Class 10 Monohybrid Cross: Punnett Square and Ratios

Check the assessment scope

State the model first

Use T for a dominant tallness allele and t for a recessive allele in a simplified Mendelian pea-plant example. TT and Tt are tall in this model; tt is short. These symbols describe this chosen trait, not a universal rule that every visible trait has one dominant and one recessive version.

Cross two heterozygous parents: Tt × Tt

Each parent contributes one allele. Under equal gamete probabilities and random fertilisation, the four cells are equally likely combinations.

Swipe across the table to see all columns.

Read genotype and phenotype separately

The genotype probabilities are ¼ TT, ½ Tt and ¼ tt: a 1:2:1 ratio. With complete dominance as assumed here, the phenotype probabilities are ¾ tall and ¼ short: a 3:1 ratio. Tt occurs twice because T from the first parent with t from the second is a different combination from t with T.

If the first three offspring are tall, the next is not forced to be short. Under the same independent model its probability of being short remains ¼. Small samples can differ substantially from the expected ratio.

Find the hidden assumption

A tall plant might be TT or Tt. You cannot select its genotype from appearance alone in this model. If a problem gives the genotype, use it; if it supplies offspring evidence, explain what that evidence supports.

For revision, write one sentence for each square: which allele came from which parent and what phenotype follows under the stated dominance rule. This prevents the grid from becoming four memorised boxes with no biological interpretation.

Check your understanding with a changed example

Change the cross to Tt × tt under the same complete-dominance model. List each parent’s possible gametes, then predict the genotype and tall-to-short phenotype ratios before reading the check.

Choose what to study next

Sources and scope

This is a focused learning resource, not a complete chapter or a prediction of board questions. Use your current syllabus and the paper-specific marking scheme for assessment requirements.