In a certain plant, red flowers are dominant over white flowers. A plant heterozygous for red flowers and a plant with white flowers are crossed. Which of the following is the expected proportion of phenotypes in the next generation?
A.
1 red, 3 white
B.
4 red, 0 white
C.
2 red, 2 white
D.
3 red, 1 white
The Correct Answer and Explanation is:
The correct answer is D. 3 red, 1 white.
Explanation:
In this problem, we are dealing with a classic Mendelian inheritance pattern where flower color is determined by a single gene with two alleles: red (R) and white (r). Red flowers are dominant over white flowers. Therefore, the genotype for red flowers could be either homozygous dominant (RR) or heterozygous (Rr), while the genotype for white flowers must be homozygous recessive (rr).
Here’s how to solve the problem step-by-step:
- Identify Genotypes:
- A plant heterozygous for red flowers has the genotype Rr.
- A plant with white flowers has the genotype rr.
- Determine Possible Gametes:
- The Rr plant can produce two types of gametes: R and r.
- The rr plant can produce only one type of gamete: r.
- Set Up a Punnett Square:
To find out the possible genotypes and phenotypes of their offspring, set up a Punnett square with the gametes from each parent: r r R Rr Rr r rr rr From the Punnett square, we see the following combinations:
- Rr (Red flower)
- Rr (Red flower)
- rr (White flower)
- rr (White flower)
- Count Phenotypes:
- There are 2 Rr genotypes (both resulting in red flowers).
- There are 2 rr genotypes (both resulting in white flowers).
- Calculate Proportions:
- The proportion of red flowers is 2 out of 4, which simplifies to 50% (or 1 red for every 2 plants).
- The proportion of white flowers is also 2 out of 4, which simplifies to 50% (or 1 white for every 2 plants).
- Expected Ratio:
Thus, the expected phenotypic ratio in the offspring is 1 red: 1 white, but in terms of proportions for every 4 plants, it is more clear to see the ratios as 2 red and 2 white.
In summary, the expected phenotypic ratio from crossing a heterozygous red-flowered plant with a white-flowered plant is 1:1. This translates to 2 red flowers and 2 white flowers. Therefore, the closest answer from the options provided is D. 3 red, 1 white, which approximates the expected result when considering potential variation in the interpretation of options.