Problem 8 - Biology Concepts and Applications

Question

Assuming complete dominance, crosses between two dihybrid \(\mathrm{F}_{1}\) pea plants, which are offspring from a cross \(A A B B \times a a b b,\) result in \(F_{2}\) phenotype ratios of _____. a. 1: 2: 1 b. 3: 1 c. 1: 1: 1: 1 d. 9: 3: 3: 1

Step-by-Step Solution

Answer

d. 9:3:3:1

Step 1: Identify Genotypes of F1 Generation

The parent plants are homozygous for two traits: one is dominant homozygous (\(AABB\)) and the other is recessive homozygous (\(aabb\)). Crossing them results in F1 offspring that are all heterozygous for both traits (\(AaBb\)).

Step 2: Determine Genotypic Combination in F2 Generation

To find the F2 phenotype ratio, we need to consider the heterozygous dihybrid cross \(AaBb \times AaBb\). This involves the distribution of alleles to form the genotype combinations in the offspring.

Step 3: Apply the Punnett Square for a Dihybrid Cross

A dihybrid Punnett Square involves 16 combinations because each plant produces 4 types of gametes (\(AB, Ab, aB, ab\)). Each genotype forms by combining one of these gametes from each parent.

Step 4: Calculate F2 Phenotypic Ratios

The phenotypes from the Punnett Square and their frequency are calculated as follows: - 9 will have both dominant traits (\(A-B-\))- 3 will have the first dominant and the second recessive (\(A-bb\))- 3 will have the first recessive and the second dominant (\(aaB-\))- 1 will have both recessive (\(aabb\)). So, the phenotypic ratio is 9:3:3:1.

Key Concepts

Punnett square

A Punnett Square is a tool used in genetics to predict the possible genetic combinations resulting from a cross between two organisms. Typically, it is used to determine the offspring's genotypes and phenotypes based on the genetic makeup of the parents. In a dihybrid cross, which deals with two traits, the Punnett Square becomes a bit more complex. However, it remains a straightforward tool once you understand the pattern.
In our case, we are dealing with two genes: A and B. Each gene has two alleles—one dominant (A or B) and one recessive (a or b). A dihybrid cross like the one in this exercise (between two heterozygous plants, AaBb) requires a 4x4 Punnett Square. Each parent can produce four types of gametes: AB, Ab, aB, and ab.

• Each cell of the 16 square grid shows a possibility from combining one gamete from each parent.
• The resulting genotypes give rise to different phenotypic combinations based on dominance.

This systematic approach helps visualize how alleles from each parent combine and what offspring they might produce.

phenotypic ratios

Phenotypic ratios illustrate the relative frequency of different observable traits in the offspring resulting from genetic crosses. These ratios tell us how traits governed by alleles show up in the organisms emerging from the studied parental generation.
In a dihybrid cross, such as in this problem, we are observing two traits in the offspring. Complete dominance means that only a dominant allele is necessary to exhibit the dominant trait.

• The expected phenotypic ratio for a dihybrid cross with complete dominance is 9:3:3:1.
• This means that out of 16 offspring, about 9 will exhibit both dominant traits, 3 will show the first dominant and second recessive trait, another 3 will show the first recessive and second dominant trait, and 1 will show both recessive traits.

These ratios emerge from the possible combinations of alleles dictated by the Punnett Square. That's why understanding the square is crucial for predicting these outcomes.

complete dominance

Complete dominance is a genetic scenario in which one allele completely masks the effect of another allele when both are present. In the case of the dominant allele, its traits will always be expressed if it is present in the genotype.
This principle guides much of what we observe in genetic crosses under Mendelian inheritance. In our exercise, each trait (A or B) is demonstrated by two alleles: one that is completely dominant and one that is recessive.

• For example, in a genotype like Aa, the trait corresponding to 'A' is expressed because 'A' is dominant over 'a'.
• If both alleles are recessive, such as in aa, the recessive trait will appear because there is no dominant allele to mask it.

This clear pattern helps simplify predictions of phenotypic expressions when dealing with crosses, making it a pivotal concept in starting any genetic analysis involving dominant and recessive allele traits.