In a dihybrid cross, how many different genotypes can be formed from the combination of two heterozygous indi… - MP NHM CHO-2024 (Shift-2)

MP NHM CHO-2024 (Shift-2)

Pathology & Genetics

Medium

Appeared in: MP NHM CHO-2024 (Shift-2)

Explanation

A dihybrid cross involves tracking two separate genetic traits simultaneously.
When two individuals heterozygous for both traits (e.g., AaBb x AaBb) are crossed, the alleles for each trait assort independently, as stated by Mendel's Law of Independent Assortment.
For each single trait (e.g., Aa x Aa), there are 3 possible genotypes: homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa).
The total number of genotypes in a dihybrid cross is the product of the possibilities for each trait: 3 genotypes for the first trait × 3 genotypes for the second trait = 9 total genotypes.

Option A: This would only occur if there was no genetic variation, such as in a cross between two identical homozygous individuals (e.g., AABB x AABB).
Option B: This number does not represent the number of genotypes in a dihybrid cross. It corresponds to the number of unique gametes produced by an individual heterozygous for three traits (2³ = 8).
Option D: This number is incorrect for a dihybrid cross. It represents the number of possible phenotypes in a pentahybrid cross (2⁵ = 32), assuming complete dominance.

Visual 1: Punnett Square - A 4x4 Punnett square for a dihybrid cross (AaBb x AaBb), clearly showing the 16 boxes and how they combine to form the 9 unique genotypes.
Visual 2: Diagram - A flowchart showing the calculation: (Genotypes for Trait 1) x (Genotypes for Trait 2) = Total Genotypes, with the numbers (3 x 3 = 9) filled in.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Dihybrid Cross Genotypes as background academic context rather than a clinical decision trigger.
Understanding dihybrid crosses is fundamental to genetic counseling for predicting the probability of offspring inheriting genetic disorders controlled by two different genes.
This principle is used in agriculture and animal breeding to predict the outcomes of crosses and select for desirable combinations of traits.
What if? - If the question asked for the number of phenotypes instead of genotypes (assuming complete dominance for both traits), the answer would be 4 (dominant-dominant, dominant-recessive, recessive-dominant, recessive-recessive), following a 9:3:3:1 ratio.

How to Approach the Question

First, identify the key terms: 'dihybrid cross' (two traits) and 'heterozygous individuals' (e.g., AaBb).
Recall or determine the number of possible genotypes for a single heterozygous cross (monohybrid cross: Aa x Aa). This yields 3 genotypes: AA, Aa, and aa.
Recognize that in a dihybrid cross, the two traits assort independently.
To find the total number of genotypes, multiply the number of genotype possibilities for each trait.
Calculate: 3 (genotypes for trait 1) × 3 (genotypes for trait 2) = 9.
Select the option that matches your calculation.

Concept Tested & Keywords

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