**Determining gene sequence in a three-point mapping cross.**

Of the 3 possible arrangements,
arrangement `III`

(with *w* in the middle) yields the
observed
**DCO** (least frequent) phenotypes: `yellow`

, `echinus`

flies and `white`

flies.

**Mapping gene distances.**

The map distance between two genes is the **sum** of all **SCO** and **DCO** frequencies between them.

The distance between *y* and *w* is the **sum** of the frequencies
of offsprings (3)/(4) and (7)/(8): 1.50% + 0.06% = 1.56 **mu**.

The distance between *w* and *ec* is the **sum** of offsprings (5)/(6) and (7)/(8):
4.00% + 0.06% = 4.06 **mu**.