Problem 1 - Campbell Biology

Question

DRAW IT Two pea plants heterozygous for the characters of pod color and pod shape are crossed. Draw a Punnett square to determine the phenotypic ratios of the offspring.

Step-by-Step Solution

Answer

The phenotypic ratio is 9:3:3:1 for (green smooth):(green constricted):(yellow smooth):(yellow constricted).

Step 1 - Define the Genotypes

Both pea plants are heterozygous for pod color and pod shape. Let:- Green pod color (G) be dominant over yellow (g)- Smooth pod shape (S) be dominant over constricted (s)Therefore, the genotype for both plants is GgSs.

Step 2 - Set up the Punnett Square

Create a 4x4 Punnett square. Each cell will represent a possible genetic combination for the offspring. There are 16 possible combinations.

Step 3 - List the Possible Gametes

Each parent can produce four types of gametes. For GgSs, the possible gametes are GS, Gs, gS, and gs. Write these gametes along the top and side of the Punnett square.

Step 4 - Fill in the Punnett Square

Combine each row and column to fill in the Punnett square with the resulting genotypes. For example, the top left cell would combine GS from the top with GS from the side to give GGSS.

Step 5 - Determine the Phenotypes

Analyze the genotypes in the Punnett square to determine the phenotypes. The phenotypes based on the dominance rules are:- Green and smooth (G_S_)- Green and constricted (G_ss)- Yellow and smooth (ggS_)- Yellow and constricted (ggss)

Step 6 - Count the Phenotypic Ratios

Count the occurrences of each phenotype combination in the Punnett square to determine the phenotypic ratios. You`ll get: 9 green smooth (G_S_), 3 green constricted (G_ss), 3 yellow smooth (ggS_), and 1 yellow constricted (ggss).

Key Concepts

Punnett square

A Punnett square is a helpful tool in genetics that allows us to predict the possible genetic combinations of offspring from two parents. It is a grid, where each cell represents a possible combination of alleles. We use letters to denote alleles. For example, 'G' for green pod color and 'g' for yellow pod color. In a 4x4 Punnett square, we consider all the possible combinations from both parents, ensuring each trait combination is represented. This helps us to visualize how traits are inherited and what traits the offspring might have.

Genotype combinations

Genotype will show the genetic makeup of an organism, including the combination of alleles inherited from the parents. In our exercise, both pea plants are heterozygous for two traits: pod color and pod shape. Therefore, their genotypes are GgSs. Each parent can produce four types of gametes due to the combinations of their alleles. Specifically, the possible gametes are GS, Gs, gS, and gs. By placing these gametes along the top and side of the Punnett square, we can fill in the grid to see all possible outcomes. For instance: GS with GS gives GGSS, demonstrating a combination where both traits are dominant.

Phenotypic ratios

Phenotypic ratios will tell us the frequency of different physical traits in the offspring. From our completed Punnett square, we can identify the phenotypes based on the combinations of alleles. The phenotypes for our traits (pod color and pod shape) are determined by dominance rules. After counting the occurrences of each combination in the Punnett square, we get the phenotypic ratios:

• 9 green and smooth (G_S_)
• 3 green and constricted (G_ss)
• 3 yellow and smooth (ggS_)
• 1 yellow and constricted (ggss)

. This ratio (9:3:3:1) helps us understand the physical traits distribution among the offspring.

Dominance rules

Dominance rules help us understand which traits will be expressed in the presence of others. A dominant allele will mask the presence of a recessive allele. For example, in our pea plants, the green pod color (G) is dominant over yellow (g), and smooth pod shape (S) is dominant over constricted (s). This means any time there is at least one 'G' or 'S' present, the dominant trait will be expressed. Therefore, in our genotypes, Gg or GG will result in green pods, while Ss or SS will result in smooth pods. Through understanding these rules, we can predict the phenotype of an organism from its genotype.