single gene traits - disease allele at 1 locus
- binary outcomes
- mendelian inheritance
ex/ Tay-sachs PKU, HD, CF
complex/multifactorial traits - depend on multiple loci + environment + epigenetic
influence
- oligogenic, polygenic, multifactorial
- DONT exhibit mendelian inheritance
ex/ ASD, heart disease, intelligence, schizophrenia, height, athletic abilities, facial
features, procrastination, being a jerk
*more common than single gene traits
oligogenic traits - risk alleles at a few loci
polygenic traits - risk alleles at many loci
multifactorial - genetic and nongenetic factors effect phenotype
polygenic theory - increase in # of loci produces an ~ normal distribution
- can be applied to all multifactorial traits
- relevant when continuous QUANTITATIVE traits depend on the additive effect of
multiple individual loci
- explains why mendelian patterns can show a normal distribution
threshold effects - QUALITATIVE traits (i.e., congenital heart disease) binary outcomes
are not the condition itself but the risk of a conditions therefore the condition only
appears if the risk produced at multiple loci exceeds threshold for that disease
binary outcomes and threshold - shifting the curve right (more risk variants in genotype)
increases proportion of individual over threshold therefore increasing prevalence in a
group
oligogenic disease and threshold - each risk allele contributes to overall risk therefore
increase in risk allele increases risk
*no single allele is necessary or sufficient to produce outcome*
familial aggregation - if there is a genetic component to a trait, the more alleles 2 people
share, the more similar their risk
- P(trait) higher in relatives than pop
,- ~50% between aprents and children and siblings
2 ways to determine if there is a genetic influence - 1. relative risk ratio
2. case-control studies
relative risk ratio - = prevalence in relatives of an affected person/ prevalence in general
population
RR = 1 = no greater risk in relatives than general pop
RR > 1 = significant genetic component
case-control studies - odds of having an affected relative in patient vs control
- if odds are same, no genetic component
- family histories compares, matched for ehtnicity, geography, etc, often spouses are
used
problem with family studies\ - relatives more likely to share a common environment
heritability - fraction of variance of a quantitative trait due to allele variation
- estimate
- genetics affects response to environment therefore not completely independent
indications of a strong genetic component - - risk in MZ twin > first degree relatives >
distant relatives
- more affected relatives increases ones risk
congenital heart defects - - many causes
- complicated genetics
- majority are multifactorial
- some LoF CNVs increase risk
- partly disrupting several genes in same path is similar to completely eliminating
function of 1 gene in path
- various maternal factors increase risk
- defects analogous to minor abnormality in hand development
CHD causes - 62% unknown
15% de novo CNV
12% chromosome syndrome
10% de novo gene mutaiton
1.3% inherited gene mutation
, cystic fibrosis - classic single gene disorder BUT expressivity modified by other genes
and environment
- allelic heterogeneity and heterozygote advantage
- mutation may be weak, strong or null based on Cl conductance determine viscosity of
mucus/severity of disease BUT severity of allele correlated with severity of pancreatic
NOT lung phenotype
- strong allele can produce weak lung phenotype and vice versa
CF modifiers for lung damage - MBL2, TGFB1
- mutations modify risk of severe lung phenotype
- DONT cause CF
- similar to oligogenic effects of CHD
is variable expressivity a complex trait? - YES
(include single-gene disorders)
hereditary hemochromatosis - = late onset AR iron overload
- single gene disorder
- penetrance depends on iron intake/output
- strong non-genetic contribution (meat consumption)
is penetrance multifactorial? - YES
can single gene disorder be complex traits? - YES, most are!
twins - experiments of nature
- share uterine environment, household, diet, parents making them best control for
environment
- vary genetic component in quantifiable manner
monozygotic twins - cleavage of fertilized zygote
- typically same sex
- share placenta
- no ethnic bias
- share 99-99.5% of alleles
sesquizygotic twins - 1 egg fertilized by 2 sperm
- zygote with mix of genotypes splits
- share 100% maternal byt < 100% paternal allele
- VERY rare
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