The recurrance risk ratio is calculated as follows:

$$ \lambda_s = \frac{\text{risk of sibling}}{\text{risk in population}}$$

where the risk of sibling refers to the risk that a sibling has to get the disease and the risk in population is the risk that the disease occurs in the population.

I do not understand how it is possible to calculate the numerator. I saw a video here that discusses how it's calculated for simple diseases that follow Mendel's laws. However, my lecturer was discussing this in the context of multifacotorial traits.

How would we calculate the risk of the disease occurring in a sibling without simply forcing the parents to breed thousands of times?

The recurrence risk ratio is calculated as follows:

$$ \lambda_s = \frac{\text{risk of sibling}}{\text{risk in population}}$$

where the risk of sibling refers to the risk that a sibling has to get the disease and the risk in population is the risk that the disease occurs in the population.

I do not understand how it is possible to calculate the numerator. I saw a video here that discusses how it's calculated for simple diseases that follow Mendel's laws. However, my lecturer was discussing this in the context of multifactorial traits.

How would we calculate the risk of the disease occurring in a sibling without simply forcing the parents to breed thousands of times?