Punnett Squares and Mendelian Genetics for the HESI A2
By Maria Santos, M.Ed. · Updated April 18, 2026
Draw a 2x2 grid. Place Parent 1's alleles on top (e.g., Bb) and Parent 2's alleles on the left (e.g., bb). Drag the top letters down and the left letters across into the 4 boxes. Capital letters (Dominant) always mask lowercase letters (Recessive). A Bb x bb cross results in 50% Bb (dominant trait) and 50% bb (recessive trait).
The Biology section of the HESI A2 is notorious for throwing complex-looking genetics problems at nursing hopefuls. But here is the good news: Elsevier (the creator of the HESI) only tests basic Mendelian genetics.
If you're still determining how Biology fits into your target school's requirements, check out our Ultimate HESI A2 Study Guide first. If you are ready for genetics, keep reading.
The "Must-Know" Genetics Vocabulary
You cannot solve a Punnett Square if you don't speak the language. The HESI will intentionally use terms like "homozygous recessive" instead of just giving you the letters. Memorize these pairings:
- Allele: A version of a gene. We represent them with letters (B or b).
- Dominant: The "bully" allele. If it's present, it wins. Represented by a capital letter (B).
- Recessive: The "weak" allele. It only shows up if there are NO dominant alleles around. Represented by a lowercase letter (b).
- Homozygous: "Homo" means same. Two of the same alleles (BB or bb).
- Heterozygous: "Hetero" means different. Two different alleles (Bb).
- Genotype: The actual genetic makeup — the letters (Bb).
- Phenotype: The *physical* result you can take a "photo" of — the actual trait (Brown hair).
How to Solve a HESI Genetics Word Problem
The HESI will give you a paragraph. Your first job is to extract the parent genotypes.
Example Word Problem:
In pea plants, tall (T) is dominant over short (t). A homozygous dominant plant is crossed with a heterozygous plant. What percentage of the offspring will be short?
Step 1: Decode the parents.
Parent 1: "Homozygous dominant" means same letters, both dominant: TT.
Parent 2: "Heterozygous" means different letters: Tt.
Step 2: Draw the Square.
Put "T T" across the top. Put "T t" down the left side.
| T | T | |
| T | TT | TT |
| t | Tt | Tt |
Step 3: Answer the specific question.
The question asks: "What percentage will be short?"
To be short (recessive), an offspring MUST be "tt". Looking at our square, there are 2 "TT" and 2 "Tt" boxes. There are ZERO "tt" boxes.
Answer: 0%
📋 From the Tutor's Desk
A classic error I see students make during our mock exams is answering the *wrong* ratio. The math is simple, but the reading comprehension is tricky. One of my students, Kevin, did a perfect heterozygous cross (Bb x Bb), resulting in 1 BB, 2 Bb, and 1 bb. The question asked for the PHENOTYPE ratio (Physical trait). He selected 1:2:1. But that is the Genotype ratio! Because "BB" and "Bb" both look exactly the same physically (they both express the dominant trait), the Phenotype ratio is actually 3:1. He lost a vital point on a question he knew how to solve simply because he didn't read "phenotype" carefully.
The "1:2:1" and "3:1" Heterozygous Guarantee
If you memorize one specific cross, let it be the fully heterozygous cross (e.g., Bb x Bb). The HESI tests this constantly because it tests your understanding of recessive masking. Without drawing the square, you should instantly know:
- Genotype Ratio: 1:2:1 (25% BB, 50% Bb, 25% bb)
- Phenotype Ratio: 3:1 (75% Dominant expression, 25% Recessive expression)
Check Your Knowledge
Don't stop at genetics. The Biology section also hits cellular anatomy hard. Try our Free HESI A2 Practice Quiz to test your broader Biology knowledge.
Self-Study Portal — Ace the HESI A2
Structured video lessons, genetics flashcards, practice tests, mistake tracking, and predicted scores — all for $19/mo.
Try Free Preview →Frequently Asked Questions
HESI A2 Study Resources
Find HESI A2 Prep Near You
Biology Not Your Strongest Subject?
Our HESI tutors drill these genetics problem until they're second nature. One-on-one sessions, customized to your weak areas.
Book a Free Consultation