Radishes may be long, round or or oval in shape. Crosses between long and oval gave 159 long and 156 oval. Crosses between oval and round gave 203 oval and 199 round. Crosses between long and...

Radishes may be long, round or or oval in shape. Crosses between long

and oval gave 159 long and 156 oval. Crosses between oval and round

gave 203 oval and 199 round. Crosses between long and round gave

576 oval, while oval by oval crosses gave 121 long, 243 oval and 119

progeny. What is the genetic control of radish shape?

Assign genotypes to the three phenotypes.

Asked on by mary677

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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The above information shows that radish shape is incompletely dominant.  The fact that there are three phenotypes is the strongest evidence for this. The other piece of important evidence is the long x round cross.  It didn't produce any long or round.  It produced oval, which is a sort of in between shape of long and round.  That's usually what incomplete dominance does.  It produces traits that are a sort of blend between the other two traits.  

If we assign genotypes to the three phenotypes, that would look something like this:

Long = LL

Oval = Ll

Round = ll

The first cross was long x oval.  That would be LL x Ll.  The only possible offspring are LL or Ll.  The expected ratio is 1:1, and your data is just barely different than that.  

The second cross was oval x round.  That's Ll x ll.  Again the expected ratio is 1:1 with Ll and ll.  That's what the results showed as well.  

The third cross was long x round.  That's LL x ll.  No matter what, hybrids (Ll) are produced.  If it was normal dominance, all of the offspring would be long. That's not what happened.  All offspring were oval, which indicates incomplete dominance.  

The fourth cross was oval x oval.  That's Ll x Ll.  That could produce LL (long), Ll (oval), and/or ll (round).  The expected genotypic and phenotypic ratios would both be 1:2:1.  Again, the data shows this was the result.   

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