Question:
A Mendelian "Dihybrid Cross" (e.g., RRYY × rryy) results in a phenotypic ratio of 9:3:3:1 in the F₂ generation. This ratio supports which law?
A Mendelian "Dihybrid Cross" (e.g., RRYY × rryy) results in a phenotypic ratio of 9:3:3:1 in the F₂ generation. This ratio supports which law?
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Remember the genetic ratios:
- Monohybrid phenotypic ratio ($3:1$) supports the Law of Dominance and Law of Segregation.
- Dihybrid phenotypic ratio ($9:3:3:1$) supports the Law of Independent Assortment.
- Monohybrid phenotypic ratio ($3:1$) supports the Law of Dominance and Law of Segregation.
- Dihybrid phenotypic ratio ($9:3:3:1$) supports the Law of Independent Assortment.
Updated On: May 21, 2026
- Law of Dominance
- Law of Segregation
- Law of Independent Assortment
- Law of Purity of Gametes
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The Correct Option is C
Solution and Explanation
Step 1: Understanding the Question:
The question asks to identify which of Mendel's laws of inheritance is validated and demonstrated by the classic $9:3:3:1$ phenotypic ratio obtained from a dihybrid cross in the $\text{F}_2$ generation.
Step 2: Detailed Explanation:
• A dihybrid cross is a genetic cross that tracks the inheritance of two different traits governed by two different genes.
• Mendel crossed homozygous dominant pea plants having round and yellow seeds ($\text{RRYY}$) with homozygous recessive plants having wrinkled and green seeds ($\text{rryy}$).
• The resulting $\text{F}_1$ generation plants were all heterozygous round and yellow ($\text{RrYy}$), as the dominant alleles masked the expression of the recessive alleles.
• When $\text{F}_1$ plants were self-pollinated ($\text{RrYy} \times \text{RrYy}$), the $\text{F}_2$ generation exhibited a phenotypic ratio of $9:3:3:1$. This represents 9 round-yellow, 3 round-green, 3 wrinkled-yellow, and 1 wrinkled-green offspring.
• This ratio occurs because the alleles for seed shape ($\text{R}$ and $\text{r}$) assort into gametes completely independently of the alleles for seed color ($\text{Y}$ and $\text{y}$).
• This independent behavior allows for the formation of novel non-parental recombinant phenotypes, which are round-green and wrinkled-yellow.
• Mathematically, the dihybrid ratio of $9:3:3:1$ is the product of two independent monohybrid crosses: $(3 \text{ dominant}: 1 \text{ recessive}) \times (3 \text{ dominant}: 1 \text{ recessive})$.
• This independent segregation and recombination of alleles for different traits is the definition of Mendel's Law of Independent Assortment.
Step 3: Final Answer:
The $9:3:3:1$ dihybrid ratio supports Mendel's Law of Independent Assortment.
The question asks to identify which of Mendel's laws of inheritance is validated and demonstrated by the classic $9:3:3:1$ phenotypic ratio obtained from a dihybrid cross in the $\text{F}_2$ generation.
Step 2: Detailed Explanation:
• A dihybrid cross is a genetic cross that tracks the inheritance of two different traits governed by two different genes.
• Mendel crossed homozygous dominant pea plants having round and yellow seeds ($\text{RRYY}$) with homozygous recessive plants having wrinkled and green seeds ($\text{rryy}$).
• The resulting $\text{F}_1$ generation plants were all heterozygous round and yellow ($\text{RrYy}$), as the dominant alleles masked the expression of the recessive alleles.
• When $\text{F}_1$ plants were self-pollinated ($\text{RrYy} \times \text{RrYy}$), the $\text{F}_2$ generation exhibited a phenotypic ratio of $9:3:3:1$. This represents 9 round-yellow, 3 round-green, 3 wrinkled-yellow, and 1 wrinkled-green offspring.
• This ratio occurs because the alleles for seed shape ($\text{R}$ and $\text{r}$) assort into gametes completely independently of the alleles for seed color ($\text{Y}$ and $\text{y}$).
• This independent behavior allows for the formation of novel non-parental recombinant phenotypes, which are round-green and wrinkled-yellow.
• Mathematically, the dihybrid ratio of $9:3:3:1$ is the product of two independent monohybrid crosses: $(3 \text{ dominant}: 1 \text{ recessive}) \times (3 \text{ dominant}: 1 \text{ recessive})$.
• This independent segregation and recombination of alleles for different traits is the definition of Mendel's Law of Independent Assortment.
Step 3: Final Answer:
The $9:3:3:1$ dihybrid ratio supports Mendel's Law of Independent Assortment.
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