & fasciated fruit, purple stems, concludes that the loci for fruit shape and stem colour on the same side of the centromere and are as such, undergoing recombination at a synapse in between the two loci to yield alternating allele combinations not seen in parent heterozygous plant.
The Recombination is causing the RF and rf gametes from the heterozygous plant to undergo genetic crossing over and also yield the Rf and rF gametes, although not at equal frequency. Hence Recombination Frequency is calculated as = (Number of Recombinants) / (Total Number of Offsprings)
= (11.5% + 11.5%) / (100%)
= 23% RF ANSWER