Mutations & Phenotype (College Board AP® Biology): Study Guide

Cara Head

Written by: Cara Head

Reviewed by: Ruth Brindle

Updated on

Mutations & phenotype

  • Mutations are random changes in DNA sequence that occur spontaneously during DNA replication or repair

    • DNA replication is a highly accurate process, but errors can arise when DNA polymerase incorporates incorrect nucleotides into the DNA sequence

  • External factors, such as UV radiation and reactive chemicals, can increase mutation rates

  • Mutations can change the type or amount of protein produced and therefore alter phenotype

  • Whether the effect of a mutation is detrimental, beneficial, or neutral depends on the environmental context

    • A phenotype that is detrimental in one environment may be beneficial in another

    • E.g. the sickle cell mutation provides resistance to malaria, making it beneficial in malaria-prone regions

  • Mutations are a source of genetic variation and are therefore essential for the evolution and adaptation of organisms

Chromosome-level mutations and phenotype

  • Errors in mitosis or meiosis can alter chromosome structure or number:

    • Nondisjunction of chromosomes leads to abnormal chromosome numbers, e.g.:

      • aneuploidy: a cell has the wrong chromosome number because one or more individual chromosomes are extra or missing

        • E.g. trisomy 21/Down syndrome and Turner syndrome

      • polyploidy: a cell has one or more extra sets of chromosomes; a body cell would normally be 2n (diploid), but a polyploid cell could be, e.g. 3n (triploid) or 4n (tetraploid)

    • Structural changes to chromosomes, e.g. inversions, translocations, duplications or deletions, can alter gene expression

  • Changes in chromosome number or structure can result in disorders with developmental limitations

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Cara Head

Author: Cara Head

Expertise: Biology & Psychology Content Creator

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding

Ruth Brindle

Reviewer: Ruth Brindle

Expertise: Biology Content Creator

Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.