beamer | One Block displayed, the other covered

Question

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?

Answer 1

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: http://tex.stackexchange.com/questions/53860/can-i-tell-beamer-that-uncover-should-be-invisible-not-merely-grayed-out