1. MR approach to analysing causal association of deodorant-use with diseases
Key paper: Dependence of Deodorant Usage on ABCC11 Genotype: Scope for Personalized Genetics in Personal Hygiene (Rodriguez et al, 2013)
rs associated with deodorant use
Homozygotes for allele A are ~5-fold less likely to use deodorant than individuals with A/G or GG
A. Mesut Erzurumluoglu (Genetic Epidemiology group)
10th May 2017

2. Mendelian genetics Production of wet earwax and axillary odour
Recessive mode of inheritance
rs in ABCC11
c.538G>A:p.Gly180Arg
Affects apocrine sweat glands by reducing secretion of odorous molecules and its precursors
~99% of Koreans have the AA genotype, which alters the composition of sweat so that they do not produce body odour. Only 2% of Europeans have this genotype 

3. Rodriguez et al, 2013
“We found strong evidence (P=3.7e−20) indicating differential deodorant usage according to the rs genotype. AA homozygotes were almost 5- fold overrepresented in categories of never using deodorant or using it infrequently. However, 77.8% of white European genotypically nonodorous individuals still used deodorant, and 4.7% genotypically odorous individuals did not. We provide evidence of a behavioral effect associated with rs This effect has a biological basis that can result in a change in the family's environment if an aerosol deodorant is used. It also indicates potential cost saving to the nonodorous and scope for personalized genetics usage in personal hygiene choices, with consequent reduction of inappropriate chemical exposures for some.”
doi: /jid

4. Deodorant-use and disease
Deodorant-use unconvincingly (rumoured to be) associated with many diseases
Breast cancer
Kidney disease
Alzheimer’s disease
Hidradenitis suppurativa
Seizures
Bone Formation Disorders

5. Mendelian randomisation (MR)
Rationale
Alleles are inherited randomly from each parent - > Random allocation of confounding factors
Using risk alleles as a proxy for exposure to calculate disease risk estimates
Natural randomised controlled trial (RCT)
Reference: Neeland & Kozlitina, Mendelian Randomization. Circulation
AT: Atrial fibrillation

6. No common cause of G and PB
Example
Example: is increased BMI associated with T2D?
Robust association
No common cause of G and PB
References: Fall et al, The Role of Adiposity in Cardiometabolic Traits: A Mendelian Randomization Analysis. PLoS Medicine
Solovieff et al, Pleiotropy in complex traits: challenges and strategies. Nat Rev Genet

7. Potential study
Individuals with AA v Individuals with AG or GG genotype for rs
Lower deodorant use in AA group -> Lower incidence of certain diseases in AA group?