Uploaded March 2025 | Updated September 2026, 1 day ago

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In this A level Biology video, we look at epistasis. First we explore what is meant by epistasis. We look at both recessive epistasis and dominant epistasis in multi-step pathways catalysed by enzymes. We then explore how coat colour in mice is determined by an epistatic effect between two genes and how this can affect the phenotype ratio in offspring.

Image credits:

Brown mouse
commons.wikimedia.org/wiki/File:Brown_pet_mouse.jpg
Tiia Monto, CC BY-SA 3.0 creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

White mouse
commons.wikimedia.org/wiki/File:White_pet_mouse_B.jpg
Pogrebnoj-Alexandroff, CC BY 3.0 creativecommons.org/licenses/by/3.0, via Wikimedia Commons

Black mouse
commons.wikimedia.org/wiki/File:201707_mouse_3D_black.svg
DataBase Center for Life Science (DBCLS), CC BY 4.0 creativecommons.org/licenses/by/4.0, via Wikimedia Commons

Epistasis (modified by S Donnelly to alter allele symbols)
commons.wikimedia.org/wiki/File:Esempio_di_epistasi.png
openstax.org/books/biology-2e/pages/12-3-laws-of-inheritance
See page for author, CC BY 4.0 creativecommons.org/licenses/by/4.0, via Wikimedia Commons