Khan Academy
Khan Academy: The Law of Segregation
Resource provides a discussion of Mendel's law of segregation with a focus on genotype, phenotype, alleles, heterozygous/homozygous, and 2 x 2 Punnett squares.
Palomar Community College District
Palomar College: Mendels' Genetics
This site from Palomar College provides a good background on exactly what Mendel did. Vocabulary links throughout make this a very good introduction article for genetics.
Other
North Dakota State Univ.: Detecting and Measuring Linkage
A good site to establish what is meant by gene linkage and sex-linkage.
Other
North Dakota State Univ.: Mendel's Law of Ind. Assort.
Mendel's second law and how he arrived at it through his experiments. Includes vocabulary and examples.
Other
North Dakota State Univ./mendel's Law of Ind. Asstort.
Good description of Mendel's findings and his second major principle.
University of Maryland
Univ. Of Maryland Baltimore Cty/biology Century
Historical perspective on Mendel's contribution. Spends a little time setting up what the current beliefs of Mendel's day were and how he changed things with what he did.