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In a cross between two plants, one parent is heterozygous for two traits (AaBb) and the other parent is homozygous recessive for both traits (aabb). Trait A exhibits incomplete dominance, while trait B exhibits complete dominance.



What is the expected phenotypic ratio of the offspring?

A.
1:1:1:1

B.
1:2:1:2

C.
3:1

D.
9:3:3:1

1 answer

To determine the expected phenotypic ratio of the offspring from the cross between the two plants, we can follow these steps:

  1. Identify the genotypes of the parents:

    • Parent 1 (heterozygous for two traits): AaBb
    • Parent 2 (homozygous recessive for both traits): aabb

  2. Determine the gametes produced by each parent:

    • Parent 1 (AaBb) can produce the following gametes: AB, Ab, aB, ab
    • Parent 2 (aabb) can only produce the following gamete: ab

  3. Set up the Punnett square:

    • The cross would be between the gametes of both parents: 
                   ab
          ----------------
      AB | AaBb  (trait A: incomplete dominance; trait B: dominant)
      Ab | Aabb  (trait A: incomplete dominance; trait B: recessive)
      aB | aaBb  (trait A: recessive; trait B: dominant)
      ab | aabb  (trait A: recessive; trait B: recessive)

  4. Determine the phenotypes:

    • AaBb: Trait A (incomplete dominance): expression depends on genotype; dominant phenotype if AA or Aa, but a blended phenotype if heterozygous (A and a lead to an intermediate trait). For B, Bb is dominant, so shows dominant phenotype.
    • Aabb: Dominant for A and recessive for B (one of the phenotypes from trait A and recessive from trait B).
    • aaBb: Recessive for A and dominant for B (shows dominant phenotype B and recessive phenotype A).
    • aabb: Recessive for both traits (shows recessive phenotype for both traits A and B).

  5. Count the phenotypes:

    • We have one of each phenotypic combination:
      • 1 AaBb
      • 1 Aabb
      • 1 aaBb
      • 1 aabb

The phenotypic outcomes based on the dominant and recessive traits:

  • Trait A: 1 (one with dominant A, two with recessive a), 2 mixed pheno (Aa will have intermediate mix)
  • Trait B: 2 dominant (Bb) and 2 recessive (bb)

So overall, the phenotypic ratio turns out to be:

  • 1 AaBb : 1 Aabb : 1 aaBb : 1 aabb

This leads us to a resulting ratio of 1:1:1:1.

Thus, the expected phenotypic ratio of the offspring is A. 1:1:1:1.