There are lots of web sites that do a fair to poor job of teaching genetics. I don't know any web site that does a good job of teaching genetics.

Inexpensive but good books:

Charles Pritzel, Genetics for herpers. Around $15 USA; do a web search.
Stansfield & ?, Schaum's introduction to genetics. Around $15 USA new from Amazon, cheaper from used book outlets.
Wilmer Miller, A survey of genetics. Free download, http://www.ringneckdove.com (see contents page)
NPA Project on genetics. Aimed at pigeon breeders but the principles are the same for pigeons, mice, snakes, etc. Also at http://www.ringneckdove.com