[Tex/LaTex] beamer | One Block displayed, the other covered

beamer

I want that when the first block is displayed, the second is gray, and vice versa.

\documentclass{beamer}
\usepackage[frenchb]{babel}
\usepackage[latin1]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{epsfig,pslatex,colortbl}
\usepackage{graphics}
\usepackage{graphicx}%pour les includegraphics
\usepackage{array}
\usepackage{beamerthemesplit} 
\usepackage{multicol}

\usetheme{PaloAlto}
\usecolortheme{orchid}
\begin{document}
\begin{frame}[t]{General formalization of haplotyping.}
\begin{block}{Inputs}
\begin{itemize}
\item A \alert{genotype matrix} $G$.
\item The \alert{rows} of the matrix are \alert{taxa / individuals}.
\item The \alert{columns} of the matrix are \alert{SNP sites /
    characters}. 
\end{itemize}
\end{block}
\begin{block}{Outputs}
\begin{itemize}
\item A \alert{haplotype matrix} $H$.
\item Pairs of rows in $H$ \alert{explain} the rows of $G$.
\item The haplotypes in $H$ are \alert{biologically plausible}. 
\end{itemize}
\end{block}
\end{frame}
\end{document}

When I use the code above, the two blocks appear together. But, I want the first block is displayed while the second is covered. Then, The first is covered and the second is displayed.

any help?

Best Answer

You can use \uncover command for this.

\uncover<1>{Text only shown in first slide.}
\uncover<2-3>{Text only shown in second and third slide.}
\uncover<1,3>{Text only shown in first and third slide.}

The difference between appearance for hidden elements (gray or completely hidden) is a configuration option:

\setbeamercovered{invisible}
\setbeamercovered{transparent}

etc. See Beamer User Guide, Section 17.6.

Source btw: Can I tell Beamer that `\uncover` should be invisible, not merely grayed out?

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