Genetics: Predicting Inheritance With Punnett Squares
Master genetics — dominant and recessive alleles, genotype vs. phenotype, Punnett squares, and non-Mendelian patterns like incomplete dominance and sex linkage — for ACT Science and biology.
The Short Version
- An allele is a version of a gene; dominant alleles (capital letter) mask recessive ones (lowercase).
- Genotype is the allele combination (e.g., Bb); phenotype is the visible trait.
- A Punnett square predicts offspring; a Bb × Bb cross gives a 3:1 phenotype ratio.
- Non-Mendelian patterns include incomplete dominance, codominance, and sex linkage. ACT Science / biology background.
Gregor Mendel figured out the basic rules of heredity by breeding pea plants, and those rules still power genetics today. The core idea: traits are controlled by genes, which come in versions called alleles. Some alleles are dominant and some recessive, and offspring inherit one of each from their parents. A simple grid — the Punnett square — lets you predict the odds of each outcome. A few patterns break Mendel's simple rules, and those have their own logic too.
This guide covers the vocabulary, Punnett squares, ratios, and non-Mendelian inheritance, with worked and practice questions matched to the level seen in ACT Science and biology at Northside Tutoring.
Why Genetics Matters
Genetics is a frequent topic in ACT Science biology passages and a major unit in high-school and AP Biology. Punnett-square reasoning is also a satisfying blend of biology and probability. (The SAT has no science section.)
The Essential Vocabulary
| Term | Meaning |
|---|---|
| Allele | a version of a gene (e.g., B or b) |
| Dominant | masks the recessive; written capital (B) |
| Recessive | shows only when paired with itself (bb) |
| Genotype | the allele pair (BB, Bb, or bb) |
| Phenotype | the visible trait |
A homozygous genotype has two of the same allele (BB or bb); a heterozygous one has two different alleles (Bb).
The Punnett Square
A Punnett square crosses the alleles from each parent. For two heterozygous parents (Bb × Bb), put one parent's alleles across the top and the other's down the side, then fill each cell:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Reading the Ratios
From that Bb × Bb cross, the four cells are BB, Bb, Bb, bb. The genotype ratio is 1 BB : 2 Bb : 1 bb (1:2:1). Since B is dominant, three of the four show the dominant trait and one shows the recessive — a phenotype ratio of 3:1. So there's a 25% chance of the recessive trait (bb).
The classic 3:1
A cross of two heterozygotes (Bb × Bb) is the most-tested case: it yields a 3:1 dominant-to-recessive phenotype ratio, with a 1-in-4 chance of the recessive trait. Recognizing this instantly answers many genetics questions.
Non-Mendelian Patterns
Some traits don't follow simple dominance:
- Incomplete dominance: the heterozygote is a blend — a red and a white flower yield pink.
- Codominance: both alleles show fully — like AB blood type, where both A and B appear.
- Multiple alleles: a gene has more than two versions in the population (blood type has A, B, and O).
Sex-Linked Traits
Genes on the sex chromosomes (especially the X) follow special patterns. Because males have only one X, X-linked recessive traits like red-green color blindness appear more often in males — a single recessive allele is enough to show the trait, with no second X to mask it.
Where You'll See This — Test by Test
Genetics appears in ACT Science biology passages and is a core biology unit; the SAT has no science section and the SSAT doesn't test it. Punnett-square reasoning blends biology with probability.
ACT Science
Genetics passages and Punnett-square reasoning appear on the ACT Science section.
Explore ACT Tutoring → K-12 CurriculumBiology
Mendelian and non-Mendelian inheritance are central to high-school and AP Biology.
Explore Science Tutoring → College AdmissionsSAT
No SAT science section; genetics isn't tested there among admissions exams.
Explore SAT Tutoring → K-12 CurriculumSchool Science
A foundational life-science topic blending biology and probability.
Explore Science Tutoring →Watch the Lesson
Sometimes a diagram needs a voice. In the short video below, one of our Northside tutors walks through the core idea and works through test-style problems in real time.
Genetics — In Plain English
A live walkthrough from our tutoring team.
— Featuring a Northside Tutoring instructor
Worked Example Problems
These problems are calibrated to the difficulty you'll actually see on test day. Try each one before opening the solution.
In a Bb × Bb cross, what fraction of offspring show the recessive trait?
Show solution
Only bb shows the recessive trait — 1 of 4 cells.
What is the phenotype ratio of a Bb × Bb cross (B dominant)?
Show solution
Three show the dominant trait (BB, Bb, Bb) to one recessive (bb): 3:1.
Is the genotype Bb homozygous or heterozygous?
Show solution
Two different alleles — heterozygous.
A red flower (RR) crosses with a white flower (WW) and all offspring are pink. What inheritance pattern is this?
Show solution
The heterozygote is a blend, so this is incomplete dominance.
Why are X-linked recessive traits more common in males?
Show solution
Males have only one X chromosome, so a single recessive allele shows — there's no second X to mask it.
Common Mistakes to Avoid
Three points students often miss
- Confusing genotype and phenotype. Bb is a genotype; the trait you can see is the phenotype.
- Forgetting the 3:1 vs. 1:2:1 distinction. The phenotype ratio of Bb × Bb is 3:1; the genotype ratio is 1:2:1.
- Treating all traits as simple dominance. Incomplete dominance, codominance, and sex linkage follow different rules.
Practice Problems — You Try
Three problems below. Work each before checking the solution.
In a Bb × bb cross, what fraction of offspring are bb?
Show solution
The cells are Bb, Bb, bb, bb — so 2 of 4, or 1/2.
What is the difference between genotype and phenotype?
Show solution
Genotype is the allele combination (e.g., Bb); phenotype is the visible trait it produces.
Two parents with AB and O blood types have children. What blood types are possible?
Show solution
AB parent gives A or B; O parent gives O. Children are AO (type A) or BO (type B) — so type A or type B, but never AB or O.
The Northside Method — How We Teach This 1-on-1
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