Genes, Chromosomes and Alleles

Mendel deduced the rules of inheritance without knowing what genes were made of. We now know that alleles are variants of DNA sequences carried on chromosomes, and that the physical behavior of chromosomes explains Mendel's laws. This lesson connects the abstract idea of the gene to its molecular and chromosomal reality.

The Genome

The genome is the complete set of genetic information in an organism, including all the genes and the non-coding DNA. In humans the nuclear genome consists of about 3.2 billion base pairs distributed across 23 pairs of chromosomes, plus a small circular mitochondrial genome. The Human Genome Project, completed in draft form in 2003, sequenced this entire set and revealed that humans have roughly 20,000 protein-coding genes, far fewer than scientists had once pre