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List the problems of classical twin studies (for estimating heritability derived from comparison of monozygotic and dizygotic twins).
- Non-additive genetic variance: ASK
- Epistasis: The effect of a particular allele at a locus may depend on the allele expression at a different locus.
- Random X-chromosome inactivation in female twins: Although MZ twins are nominally identicvcal in their genomes, this will not be true at the level of expression, owing to the random nature of X-chromosome inactivation.
- Assortative mating: Standard model assumes random mating in a population. However, it is often not (eg. similar height people may marry). DZ twins may share more or less than 50% of genes because of this.
- Differences in similarities of environments: MZ twins often have a different environment to DZ twins (MZ twins tend to be closer).
Give 2 examples of genes that have been studied in candidate gene association studies.
- Schizophrenia: DRD4 (encoding for receptors for dopamine) and COMT (encoding an enzyme that degrades dopamine)
- Anxiety: Serotonin transporter gene
Why could candidate gene association studies be problematic?
- WHereas in the case of diseases of rods and cones we have agood idea of where the pathology lies (in the proteins found only in rods and cones),
- it is hard to know where the pathology/variation lies in the first place for psychological conditions like schizophrenia and depression.
What locus is associated with encoding light-absorbing proteins in cone cells?
There are two types of epigenetics-related modifications that can occur to DNA. What are they?
- DNA methylation: addition of methyl group to part of a DNA molecule, which prevents certain genes from being expressed
- Histone modification: binding of epigenetic factors (eg. methyl) to tails of histone (proteins around which DNA wraps around). Modifications can affect how tightly the DNA is wrapped around the histone protein, affecting also the accessibility of DNA.