Check out the previous article to read about Chromosomes 1-11 and Chromosome 23 (Sex chromosomes). Click The Epic Story of Human Chromosomes (Part – 1)

Chromosome 12

How a fertilised egg develops into a complex body with right organs at right place? How an individual cell knows which part of the body it should become? Ridley encounters these questions in this chapter. As the fertilised egg grows into an embryo, it gradually develops into asymmetries – a head-tail axes and a front-back axes based on the gradients in the chemical products of maternal genes. Here comes the role of homeotic genes in our DNA: the developmental genes to construct our body. These genes orderly stay together on a chromosome as clusters with each gene specifying different parts of the body in the same order as the body part. (One large cluster of these genes lies on the middle of chromosome 12). Furthermore, all these genes have a common sequence which makes proteins to switch on/off other homeotic genes. For example, if the cells in an embryo get signalled that they are in the front end, they develop the characteristics of head by switching head genes on. This step by step process goes until the full organ develops.

Chromosome 13

In this chapter, Ridley discusses how genes can reveal secrets about our pre-history, occupation, languages and cultural behaviours. For instance, he picks up ‘the breast cancer’ gene BRCA2 on chromosome 13, where the mutated version of this gene is more common in Jewish people, who are reluctant to mix up with other nations (due to religious reasons), thus retaining their genetic integrity. He even talks about the sparking correlation between genetic variation and language evolution in deducing migration patterns of humans in history.

Chromosome 14

Since the origin of life, genetic code have been copied billions of times, yet for a particular organism, cells cease to divide after a few hundreds of copies. Why does the cells grow old and die even though the genome is immortal? Why death is inevitable for us? Ridley encounters these questions in this chapter. The answer lies in the gene TEP1 on our chromosome 14. It makes a protein called telomerase, which can repair the frayed ends of chromosomes (telomeres), after the DNA gets copied. However, in normal human development, the genes that make telomerase are switched off in all, which is the principal reason for death. Ridley further discusses the rate of ageing in other animals in evolutionary point of view.

Chromosome 15

We inherit two copies of our genes from our parents (one from our mother and another from father). Usually both copies of our genes stay active. But, in some cases, only one of the two copies (either mother’s or father’s) is normally turned on purposefully as if they have paternal and maternal stamps. This chapter talks about this phenomena called genomic imprinting. This is evident from the fact that during embryo development, paternal genes are responsible for making placenta where as maternal genes make large parts like head and brain. Ridley further talks about the genetic diseases Prader-Willi syndrome and Angelman’s syndrome, both are caused by the lack of same chunk of DNA on chromosome 15. Depending on which parent the missing chunk of chromosome came from, determines which disease a child ends up receiving.

Chromosome 16

Ridley dwells into the mysteries of brain functions. Neurologically, what exactly is learning ? What changes happen in brain when we memorise something? Ridley talks about two prominent genes, CREBBP on chromosome 16 and CREB gene on chromosome 2. These genes make proteins that can switch on genes which alter the shape and function of synapses (junctions between nerve cells). When an electric signal reaches a synapse, it transfers into a chemical signal (AMP molecule), before resuming its electrical journey. Learning and memorising seems to be a change in the properties of these junctions i.e the strengthening of synapses, which leads to better efficiency of neural transmission.

Chromosome 17

This chapter tells the remarkable story of cancer through the eyes of genes. Oncogenes in our DNA encourage cells to grow, which is crucial during embryo growth, healing a wound etc. On the contrary, tumour suppressing genes detect excessive cell growth and shuts them off. Cancer is a situation where, (by some mutations) oncogenes are jammed on and tumour suppressing genes are jammed off, causing uncontrolled cell division. Generally, tumour suppressors detect abnormal behaviour in cells and issues instructions to different genes to dismantle the cell from the inside to commit suicide, known as apoptosis. Ridley talks about two such tumour suppressing genes TP53 and APC found on chromosome 17.

Chromosome 18

This chapter is about genetic engineering: it’s past, present and future. Ridley mentions a tumour suppressor gene on chromosome 18 and discusses the necessities and possibilities of gene manipulating. Cutting, pasting and editing of DNA were indeed became true by recombinant DNA technology developed from 70s. However, gene therapy i.e the delivery of ‘corrected genes/DNA’ to human genome is much challenging due to ethical issues as well as the risk involved in methodologies.

Chromosome 19

Ridley discusses the genetic diagnosis of two common diseases : coronary artery disease and Alzheimer’s disease. Both are caused by the faulty versions of apolipoprotein (APO) genes on chromosome 19. Cholesterol and triglyceride fats from the food we eat are supplied to cells through blood by lipoproteins. APOE and APOB proteins bridge the introduction between lipoproteins and receptors of cells for safe delivery of triglycerides and cholesterol. If these proteins do not work (due to mutations in APOE and APOB genes), then the cholesterol and fat stay in bloodstream and can built up on the walls of arteries, leading to the coronary artery disease. By genetic diagnosis, we can identify those people who have faulty APO genes (APOE2 and APOE4) and can warn them from taking cholesterol rich diets, thus saving their lives. Ridley then talks about the cons of genetic screening. For instance, what if companies implement genetic diagnosis in their job recruitments to select healthy candidates ? Don’t you think that it can cause an outrage on discrimination?

Chromosome 20

Ridley presents the case of a mysterious brain disease called scrapie found in sheep (in 18th century), that seemed to be infectious but no microbes were found. Similar brain diseases were found in cannibalistic people in Papua New Guinea in1950s. However in 1982, Stanley Prusiner discovered a protein that was found in scrapie-like animals and more importantly it resisted digestion by protease enzymes. The gene was PRP (protease resistant protein) which was found on chromosome 20. The significant thing about it is that it produces a prion, a protein, that changes its shape to a tough and sticky form. It can not be destroyed and it changes other prions into versions of itself. Thus the disease can be both transmitted and genetic caused.

Chromosome 21

By now, previous chapters have made us familiar with gene based diseases, genetic diagnosis and gene therapy to repair mutations. But, this chapter takes us to the dark side of genetics: the controversial concept of Eugenics. As we are able to detect the mutated versions in genes of an embryo through genetic diagnosis, we now have the option to either allow or forbid the birth of babies (humans) with genetic disorders, right? Ultimately this leads to the debate of abortion. This chapter broadly talks about this widespread dispute on eugenics among the state, scientific, social and religious communities. Ridley starts the discussion with the example of Down syndrome, a genetic disorder caused by the presence of three copies of chromosome 21. Children with this disorder are mentally retarded, have short stature with hearing and vision difficulties.

Chromosome 22

The final chapter of the book talks about us, our self, the free will and mind which is unique to everyone. Even though our genes have made us into a complex structure with specialised features- our unique behaviours, our character and act of freedom are not grounded on any gene. Ridley ends the book by advocating scientific determinism and argues that our behaviour is unpredictable in short term but can be determined in long term. This, however doesn’t solve the question of free will. For example, you have a choice to continue or stop reading this article right now. None of your genes determine your decision, only your will does.


In the end, this is a fantastic book I’ve read on biology so far. It helped me to dive deep into the world of genes and made me familiar with the secrets of life that I never heard of.

The journey of lifeless atoms to coordinate themselves and to finally produce complex conscious beings like us – was breathtaking all the way !