To solve the problem of finding the phenotypic ratio for a dihybrid cross between two heterozygotes, we need to understand a few basic concepts in genetics.
Firstly, a dihybrid cross examines the inheritance of two different traits. In this scenario, the parents are heterozygous for both traits, meaning each parent has two different alleles for each trait. Let's assume the traits involve the alleles A/a and B/b:
- The genotype of each parent is AaBb.
When these parents are crossed, they can produce offspring with different combinations of alleles. To determine the phenotypic ratio, we need to consider all possible combinations and their associated phenotypes.
A Punnett square is often used to identify these combinations. For a dihybrid cross (AaBb x AaBb), the Punnett square is a 4x4 grid, representing all combinations of the two traits.
Producing the possible offspring combinations, we get:
- 1/16 AABB
- 2/16 AABb
- 2/16 AaBB
- 4/16 AaBb
- 1/16 AAbb
- 2/16 Aabb
- 1/16 aaBB
- 2/16 aaBb
- 1/16 aabb
Counting these combinations with their respective phenotypes:
1. Dominant for both traits (A_B_): 9 combinations.
2. Dominant for the first trait, recessive for the second (A_bb): 3 combinations.
3. Recessive for the first trait, dominant for the second (aaB_): 3 combinations.
4. Recessive for both traits (aabb): 1 combination.
Summarizing these groups, we obtain the phenotypic ratio:
- 9 dominant for both traits
- 3 dominant for the first trait and recessive for the second trait
- 3 recessive for the first trait and dominant for the second trait
- 1 recessive for both traits
So, the correct phenotypic ratio is 9:3:3:1.
Therefore, the correct answer is C. [tex]$9:3:3:1$[/tex].
