Science · Suggested Class 10 · school sequences vary
Four boxes do not promise four children with one of each outcome. A Punnett square lists probabilities in a model, not a family’s guaranteed results.
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In a simple single-gene model, each parent contributes one allele. A Punnett square combines possible gametes; genotype and phenotype ratios describe probabilities under stated assumptions.
Original BananaLearn guide · 7 November 2026
On a narrow screen, swipe the diagram or open it full size.
Use a simplified pea-height model with alleles T and t. Assume T produces the dominant tall phenotype in TT or Tt, while tt is short. The model concerns one gene, two alleles, complete dominance and equal gamete probabilities.
Real traits can involve many genes and environmental effects. Do not use this simple model to make confident predictions about complex human traits such as eye colour or intelligence.
Genotype is the allele combination, such as TT, Tt or tt. Phenotype is the expressed trait in the model, such as tall or short. TT and Tt differ in genotype but share the tall phenotype under the assumption.
Dominant does not mean better, stronger or necessarily more common. It describes how the heterozygote’s trait appears in this specified model.
A TT parent contributes T to each gamete in the model; tt contributes t. A Tt parent can contribute T or t, each with probability 1/2. Each gamete carries one allele at the locus.
Do not put a whole Tt pair into one gamete. The two parental alleles separate during gamete formation; the pair is restored when the two gametes combine.
On a narrow screen, swipe the diagram or open it full size.
Place T and t from one parent along the top and T and t from the other down the side. The four combinations are TT, Tt, Tt and tt. Each box has probability 1/4 under these assumptions.
The genotype probabilities are 1/4 TT, 1/2 Tt and 1/4 tt. Phenotype probabilities are 3/4 tall and 1/4 short. The two Tt boxes represent different gamete combinations leading to the same genotype.
TT × tt gives Tt in every box, so all offspring are heterozygous and tall in this model. Tt × tt gives Tt and tt with probability 1/2 each.
A square with four identical labels is not an error if the parental gametes require it. Reducing repeated rows or columns is fine as long as probabilities remain correctly weighted.
For Tt × Tt, four independent offspring do not have to include exactly one tt. Each offspring has a 1/4 short probability; previous outcomes do not force the next one to restore a ratio.
In a larger ideal sample, observed proportions may be nearer the model’s probabilities, but random variation remains. A sample count supports evaluating a model; it is not a guarantee for the next case.
A tall pea in this model could be TT or Tt. Appearance alone does not distinguish them. Crossing with tt can provide evidence: any short offspring shows that the tall parent supplied t.
A small sample of all tall offspring does not prove the unknown parent is TT. A Tt parent might, by chance, contribute T in all those observed cases. State that limitation.
Record the trait assumptions, parental genotypes, gamete options and probabilities, offspring genotypes and phenotypes, and an explanation of what the square cannot predict.
For practice, draw the same cross with parents swapped along the axes and check that probabilities are unchanged. This single-trait guide does not cover linked genes, incomplete dominance or multigene inheritance.
See the reasoning
Try, explain, check
Try each task before opening its explanation. These are original BananaLearn questions, not official exam or past-paper questions. Where several answers can work, the explanation gives the criteria.
An allele combination
Examples here are TT, Tt and tt.
An expressed trait under the model
Tall or short are the examples.
TT and tt
Their two alleles match.
Tt
The alleles differ.
T or t
Each carries one allele at this locus.
1/4
Only the t-from-each combination gives tt.
1/2
Two of the four equally likely boxes give Tt.
3/4
TT and Tt are tall under complete dominance.
No
It describes expression in a heterozygote, not population frequency.
No
Each independent outcome is probabilistic.
No
Both are tall in this model.
No
A Tt parent can yield that sample by chance.
Copy these headings and fill them for one worked task, then for a fresh attempt. Use your own evidence rather than copying the model answer.
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Original BananaLearn explanations, examples, illustrations and practice. Source facts checked 7 November 2026. Suggested levels are learning guides, not a claim that every board or textbook uses the same sequence.
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