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

Mutations & phenotype

• Mutations occur spontaneously and randomly during DNA replication

• DNA base sequences determines the sequence of amino acids that produce a protein, therefore mutations in a gene can sometimes lead to a change in the amino acid sequence coded for by the gene

• The genetic code is degenerate: more than one triplet codes for the same amino acid, therefore many mutations will not cause a change in the amino acid sequence

• Some gene mutations change triplets of bases downstream from (after) the mutation point in the DNA sequence

  • This will result in a nonfunctional polypeptide

• Where a polypeptide is left altered, the resulting phenotype may also be affected

Cause and effect of mutations

• Random mutations in DNA can stem from errors in DNA replication or DNA repair mechanisms

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

  • Several DNA repair pathways exist to correct damage caused by external factors such as UV radiation and reactive chemicals, when the repair mechanisms fail mutations can occur

• Mutations are also caused by errors in mitosis or meiosis

• This can result in changes in phenotype

  • Changes in chromosome number often result in new phenotypes

    • Human disorders with developmental limitations are common when there is a chromosome number change, e.g., Down syndrome/Trisomy 21 and Turner syndrome

  • The creation of polyploids can have significant effect on the phenotype

    • Polyploid organisms have more than two sets of chromosomes

    • The phenotypes that arise from polyploids often exhibit greater size, growth rate, hardiness, or productivity

    • This is known as polyploid vigor

    • However, triploidy is known to cause sterility in the organisms due to problems during meiosis

Consequences of mutations

• The effect of a mutation depends on the environmental context

  • It can change based on external conditions, available resources, and selective pressures

• Mutations can be detrimental, beneficial, or neutral

  • Detrimental mutations:

    • These are often harmful to the organism and will likely affect reproduction and survival

    • However a phenotype that is detrimental in one environment may be beneficial in other, e.g. a mutation to winged insect may be beneficial to insects living in a windy coastal environment so they avoid be blown out to sea during flight

  • Beneficial mutations:

    • This type of mutation enhances survival or reproduction of an organism in a given environment

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

  • Neutral mutations:

    • A mutation that no immediate effect on an organism's survival or reproduction

    • The mutations may have occurred in a non-coding regions of DNA or the amino acid sequence remains unchanged and therefore still codes for the same protein

• Mutations are the primary 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

Examples of gene mutations

Cystic fibrosis and CFTR gene

• Cystic fibrosis is a genetic disorder of cell membranes resulting from a mutation event on the CFTR gene

  • It results in disruptions to ion transport in cells, particular those in the respiratory tract and reproductive organs; the body produces large amounts of thick, sticky mucus in these areas

  • Over time, this may damage the lungs and stop them from working properly

• The mutation causes a four-base deletion in the DNA sequence of the CFTR gene

• This causes deletion of one amino acid (phenylalanine) in the polypeptide chain of the CFTR protein

  • This renders the protein unable to attach to the plasma membranes

• Cystic fibrosis is caused by a recessive allele

  • This means individuals who are heterozygous won’t be affected by the disorder but are ‘carriers’

  • Individuals who are homozygous recessive will have the disorder

Adaptive melanism in pocket mice and MC1R gene

• Adaptive melanism in rock pocket mice (Chaetodipus intermedius) allows some populations of pocket mice to develop dark-colored (melanic) fur,

• This provides camouflage in volcanic rock environments which increases their chances of survival

• The change in phenotype is caused by mutations in the MC1R (Melanocortin 1 Receptor) gene

  • The MC1R gene encodes a protein that regulates melanin production in hair and skin cells

  • Mouse individuals with MC1R mutations produce more pigment proteins which leads to darker fur and therefore better camouflage from predators

  • Over generations, natural selection favors mice with dark fur in these darker environments