Da dashed auch alle weiteren Kanten und Knoten im Baum beeinflusst, müssen diese mit solid in den Ausgangszustand zurückgesetzt werden.
\documentclass[tikz, border=5pt]{standalone}
\begin{document}
\begin{tikzpicture}[
thin,
level 1/.style={sibling distance=40mm},
level 2/.style={sibling distance=25mm},
level 3/.style={sibling distance=20mm},
every circle node/.style={minimum size=0mm, inner sep=0mm}
]
\node[circle,fill] (root) {}
child { node [circle,draw] {}
child { node [circle,fill] {}
edge from parent
}
child { node [circle,draw] {}
edge from parent
}
edge from parent
}
child { node [circle,fill] {}
child [dashed] { node [circle,fill] (y2) {}
child [solid] { node [circle,draw] {}
edge from parent
}
child [solid] { node [circle,draw] {}
edge from parent
}
edge from parent
}
child [dashed] { node [circle,draw] (z4) {}
child [solid] { node [circle,draw] {}
edge from parent
}
child [solid] { node [circle,draw] {}
edge from parent
}
edge from parent
}
};
\end{tikzpicture}
\end{document}
Wenn der Baum ohne Knoten gezeichnet werden soll, dann kannst Du den Quelltext viel schlanker gestalten.
\documentclass[tikz, border=5pt]{standalone}
\begin{document}
\begin{tikzpicture}[
thin,
level 1/.style={sibling distance=40mm},
level 2/.style={sibling distance=25mm},
level 3/.style={sibling distance=20mm}
]
\coordinate
child {
child
child
}
child {
child [dashed] {% (y2)
child [solid]
child [solid]
}
child [dashed] {% (z4)
child [solid]
child [solid]
}
}
;
\end{tikzpicture}
\end{document}