Punnett squares and genetics for the TEAS 7: worked examples you can copy

What ATI's outline says about genetics

ATI's TEAS 7 content outline lists objective S.2.3, "Apply concepts underlying Mendel's laws of inheritance," among five biology objectives, next to S.2.2 on how genetic material relates to protein structure. Biology has 9 scored questions in total, so genetics is a small but reliable slice of the science section.

The verb matters. "Apply" means you will be asked to work a cross and give a ratio or a probability, not only define a word. That is why practicing a handful of crosses on scratch paper beats rereading definitions.

Genotype, phenotype and the words in the question stem

The genotype is the genetic makeup, written as letters such as Aa. The phenotype is the observable trait, such as purple flowers. Homozygous means two identical alleles (AA or aa) and heterozygous means two different alleles (Aa). A dominant allele shows whenever it is present, and a recessive trait shows only in aa.

The trap: AA and Aa have different genotypes but the same phenotype. If a question asks what fraction of offspring "show the dominant trait," count AA and Aa together. If it asks what fraction are "heterozygous," count only Aa.

Worked example: monohybrid cross and test cross

Purple (P) is dominant over white (p). Cross Pp x Pp. Put P and p across the top, P and p down the side, and fill the four boxes: PP, Pp, Pp, pp. Three of four boxes have at least one P, so 75 percent are purple and 25 percent white. Two of four are Pp, so the chance of a heterozygous offspring is 50 percent.

Now cross a purple plant of unknown genotype with a white pp plant. This is a test cross, and OpenStax defines it as crossing the dominant-looking individual with a homozygous recessive one. If the purple parent is Pp, the boxes read Pp, Pp, pp, pp, a 1:1 ratio, so any white offspring prove the parent was heterozygous.

Does the TEAS have dihybrid crosses?

ATI does not publish sample items showing whether dihybrid crosses appear. Its objective covers Mendel's laws, and the law of independent assortment is the one a dihybrid cross demonstrates, so it is sensible to know the result. Crossing AaBb x AaBb gives a 16-box grid with a 9:3:3:1 phenotype ratio.

You rarely need the full 16 boxes. Use the product rule: treat each gene as its own monohybrid cross and multiply. The chance of an aabb offspring is 1/4 x 1/4 = 1/16. The chance of showing both dominant traits is 3/4 x 3/4 = 9/16. An AaBb parent makes 2 to the power of 2, or 4, gamete types: AB, Ab, aB and ab.

Patterns that break the 3:1 ratio

In incomplete dominance the heterozygote looks intermediate between the parents, so a heterozygous cross gives a 1:2:1 phenotype ratio that matches the genotype ratio. In codominance both alleles show at once, as in MN blood groups, where heterozygotes carry both M and N antigens.

X-linked traits follow a different pattern because males are hemizygous: they have one X, so one recessive allele is enough. Worked example: a carrier mother (X^H X^h) and an unaffected father (X^H Y). Each son has a 50 percent chance of being affected; no daughter is affected, but each has a 50 percent chance of being a carrier.

Sources

ATI TEAS Version 7 Content Outline (PDF). Biology objectives S.2.2 and S.2.3 (Mendel's laws) wording; 9 scored biology questions (checked November 2026).

OpenStax Biology 2e, 12.2 Characteristics and Traits. Genotype vs phenotype; homozygous and heterozygous; 1:2:1 and 3:1 ratios; test cross definition; incomplete dominance; codominance (MN); X-linked hemizygous males.

OpenStax Biology 2e, 12.3 Laws of Inheritance. Laws of segregation and independent assortment; dihybrid 9:3:3:1 ratio with 16-box grid; product rule and forked-line methods; 2 to the n gamete types.