\tikzstyle{input_neuron}=[circle,draw=blue!50,fill=blue!10,thick,minimum size=1mm]
\begin{center}
\begin{tikzpicture}
	\tikzset{
		line/.style={draw}
	}
	
	% 1
	\node [label={\tiny Intelligence},input_neuron] (ain2) at (0,4)  {$I$} ;
	\node [label={180:\tiny Grade},input_neuron] (ain4) at (0,2.5)  {$G$};	
	
	\path [line, line width=0.5mm] (ain2) -- (ain4);
	
	% 2
	\node [label={\tiny Intelligence},input_neuron] (g) at (3,4)  {$I$} ;
	\node [label={270:\tiny Grade},input_neuron] (h) at (2,2.5) {$G$} ;
	\node [label={270:\tiny SAT},input_neuron] (i) at (4,2.5) {$S$} ;
	
	\path[line, line width=0.5mm] let \p1=(g.south), \p2=(h.north) in (g.south) -- +(0,-12pt) -| (h.north);
	\path [line, line width=0.5mm] let \p1=(g.south), \p2=(i.north) in (g.south) -- +(0,-12pt) -| (i.north);

	% 3
	\node [input_neuron] (a) at (1.5,1)  {$C$} ;
	\node [input_neuron] (b) at (0,-0.5) {$\tiny X_1$} ;
	\node [input_neuron] (c) at (1.5,-0.5) {$\tiny X_2$} ;
	\node  (e) at (2.55,-0.5) {\textbf{\ldots}} ;
	\node [input_neuron] (d) at (3.5,-0.5) {$\tiny X_n$} ;

	\path[line, line width=0.5mm] let \p1=(a.south), \p2=(b.north) in (a.south) -- +(0,-12pt) -| (b.north);
	\path [line, line width=0.5mm] (a) -- (c);
	\path [line, line width=0.5mm] let \p1=(a.south), \p2=(d.north) in (a.south) -- +(0,-12pt) -| (d.north);
	
\end{tikzpicture}
\end{center}