Purple flowers are dominant to white flowers ((P = purple, p = white). You have a purple flowered
plant.
a. How would you determine if the genotype is homozygous dominant or heterozygous?
b. What would be the genotype of the flower that is crossed with the unknown genotype?
c. If the Purple flower was homozygous purple, what would you expect the genotypic and
phenotypic ratios of the resultant cross from 5b to be?
Genotypic ratio:
Phenotypic ratio:
d. If the Purple flower was heterozygous purple, what would you expect the genotypic and
phenotypic ratios of the resultant cross form 5b to be?
Genotypic ratio:
Phenotypic ratio:
The Correct Answer and Explanation is :
Let’s break this down step by step for each part of the question.
a. How would you determine if the genotype is homozygous dominant or heterozygous?
To determine whether the purple-flowered plant is homozygous dominant (PP) or heterozygous (Pp), you can perform a test cross. A test cross involves crossing the purple-flowered plant with a homozygous recessive (pp) plant, which is white-flowered. The resulting offspring’s phenotypes will reveal the genotype of the purple-flowered parent.
- If the purple-flowered parent is homozygous dominant (PP), all the offspring will inherit at least one P allele from the parent and one p allele from the white-flowered (pp) parent. The offspring will all be heterozygous (Pp) and show purple flowers.
- If the purple-flowered parent is heterozygous (Pp), about half of the offspring will inherit a P allele (giving them purple flowers) and the other half will inherit a p allele (giving them white flowers). This results in a 1:1 ratio of purple to white flowers.
b. What would be the genotype of the flower that is crossed with the unknown genotype?
The flower that is crossed with the unknown genotype should be homozygous recessive (pp), because this is the only way to reveal the true genotype of the purple-flowered plant through the offspring’s phenotypes.
c. If the Purple flower was homozygous purple, what would you expect the genotypic and phenotypic ratios of the resultant cross from 5b to be?
- Genotypic ratio: If the purple-flowered parent is homozygous dominant (PP) and is crossed with a homozygous recessive (pp) parent, all offspring will be heterozygous (Pp). Therefore, the genotypic ratio will be 100% Pp.
- Phenotypic ratio: Since the purple trait (P) is dominant to white (p), all the offspring will show purple flowers. Therefore, the phenotypic ratio will be 100% purple.
d. If the Purple flower was heterozygous purple, what would you expect the genotypic and phenotypic ratios of the resultant cross from 5b to be?
- Genotypic ratio: If the purple-flowered parent is heterozygous (Pp) and is crossed with a homozygous recessive (pp) parent, the offspring will inherit either a P or a p allele from the purple-flowered parent and a p allele from the white-flowered parent. This results in a 1:1 ratio of Pp and pp. Therefore, the genotypic ratio will be 50% Pp and 50% pp.
- Phenotypic ratio: Since the purple flower is dominant, the Pp offspring will show purple flowers, while the pp offspring will show white flowers. Therefore, the phenotypic ratio will be 50% purple and 50% white.
Explanation (300 words):
In this genetic problem, we are dealing with the inheritance of flower color in a plant. The dominant trait is purple flowers (P) and the recessive trait is white flowers (p). The key to determining the genotype of the purple-flowered plant is through a test cross. This cross involves mating the purple-flowered plant with a white-flowered plant (which has the recessive homozygous genotype pp). The phenotypic outcomes of the offspring will help us determine whether the purple-flowered plant is homozygous dominant (PP) or heterozygous (Pp).
If the purple-flowered plant is homozygous dominant (PP), all offspring from the cross will inherit one dominant P allele from the purple-flowered plant and one recessive p allele from the white-flowered plant. This results in all offspring being heterozygous (Pp) and displaying purple flowers.
On the other hand, if the purple-flowered plant is heterozygous (Pp), the offspring will inherit either a P allele (for purple flowers) or a p allele (for white flowers) from the purple-flowered plant and a p allele from the white-flowered plant. The offspring will show a 1:1 ratio of purple to white flowers, with half the offspring being purple (Pp) and half being white (pp).
The test cross method allows us to reveal the genotype of the purple-flowered plant based on the phenotypes of the offspring. If all offspring are purple, the purple-flowered plant is homozygous dominant (PP). If the offspring show both purple and white flowers in a 1:1 ratio, the purple-flowered plant is heterozygous (Pp).