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Genes & gene-environment interactions

Ways to investigate whether a trait is determined by genes

• Twin studies
  • Used to study how much a certain trait or behavior is caused by genetics and how much is caused by the environment
  • Monozygotic share 100% of their genes, they have the same genetic material » the egg split after it was fertilized
  • Dizygotic twins are from two different eggs, fertilized by two different sperm cells » they share 50% of their genes
  • Dizygotic twins are more alike than regular siblings because they share more of the same environment » equal environment assumption (but: twins also have an unshared environment)
• Adoption studies
  
  • Adoptive child shares genes with biological parents but environment with adoptive parents
• From twin- and adoption studies we can calculate a heritability rate » 0-1 (1 = 100% influenced by genes, 0 = genes don’t play any role)
• Genetic amplification: increasing of heritability rate from childhood to young adulthood
• Active model of selected environments: we actively choose our environment, and that’s often based on our genetic predisposition

How do genes work and interact

• Genotype: the genetic material inherited from our parents
• Phenotype: observable expression of the genotype (e.g. height, behavior)
• Environment: everything other than the genes
• Parents genotype – child genotype relation (transmission):
  
  • We have 100 trillion cells, each of these cells contains a nucleus (center of the cell) » in the nucleus are 46 paired chromosomes (23 pairs) consisting of curled up DNA
  • Gamete: sex cells » they determine the sex of the baby and consist of 23 chromosomes (instead of 46)
  • Zygote: 2 gametes (egg and sperm cell) together » contains 46 chromosomes again
  • Why are siblings not identical (why are the gametes (and thus zygotes) not the same)?
    
    • Random assortment » in gamete creation and fertilization
    • Crossing over: sections of DNA switch from chromosomes (parts of the DNA are swapped, and when the cells divide, they’re no longer identical)
    • Mutations: accidental changes in DNA (caused by spontaneous error or environmental influences)
• Child’s genotype – child’s phenotype:
  
  • Genes are segments of DNA (of a specific type and at a specific location on the chromosome)
  • How do genes work: genes tell cells what to do » by making proteins (DNA is the ‘cookbook’ that contains recipes for making specific proteins)
  • The cells do not all the same because the genes in the cell are not all working at the same time. Genes are switched on and of (especially in the embryological development). The switching is done by regulator genes. The switching always happens in a chain of events (e.g. switching on a gene causes other genes to switch of)
  • Mandelian inheritance pattern: there are two variants to two different alleles (dominant and recessive)
  • Pologenetic inheritance: the phenotype is determined by many different genotypes
• Child’s environment – child’s phenotype:
  
  • The combination of genes and environment determine the phenotype (they interact)
  • Many different genes work together in very complex patterns
  • Genome: all the genes together
• Child’s phenotype – child’s environment
  
  • Children create their own environment (e.g. children that like to read, read more books and are thus more exposed to reading which will further enhance their reading skills)
• Child’s environment – child’s genotype
  
  • Epigenetics: the study of stable changes in gene expression that are mediated by the environment » most common is methylation (the DNA sequence isn’t changed, but the expression is)
  • The structure of the genetic code is fixed (genotype) but the expression can be altered by the environment (epigenetics)

Gene-environment interaction vs. epigenetics

• Gene-environment interaction: a certain genotype makes you more vulnerable to environmental influences. No changes in genetic activity or expression.
• Epigenetics: the environment changes the activity/expression of the genes