\documentclass{article}

\usepackage{tikz}


\begin{document}
\pagestyle{empty}

\def\layersep{2.5cm}

\begin{figure}
\begin{tikzpicture}[shorten >=1pt,->,draw=black!50, node distance=\layersep]
    \tikzstyle{every pin edge}=[<-,shorten <=1pt]
    \tikzstyle{fmmde}=[circle,fill=black!25,minimum size=17pt,inner sep=0pt]
    \tikzstyle{component}=[fmmde, fill=green!50];
    \tikzstyle{failure}=[fmmde, fill=red!50];
    \tikzstyle{symptom}=[fmmde, fill=blue!50];
    \tikzstyle{annot} = [text width=4em, text centered]

    % Draw the input layer nodes
    %\foreach \name / \y in {1,...,4}
    % This is the same as writing \foreach \name / \y in {1/1,2/2,3/3,4/4}
    %    \node[component, pin=left:Input \#\y] (I-\name) at (0,-\y) {};

    \node[component] (C-1) at (0,-1) {$C^0_1$};
    \node[component] (C-2) at (0,-3) {$C^0_2$};
    %\node[component] (C-3) at (0,-5) {$C^0_3$};
    %\node[component] (K-4) at (0,-8) {$K^0_4$};
    %\node[component] (C-5) at (0,-10) {$C^0_5$};
    %\node[component] (C-6) at (0,-12) {$C^0_6$};
    %\node[component] (K-7) at (0,-15) {$K^0_7$};

    % Draw the hidden layer nodes
    %\foreach \name / \y in {1,...,5}
    %    \path[yshift=0.5cm]
            \node[failure] (C-1a) at (\layersep,-1) {a};
            \node[failure] (C-1b) at (\layersep,-2) {b};
            \node[failure] (C-2a) at (\layersep,-3) {a};
            \node[failure] (C-2b) at (\layersep,-4) {b};

    % Draw the output layer node

    % Connect every node in the input layer with every node in the
    % hidden layer.
    %\foreach \source in {1,...,4}
    %    \foreach \dest in {1,...,5}
    \path (C-1) edge (C-1a);
    \path (C-1) edge (C-1b);
    \path (C-2) edge (C-2a);
    \path (C-2) edge (C-2b);

    %\node[symptom,pin={[pin edge={->}]right:Output}, right of=C-1a] (O) {};
    \node[symptom, right of=C-1a] (s1) {s1};
    \node[symptom, right of=C-2a] (s2) {s2};



    \path (C-2b) edge (s1);
    \path (C-1a) edge (s1);

    \path (C-2a) edge (s2);
    \path (C-1b) edge (s2);

    \node[component, right of=s1] (DC) {$C^1_1$};

    \path (s1) edge (DC);
    \path (s2) edge (DC);
    


    % Connect every node in the hidden layer with the output layer
    %\foreach \source in {1,...,5}
    %    \path (H-\source) edge (O);

    % Annotate the layers
    \node[annot,above of=C-1a, node distance=1cm] (hl) {Failure modes};
    \node[annot,left of=hl] {Base Components};
    \node[annot,right of=hl](s) {Symptoms};
    \node[annot,right of=s](dcl) {Derived Component};
\end{tikzpicture}
% End of code
\caption{DAG representing failure modes and symptoms $FG^0_1 \rightarrow DC^1_1$}
\end{figure}

\end{document}