Punnett squares help students predict how traits may be inherited from parents to offspring. This cheat sheet covers alleles, genotypes, phenotypes, dominant and recessive traits, and probability in genetics. Students need these tools to solve inheritance problems clearly and to understand that genetic outcomes are predicted as chances, not guarantees.
The main idea is that each parent contributes one allele for a gene, and allele combinations determine possible offspring genotypes. A monohybrid cross tracks one trait, while a dihybrid cross tracks two traits at the same time. Probability can be written as a fraction, decimal, or percent, and each offspring has an independent chance of receiving any predicted allele combination.
Key Facts
- A genotype is the allele combination for a trait, such as TT, Tt, or tt.
- A phenotype is the observable trait, such as tall plants or short plants.
- A dominant allele is shown with a capital letter and can appear in the phenotype when only one copy is present, such as Tt.
- A recessive allele is shown with a lowercase letter and appears in the phenotype only when two recessive copies are present, such as tt.
- In a monohybrid cross of Tt x Tt, the genotype ratio is 1 TT : 2 Tt : 1 tt and the phenotype ratio is 3 dominant : 1 recessive.
- Probability = number of desired outcomes / total number of possible outcomes.
- To convert probability to a percent, use percent = fraction x 100, so 3/4 = 75%.
- In a dihybrid cross of AaBb x AaBb with complete dominance, the common phenotype ratio is 9 : 3 : 3 : 1.
Vocabulary
- Allele
- An allele is a version of a gene, such as A or a.
- Genotype
- A genotype is the pair of alleles an organism has for a specific trait.
- Phenotype
- A phenotype is the physical or observable form of a trait produced by a genotype and the environment.
- Homozygous
- Homozygous means an organism has two identical alleles for a gene, such as BB or bb.
- Heterozygous
- Heterozygous means an organism has two different alleles for a gene, such as Bb.
- Punnett Square
- A Punnett square is a diagram used to predict possible allele combinations in offspring.
Common Mistakes to Avoid
- Confusing genotype with phenotype is wrong because genotype describes alleles, while phenotype describes the visible or expressed trait.
- Writing recessive alleles as capital letters is wrong because capital letters represent dominant alleles and lowercase letters represent recessive alleles.
- Assuming a 75% dominant prediction means exactly 3 out of every 4 offspring will show the trait is wrong because probability predicts chances for each birth or offspring event.
- Putting both parent alleles together in one box is wrong because each box should show one allele from each parent, forming one possible offspring genotype.
- Forgetting to list all gamete combinations in a dihybrid cross is wrong because missing gametes leads to an incomplete Punnett square and incorrect ratios.
Practice Questions
- 1 In pea plants, T is tall and t is short. Cross Tt x Tt. What are the genotype ratio and phenotype ratio?
- 2 A black fur allele B is dominant over a white fur allele b. Cross Bb x bb. What percent of offspring are expected to have black fur?
- 3 In a cross of Aa x Aa, what is the probability that an offspring will be homozygous recessive? Give your answer as a fraction and a percent.
- 4 Why can two heterozygous parents with the same dominant phenotype have an offspring with the recessive phenotype?