A **double crossover** between two nonsister
chromatids
produces two **double-crossover** gametes.

By the **product**
**law**, the probability of two **crossovers** occurring is equal to the **product**
of the individual probabilities; so a **double crossover** (**DCO**) is much rarer than a
**single crossover** (**SCO**).

- What are possible
**SCO** gametes?