Question

Jade creates a Punnett square to show the cross between a parent with freckles and a parent without freckles. The completed Punnett square shows the possible genotypes and phenotypes of the offspring. Jade, however, made a mistake as she was making the Punnett square. In which part of the Punnett square did Jade make her error? Be sure to explain your answer

Answer 1

Jade’s mistake is in the gamete labels (the row and/or column headers) of the Punnett square.

Why: each gamete gets only one allele for the trait. If freckles (F) is dominant and no freckles (f) is recessive, a parent without freckles must be ff and produces only f gametes. A parent with freckles could be FF or Ff; if she is heterozygous (Ff) her gametes are F and f. The headers of the Punnett square must therefore be single alleles (F and f on one side, f and f on the other if the other parent is ff).

Correct 2 × 2 square for Ff × ff:

- Gametes: top f, f ; side F, f
- Boxes: Ff, Ff, ff, ff
- Result: 50% freckles (Ff) and 50% no freckles (ff).

If Jade put whole genotypes (like “Ff” or “ff”) in the gamete headers instead of single alleles, or otherwise labeled the gametes incorrectly, the offspring genotypes/phenotypes she got would be wrong.