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.
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.
Khan Academy
Khan Academy: Natural Selection in Populations
How natural selection works at the level of genes, alleles, genotypes, & phenotypes.
Khan Academy
Khan Academy: Allele Frequency & the Gene Pool
Learn how to find allele frequency and how it's different from genotype frequency.