Evaluating data on phenotypes

• both genetic and environmental factors influence the phenotype of an organism
• evaluating data about influences on phenotypes:

  • the phenotype (characteristics) or an organism is the result of the organism's genotype and the interaction of its genotype with the environment
  • not always clear how much a phenotype is influenced by genes and how much it is influenced by the environment
• example - overeating:

  • overeating was thought to be caused only by environmental factors, like an increased availability of food in developed countries
  • later discovered that food consumption increases brain dopamine levels in animals
  • once enough dopamine was released, people would stop eating
  • researchers discovered that people with one particular allele had 30% fewer dopamine receptors
  • people with this particular allele were more likely to overeat - wouldn't stop when dopamine levels increases
  • based on this evidence - scientists now think overeating has both genetic and environmental causes
• example - antioxidants:

  • many foods in our diet contain antioxidants - compounds thought to play a role in preventing chronic diseases
  • foods such as berries contain a high level of antioxidants
  • scientists thought that berries produced by different species of plant contained different levels of antioxidants bc of genetic factors
  • experiments were carried out to see if environmental conditions affected antioxidant levels - found that environmental conditions caused a great level of variation
  • scientists now believe that antioxidant levels in berries are due to both genetic and environmental factors
• Twin studies:

  • studies of identical twins - extremely useful when trying to determine what is due to environmental factors and what is due to genetic factors
  • twins are genetically identical, so any difference in phenotype must be entirely due to environmental factors
  • if a characteristic is very similar in identical twins - genetics probs plays a more important role
  • if a characteristic is different between the twins - environment must have a larger influence
• identical twins have very similar epigenetic marks when they are born and in the first years of their life

  different epigenetic changes occur in each twin as they get older

  environmental factors that can affect the epigenome (epigenetic marks that have been added to the entire genome) include diet, physical exercise and stress
• example - studying twins can be useful for determining the importance of genetic and environmental factors in the development of certain diseases:

  • comparisons between the prevalence of Alzheimer's disease in MZ twins and DZ twins have shown that the disease has a genetic risk
  • however, the disease is not always found in both identical twins - suggests that environmental factors also play a part
  • thought that genetics account for about 80% of this risk
• Data that comes from twin studies involving a large sample size (i.e. lots of pairs of twins) is better for drawing valid conclusions than data based on a small sample size - bc a large sample size is more representative of the population