MR analyses in which the genetic IV-risk factor association and genetic IV-outcome association are from the same sample and individual-level data is used to derive the MR estimate.
Genetic IVs must fulfil the same IV assumptions. The gene-outcome model must be correctly specified, but the gene-exposure model can be miss-specified. One-sample MR is more prone to data overfitting than two-sample MR. Though, two-sample MR may be similar to one-sample if there is overlap of participants in the two samples. In one-sample MR, weak instrument bias will tend to bias towards the confounded multivariable regression estimate.

References
Other terms in 'Definition of MR and study designs':
- Bidirectional MR
- Binary exposure MR
- Factorial MR
- Instrumental variable (IV)
- Mendelian randomization (MR)
- Multivariable MR
- Two-sample MR
- Two-sample MR with individual participant data (IPD)
- Two-sample MR with summary (aggregate) data
- Two-step/Mediation MR