Punnett Squares and Monohybrid Ratios
Use a Punnett square to predict genotype and phenotype probabilities for a single-gene cross with complete dominance. Distinguish homozygous and heterozygous genotypes and explain variation in observed offspring counts.
What a learner can do afterwards
- Completes a Punnett square for a single-gene cross with complete dominance and states the expected phenotype ratio.
- Uses homozygous for two identical alleles and heterozygous for two different alleles, without confusing genotype with phenotype.
- Explains why a small offspring sample may not match the predicted ratio exactly.
1 · Read
Mutations can create new alleles, which are versions of a gene. Here we follow one gene in diploid pea plants, with one allele inherited from each parent. Two identical alleles are homozygous: TT is homozygous dominant and tt is homozygous recessive. Two different alleles, Tt, are heterozygous. Genotype means the allele combination; phenotype means the observable characteristic.
Use complete dominance here: T gives tall plants and t gives short plants. TT and Tt are tall; tt is short. A tall phenotype alone cannot distinguish TT from Tt. For seed shape, R gives round seeds and r gives wrinkled seeds: RR and Rr are round; rr is wrinkled. Other traits may follow different inheritance patterns.
Gametes are reproductive cells. Each carries one allele for this gene. A heterozygous parent passes either allele with equal chance. To fill a Punnett square, write one parent's alleles across the top and the other's down the side. Combine one column allele with one row allele in every box. Write the capital letter first. Count genotypes, then group them by phenotype.
Cross Rr with Rr. Each box is equally likely.
| Gamete | R | r |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
There are two routes to Rr. The genotype ratio is 1 RR to 2 Rr to 1 rr. RR and rr are homozygous, so half the boxes are homozygous. RR and Rr are round, giving 3 round to 1 wrinkled.
A homozygous parent has only one allele type to pass on. In RR crossed with rr, one parent always gives R and the other always gives r. Every offspring is Rr and has round seeds. Use the actual parental alleles for each cross. A different cross can give a different ratio.
Each box gives a probability, not a fixed family count. In this model, fertilizations are independent: earlier offspring do not change the next one's chance. Expected count equals total offspring multiplied by probability. Rr by Rr gives an expected 200 x 1/4 = 50 wrinkled plants. Actual counts can differ. Larger samples tend to have proportions closer to predictions, but exact agreement is never required.
Combine one allele from each parent in every box. Count genotypes, apply the stated dominance rule and treat the resulting ratios as probabilities.
Lesson revisions, questions and flashcards written for this dataset. OpenStax inheritance records were consulted for scientific facts and assessment design.
Readings, extra questions and flashcards written for this dataset, with OpenStax records consulted for facts and assessment design. Video reference: Khan Academy on YouTube.
More to read
Keep one contribution from each parent
Every box in a Punnett square must contain one allele from each parent. Begin with the parental genotypes, list their possible gamete alleles and combine the entries. When both parents are heterozygous, a mixed genotype can arise in two ways: the dominant allele can come from either parent. Both routes count. After finding the genotype probabilities, apply the stated dominance rule to predict phenotypes. Changing either parent's genotype can change those probabilities. A 3-to-1 phenotype ratio is therefore a result for a particular cross and model, not a rule for every single-gene cross.
A phenotype may fit more than one genotype
An allele combination is a genotype. An observable characteristic is a phenotype. Under complete dominance, a homozygous dominant genotype and a heterozygous genotype can produce the same phenotype. Looking at that characteristic cannot distinguish the two. Within this model, the recessive phenotype identifies the recessive homozygous genotype. Check the model before using either conclusion. Some characteristics involve several genes, and some single-gene traits do not follow complete dominance. Human eye colour, for example, should not be treated as a one-gene dominant/recessive trait.
A predicted count is not a required count
Multiply an outcome's probability by the number of offspring to find its expected count. The count you observe can differ. If fertilizations are independent, previous outcomes do not alter the next offspring's probability. Three tall offspring do not make a short offspring due. A larger sample tends to have proportions closer to the predicted probabilities, but it does not have to match an exact ratio. Probabilities still describe individual offspring in small samples; small samples just allow more variation in the proportions observed.
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