Genetic inheritance and common ancestry


Biological reproduction, whether it involves the creation of new cells by cell division or new organisms by sexual or asexual means, requires the copying of genetic information and the appropriate distribution of the information to the descendants.  This process is elegantly facilitated by the structure of DNA.  Specifically, the fact that a DNA molecule is composed of two strands, which are perfectly complementary in base sequence, means that the molecule can be separated into two separate strands, each of which contains the same genetic information.  Thus, the process of DNA copying (termed DNA replication) involves only the creation of two new complementary DNA strands, formed upon the already-existing templates.  Though conceptually simple, DNA replication involves machinery of remarkable complexity, including modules responsible for proofreading and error correction.

 
Pedigree showing the inheritance of hemophilia in a human family. Image courtesy of Wellcome Library, London.

All of the DNA in an organism, then, has descended from the DNA in the original cell of that organism.  Likewise, all of the DNA in a family of organisms has descended from the DNA in the founding members of the family.  (Consider a family tree, or pedigree, like the one pictured on this page.)  Extended over time, this principle applies to the DNA in the members of a species, or to any grouping of organisms (such as vertebrates, mammals, or insects) that are related through common ancestors.

How, then, do organisms come to differ genetically from one another? People are clearly diverse in their characteristics, and the same is true of all species. Species themselves are diverse: for example, there are about 5000 living species of mammals, which include mice, bats, dogs, cows, whales and people, all descended from organisms that lived 300 million years ago. All of this genetic diversity is the result of the occurrence of mutation, whereby changes in a DNA sequence are introduced through errors or rearrangements during reproduction, so that the DNA in the descendants (cells or offspring) is different from that of the parent. Mutations can arise in various ways, and range from the alteration of a single base (say, a change from A to C) to the duplication or deletion of huge chunks of DNA that can include whole genes. No matter how the change comes about, it will be passed on to all the descendants of that cell or of that organism, unless the cell or organism does not reproduce.

So, through mutation, genetic diversity can arise within the body of a person, leading to cancer. Genetic diversity can arise in a human family, leading to inherited disease (such as hemophilia, pictured in the pedigree on this page) or to inherited characteristics such as height or eye color. And genetic diversity can arise in a population of organisms, leading to evolutionary change.

Identifying and understanding genetic diversity its sources and its consequences  is a major goal of modern genetics, with implications for the detection and treatment of disease, as well as for efforts to explain life's evolutionary past. The detection of genetic diversity (or similarity) among organisms requires the detection of changes (or lack thereof) in the DNA sequences of their genes and/or genomes. One very important tool for the analysis of the relatedness of DNA sequences is the alignment of sequences, for the comparison of their similarities and differences. BlastED seeks to introduce you to the structure and use of a widely-used sequence alignment tool called BLAST.

Next Section: Important questions that are addressed by DNA sequence alignment