High school biology class taught us that each of us is a composite of our parents; your mother and father each provided you with half your genes, and each parent’s contribution was equal. Gregor Mendel, often called the father of modern genetics, came up with this concept in the late 19th century, and it has been the basis for our understanding of genetics ever since.
But in the past couple of decades, scientists have learned that Mendel’s understanding was incomplete.
It is true that children inherit 23 chromosomes from their mother and 23 complementary chromosomes from their father. But it turns out that genes from Mom and Dad do not always exert the same level of influence on the developing fetus. Sometimes it matters which parent you inherit a gene from—the genes in these cases, called imprinted genes because they carry an extra molecule like a stamp, add a whole new level of complexity to Mendelian inheritance. These molecular imprints silence genes; certain imprinted genes are silenced by the mother, whereas others are silenced by the father, and the result is the delicate balance of gene activation that usually produces a healthy baby.
When that balance is upset, however, big problems can arise.
Scientific American Mind has an interesting story by Melinda Wenner about what we inherit from our parents. The article is available at http://www.scientificamerican.com/article.cfm?id=a-patchwork-mind.