A Punnett square lists each parent's possible gametes along two edges and combines them in the grid, so each cell is an equally likely offspring. Crossing two heterozygotes (Aa × Aa) gives genotypes 1 AA : 2 Aa : 1 aa and phenotypes 3 dominant : 1 recessive. Two unlinked heterozygous traits give 9:3:3:1.
How do you solve a Punnett square?
List each parent's gametes: Aa makes A and a; AaBb makes AB, Ab, aB and ab (genes unlinked).
Put one parent's gametes across the top and the other's down the side.
Combine alleles in each cell; each cell is equally likely.
Count genotypes (AA, Aa, aa) and phenotypes (dominant or recessive) and write each as a fraction of the total.
Common ratios under complete dominance
Aa × Aa: genotypes 1 AA : 2 Aa : 1 aa; phenotypes 3 dominant : 1 recessive.
A 1/4 chance applies to each offspring independently. Two carrier parents with four children may have none, one or several children with the recessive trait. Larger numbers of offspring tend to come closer to the expected ratio, but no sample size guarantees it.
Model assumptions
Autosomal genes, normal segregation, random combination of gametes, complete dominance and, for two traits, unlinked genes. Real traits can be more complex.
Questions and answers
How do you set up a Punnett square? Write one parent's gametes across the top and the other's down the side. For one gene, each parent gives one allele (Aa makes A and a). For two unlinked genes, each gamete gets one allele of each gene (AaBb makes AB, Ab, aB and ab). Fill each cell with the combined alleles, then count.
What does the 9:3:3:1 ratio mean? When two parents heterozygous for two unlinked genes (AaBb × AaBb) are crossed under complete dominance, the 16 equally likely cells give 9 with both dominant traits, 3 with the first dominant and second recessive, 3 with the reverse, and 1 with both recessive.
Are Punnett squares 100% accurate? They give probabilities under a stated model, not guarantees. A 1/4 chance applies to each offspring independently, so a family of four may have zero or several offspring with the recessive trait. Real traits can also involve linkage, incomplete dominance or many genes, which this simple model leaves out.
What is the difference between genotype and phenotype? Genotype is the pair of alleles an organism carries, such as AA, Aa or aa. Phenotype is the observable trait. Under complete dominance, AA and Aa show the same dominant phenotype, and only aa shows the recessive one.
What does heterozygous mean? Heterozygous means having two different alleles for a gene, such as Aa; homozygous means two identical alleles, such as AA or aa. A heterozygote for a recessive trait is often called a carrier because it can pass on the hidden allele.
Is genetics on the TEAS 7? Yes. ATI's content table lists 9 scored Biology items, and TEASPATH's objective map places inheritance under objective S.2.3: genes and alleles, dominant and recessive traits, genotype and phenotype, and Punnett-square probability.
Sample questions with explanations
In an invented plant, purple flowers (P) are dominant over white flowers (p). Two plants with genotype Pp are crossed. What fraction of the offspring are expected to have white flowers? Answer: B. 1/4. Pp × Pp gives PP, Pp, Pp and pp. Only pp shows the recessive white phenotype, so 1 of 4 equally likely cells, or 1/4. 3/4 is the fraction with purple flowers.
Two Aa parents are crossed under complete dominance. Among the offspring that show the dominant trait, what fraction are expected to be heterozygous? Answer: C. 2/3. The grid gives AA, Aa, Aa and aa. Three cells show the dominant trait (AA, Aa, Aa), and two of those three are heterozygous. Restricting to the dominant group changes the denominator from 4 to 3, so the answer is 2/3.
An AaBb individual is crossed with an aabb individual. The genes are unlinked and dominance is complete. What fraction of offspring are expected to show both recessive traits? Answer: C. 1/4. AaBb makes four equally likely gametes (AB, Ab, aB, ab); aabb makes only ab. Offspring are AaBb, Aabb, aaBb and aabb in equal parts, so aabb is 1/4. You can also multiply: P(aa) = 1/2 and P(bb) = 1/2, so 1/2 × 1/2 = 1/4.
TEASPATH is independent of ATI. Lab scenarios, data and questions are original practice material, not real TEAS items. Practice results are not official scores or admission predictions.
Independence
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