Figure 1. Scatterplot and fitted regression functions showing negative correlation between the maternal age at birth and the number of recombination events in offspring.

The red dashed line is the linear regression (β = −0.49, p = 0.0017, r² = 0.081). The solid blue line represents the result of the linear spline regression with knots at each distinct value of maternal age at birth and the smoothing parameter λ estimated by REML (λ = 22.29, p = 0.005, r² = 0.091).