Lesson · B.3.3
Genetics and Punnett squares
How to read alleles, genotypes and phenotypes and build a Punnett square, the single most testable skill in HESI A2 genetics.
ObjectiveB.3.3Read6 minExam weightCell Division and Genetics ≈ 20% of Biology
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- Define allele, genotype, phenotype, homozygous and heterozygous, and use them correctly in a sentence.
- Build a Punnett square for a single trait cross and read off the genotype and phenotype ratios.
- Explain why a recessive trait can skip a generation and a dominant one usually cannot.
What the lesson concludes
- A gene is a stretch of DNA, an allele is one version of that gene, and most traits on the exam involve just two alleles.
- Genotype is the pair of alleles an organism carries, phenotype is the trait you can actually observe.
- Capital letter means dominant, lowercase means recessive, and a recessive phenotype only shows up when both alleles are recessive.
- A Punnett square sorts the possible offspring combinations into a grid, one parent's alleles across the top, the other's down the side.
- A monohybrid cross between two heterozygotes gives the classic three to one phenotype ratio, three dominant to one recessive.