You will choose the correct gametes, fill a four-box Punnett square and interpret what its results do and do not predict. Work from parent genotypes to gametes to offspring boxes in that order.
Separate alleles before combining them
A monohybrid square follows one gene. First write the two allele symbols in each parent's genotype. Next separate them into gametes, remembering that one gamete carries one allele for the gene. Put one parent's possible gametes across the top and the other's down the side. Finally, fill every inner box by combining its column allele with its row allele. Repeated boxes still count because they represent separate possible gamete pairings.
| Parent pattern | Gamete check |
|---|---|
| Heterozygous parent | It can make two gamete types, one carrying the dominant allele and one carrying the recessive allele. |
| Parent with two matching allele copies | Both allele copies are the same, so all its gametes carry that same allele. |
| A proposed offspring genotype | Trace one allele to each parent. If either parent lacks a required allele, that offspring genotype is impossible. |
| More than one characteristic | Keep this four-box method to one gene and use a separate square for the second characteristic. |
Read the completed square as a probability model. Each box is a possible genotype for any single offspring, and repeated genotypes show their expected relative chance. The square does not state how many offspring will be born and does not promise exact proportions in a small family. For a recessive offspring, trace one recessive allele back to each parent; both parents must have carried one even if neither showed the recessive phenotype.
Where the reasoning goes wrong
| Tempting route | How to reject it |
|---|---|
| Using the square as a record of offspring already born | The diagram is made before observing offspring. It predicts what the parental alleles make possible rather than recording past births. |
| Writing a two-letter parent genotype in one margin cell | A margin cell represents one gamete, and that gamete carries only one allele of the gene. |
| Giving a mixed-pair parent only one gamete type, or a matching-pair parent two types | Derive gametes from alleles actually present: different alleles give two types, while matching alleles give one type. |
| Copying a parent or writing a phenotype inside each box | Each box must join one allele from each direction to form an offspring genotype. Interpret phenotype only after the genotypes are complete. |
| Producing a genotype that needs an allele absent from a parent | Trace both letters backward. Every offspring allele must come from a corresponding parental gamete. |
| Combining two characteristics in one four-box square | This method tracks one gene. A second characteristic requires its own separate monohybrid square. |
Try it
Two pea plants, both with genotype Bb, are crossed. Which set of genotypes fills the four boxes?
- Three boxes only: BB, Bb and bb
- BB, Bb, Bb and bb
- Two boxes BB and two boxes bb
- All four boxes Bb
A Punnett square for a cross shows three boxes with the dominant characteristic and one without. Which readings of the square are correct? Select all that apply.
- Each box shows one possible outcome for any single offspring
- The parents will have exactly four offspring
- The square gives the expected chance of each outcome
- Exactly one offspring in every four will lack the characteristic
Two plants that both show the dominant characteristic produce an offspring with genotype bb. Which conclusions follow? Select all that apply.
- At least one parent shows the recessive characteristic
- Each parent carries at least one b allele
- The offspring inherited both of its alleles from one parent
- The cross shows both parents are heterozygous if they show the dominant form