Survival of the Sickest Chapter 6

Our DNA is composed of about 3 billion nucleotides. All the genes in DNA are organized in 23 pairs of chromosomes: 50% from the father, 50% from the mother. The chromosomes contain information that code our body's characteristics. The dominant traits are always preferred in the case of one dominant trait and one recessive trait.

At first, scientists thought that only accidental mutations can cause genetic changes. For example, throughout the history, there were pandemics associated with the peaks of sunspot. The reason was that there was an antigenic shift/drift in our genes and it makes our bodies vulnerable to environment changes. Thus, we don't have the antibodies to fight with diseases. 

However, Barbara McClintock discovered that genetic changes can happen in a non-accidental way. McClintock observed genes in corns that sometimes the whole sequence of DNA moves from one place to another. And this phenomenon, she believed, is triggered by outside threats to the survival of the corn. Therefore, she called the genes in corns that make this change happen "jumping genes".

John Cairns also tested the mutation in genes by observing the genes of bacteria E. coli, which is a bacteria that digests the food. When Cairns fed nothing but lactose to the bacteria, the bacteria went through mutations that got rid of the lactose intolerance. He concluded that genes can choose to mutate. Other scientists propose the theory of "hypermutation", in which the genes are adapted to the environment due to massive increase in the mutation rate.

Originally, scientists found that only 3% of the genes in DNA are actually coding genes. They entitle the rest as "junk genes" at first. However, later they found that a large portion of noncoding genes are made of "jumping genes". Many strands of DNA contain viruses and bacteria that associated with human bodies a long time ago.

The Weissmann barrier distinguishes cells into germ cells and somatic cells. Changes in somatic cells can only happen in current carrier and cannot be passed on to the offspring.