Students should be able to complete a Punnett square diagram and extract…

The idea

This statement covers two related skills: completing a Punnett square and interpreting genetic crosses and family trees. To complete a Punnett square, write the alleles of one parent across the top and the alleles of the other parent down the side, then fill each cell with the allele from its row and column. Use capital letters for dominant alleles and lower-case letters for recessive alleles. From the completed square, identify genotype and phenotype ratios. Family trees use symbols to show inheritance across generations; you interpret them by tracking which phenotype appears in each generation and deducing parental genotypes. For example, if two unaffected parents have an affected child, the condition is recessive and both parents must be carriers. Always justify a deduced genotype using the evidence in the diagram.

What earns the marks

• A Punnett square shows the possible allele combinations from two parents.
• Parental alleles are placed along the top and side of the grid.
• Each cell receives one allele from each parent.
• Dominant alleles are represented by capital letters and recessive alleles by lower-case letters.
• Genotype and phenotype ratios can be read from the completed square.
• Family trees show phenotypes across generations and can be used to deduce genotypes.
• A recessive condition can be identified when unaffected parents have an affected child.

Common mistakes and how to put them right

• Putting two alleles from the same parent into one cell.

  Put it right by taking one allele from the row parent and one from the column parent.

• Using the same letter case for dominant and recessive alleles.

  Put it right by using a capital letter for the dominant allele and the matching lower-case letter for the recessive allele.

• Claiming a genotype from a family tree without evidence.

  Put it right by stating the offspring or parent phenotypes that support the deduction.

Exam tips

• Define the letters you use before completing the Punnett square.
• Check each cell contains exactly two alleles, one from each parent.
• When interpreting a family tree, work from the affected individual back to the parents to deduce genotypes.