SwellType is a custom gene data type, used by the monochrome biomorph genome and its subclass, the colour biomorph genome. A gene which has a SwellType value are called 'Gradient genes'.

Original Pascal Source

In 'Monochrome WatchMaker/Globals', SwellType is defined like this:

type
    SwellType = (Swell, Same, Shrink);

It is defined identically in Common_Exhibition.p in the Colour Watchmaker sources, line 30:

type
    SwellType = (Swell, Same, Shrink);

WatchmakerSuite (Java)

In WatchmakerSuite (Java) SwellType is an enum. It is defined by the SwellType enum.

package net.richarddawkins.watchmaker.morphs.mono.genome.type;

public enum SwellType {
    Swell, Same, Shrink
}