Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism. Often, it is impossible to determine which ...

This two-hour program is divided into 16 chapters. Choose any chapter below and select QuickTime or Real Video to begin viewing. If you experience difficulty viewing, it may be due to high demand. We ...

For more than 160 years, the National Academies have provided nonpartisan, evidence-based advice on decisions that affect our nation. Browse the listing of projects and activities to see examples of ...