A genetic engineer is going to cross two watermelon plants to produce seeds for a spring planting. He is breeding for size, and wants to have as many watermelons with the phenotype for long shape as possible. In watermelons, the allele for short shape (R) is dominant to the allele for long shape (r). Would crossing a watermelon homozygous recessive for the trait with a watermelon heterozygous for the trait give the most long watermelons possible? Explain your answer using Punnett Squares.

R stands for the short shape is dominant over the long shape r. The possible crosses are as follows

Case: Cross between Rr and rr yielding 50 % long shaped.
  r r
R Rr Rr
r rr rr
 

Case: Cross between Rr and Rr yielding 25 % long shaped.
  R r
R RR Rr
r Rr rr
 

Case: Cross between rr and rr yielding all long shaped. Therefore, the cross between two homozygous for long shape bears all long shaped watermelons. 
  r r
r rr rr
r rr rr
 

Case: All phenotype are short.
  R R
R RR RR
R RR RR

Case: Cross between RR and rr
  r r
R Rr Rr
R Rr Rr
Result: All are short shaped.

 Case: Cross between RR and Rr yielding all short shaped watermelons.
  R r
R RR Rr
R RR Rr

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