• Wright's Coefficient of Relationship ($R$):
• Defined as the proportion of genes shared identical by descent between two relatives:

$$R_{XY} = \sum \left(\frac{1}{2}\right)^L$$

where $L$ is the number of steps in each genealogical path through a common ancestor.

• Single First Cousins: Share 2 common grandparents through 1 set of siblings:

$$R = 2 \times \left(\frac{1}{2}\right)^4 = \frac{2}{16} = 0.125$$

• Double First Cousins: Two siblings of one family marry two siblings of another family. Their children share all 4 grandparents through 4 independent paths:

$$R = 4 \times \left(\frac{1}{2}\right)^4 = \frac{4}{16} = \frac{1}{4} = \mathbf{0.25}$$

• nalysis:
• (0.25): Correct.

Correct option: (3) 0.25