Mutations & Phenotype (College Board AP® Biology): Study Guide
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
A mutation may introduce new alleles into a population which provide an opportunity for natural selection or genetic drift to act
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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