You can expect a smooth chin phenotype only when there are two copies of the recessive allele (e.g., $bb$).

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In a Punnett square, a capital letter represents the dominant allele, and a lowercase letter represents the recessive allele.

Allele Representation in a Punnett Square

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

A cleft chin is an inherited trait. Wherever a gene pair contains the dominant allele (e.g., $B$), you can expect a cleft chin phenotype.

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