documentclass{article}


usepackage{ragged2e}
usepackage{tabularx, makecell}
setlength{extrarowheight}{2pt}
setlength{tabcolsep}{3pt}%

begin{document}


begin{center}
  begin{tabular}{ l  | l }
    hline
    Symbols& Description\ hline
    N& The number of SNPs \ hline
    n& The number of patients \ hline
   X_i&$i$th SNP \ hline
   Y&the endpoint  \hline
   beta_i&Effect size for$i$th SNP   \hline
   p_{i+}&The probability $X_i=1$ given $Y =+1$ \hline
   p_{i-}&The probability $X_i=1 $given $Y =-1$ \hline
   n_{+}&The number of patients experience toxicity \hline
   n_{-}&The number of patients do not experience toxicity \hline
   Z_{i+}&The number of patient has a major allele for $i$th SNP given that patient experience toxicity \hline
   Z_{i-}& makecell{The number of patient has a minor allele for $i$th SNP given that patient do not experience toxicity} \hline}
    hline
    caption{Summary of notations}
  end{tabular}
 end{center}
end{document}