In a Mendelian cross, a round yellow pea plant (RRYY) was crossed to a wrinkled green pea plant (rryy). What will be the F1 and F2 offsprings? Give their genotypic and phenotypic ratios. (IFS 2019, 15 Marks)

Introduction

In genetics, Mendelian inheritance describes how traits are inherited according to predictable patterns. The question concerns a cross between two pea plants with known genotypes for two traits: seed shape (round vs. wrinkled) and seed color (yellow vs. green). 

Given:

• Parental Genotypes:
  
  • Round Yellow Pea Plant (RRYY): Homozygous dominant for both traits (Round seed shape and Yellow color).
  • Wrinkled Green Pea Plant (rryy): Homozygous recessive for both traits (Wrinkled seed shape and Green color).

F1 Generation (First Filial Generation)

• The F1 generation results from crossing the two parental plants (RRYY × rryy).
• Gametes from Parental Plants:
  
  • Round Yellow (RRYY) can only produce RY gametes.
  • Wrinkled Green (rryy) can only produce ry gametes.
• F1 Genotypic Combination:
  
  • All F1 offspring will inherit one R and one Y allele from the round yellow parent (RRYY), and one r and one y allele from the wrinkled green parent (rryy).
  • Therefore, the F1 genotype for all offspring will be RrYy.
• F1 Phenotypic Traits:
  
  • Round seed shape (R) is dominant over wrinkled (r).
  • Yellow seed color (Y) is dominant over green (y).
  • Hence, all F1 offspring will display the Round Yellow phenotype.

Summary of F1 Generation:

• Genotypic ratio: 100% RrYy (heterozygous for both traits).
• Phenotypic ratio: 100% Round Yellow.

F2 Generation (Second Filial Generation)

• F2 offspring result from the self-pollination of F1 individuals (RrYy × RrYy).
• According to Mendel’s law of independent assortment, each gene will assort independently during gamete formation.
• Possible Gametes from F1 (RrYy) plants:
  
  • The F1 plant (RrYy) can produce four possible types of gametes: RY, Ry, rY, ry.
• Punnett Square: To calculate the F2 generation's genotypic and phenotypic ratios, we perform a Punnett square cross between two F1 plants.

Genotypic Ratio (F2 Generation)

• Punnett Square for RrYy × RrYy results in the following genotypic combinations:
  
  • 1 RRYy (homozygous for round, heterozygous for yellow).
  • 2 RrYy (heterozygous for both traits).
  • 1 RRYY (homozygous for round and yellow).
  • 2 RrYY (heterozygous for round, homozygous for yellow).
  • 1 RRyy (homozygous for round, homozygous for green).
  • 2 Rryy (heterozygous for round, homozygous for green).
  • 1 rrYY (homozygous for wrinkled, homozygous for yellow).
  • 2 rryy (homozygous for wrinkled and green).
  • 1 rrYy (homozygous for wrinkled, heterozygous for yellow).

Thus, the genotypic ratio is:

• 9 (Round Yellow): 3 (Round Green): 3 (Wrinkled Yellow): 1 (Wrinkled Green)

Phenotypic Ratio (F2 Generation)

• Round is dominant to wrinkled, and Yellow is dominant to Green.
• Therefore, the phenotypic expression in F2 would be:
  
  • Round Yellow (dominant for both traits) – includes all combinations that result in at least one dominant allele for seed shape (R) and seed color (Y).
  • Round Green (dominant for shape, recessive for color) – includes combinations with a dominant R allele and homozygous recessive yy.
  • Wrinkled Yellow (recessive for shape, dominant for color) – combinations with homozygous recessive for shape (rr) and at least one dominant Y.
  • Wrinkled Green (recessive for both traits) – combinations with homozygous recessive for both traits (rryy).

The phenotypic ratio will be:

• 9 Round Yellow: 3 Round Green: 3 Wrinkled Yellow: 1 Wrinkled Green.

Conclusion

The Mendelian cross between a round yellow pea plant and a wrinkled green pea plant will result in F1 offspring that are all round and yellow due to their heterozygous genotype. The F2 offspring will exhibit a variety of genotypes and phenotypes following the principles of Mendelian genetics, with a phenotypic ratio of 9:3:3:1.