The Basics

  • Nucleotides are the alphabet of DNA. There are only four “letters” in DNA : adenine (A), thymine (T), guanine (G) and cytosine (C). They always go by pairs, A with T, and G with C. Such pairs are called “base pairs”.
  • Almost every cell in our body contains a complete copy of our genome. The exceptions are egg/sperm cells, which only carry half of our genome, as well as red blood cells and some white blood cells, which have no DNA at all (otherwise blood transfusions would often cause an immune reaction, like organ transplants).
  • If unfolded the DNA in each cell’s nucleus would be 2 metres long. Humans have an estimated 100 trillion cells. In other words, if the all the DNA from every cell in a person’s body were patched up together they would form a strand of 200 billion kilometres, or more than 1,000 times the distance between Earth and the Sun.
  • Mitochondrial DNA is found outside the cell’s nucleus, and therefore outside of the chromosomes. It consists of only 16,569 base pairs.
  • A SNP (single nucleotide polymorphism) is a mutation in a single base pair. Depending on what section of DNA is affected these mutations can have no effect at all (it is usually the case for SNP’s defining Y-DNA and mtDNA haplogroups), a change in physical appearance (e.g. eye colour), an improvement of health (e.g. increased immunity), a increased susceptibility to a disease (e.g. diabetes), or a genetic disease (e.g. cystic fibrosis).

    The Chromosomes

    • A human genome is made of 3,000 million base pairs, split into 46 chromosomes.
    • There are in fact 23 pairs of chromosomes, each person inheriting a maternal and paternal copy of each. Pairs of chromosomes are numbered from the largest (chromosome 1) to the smallest (chromosome 21). Chromosome 22 ought to be the smallest, but it was later discovered than chromosome 21 was smaller, and the established ordered was kept.
    • The sex-determining chromosomes (X and Y) are the only pair that is not symmetrical in size. The Y-chromosome possess 60 millions bases, against 153 millions for the X chromosome.
    • The reason why the Y chromosome is so much smaller than the X chromosome is that the latter possess genes that attack the Y chromosome. In response the Y chromosome has had to shut down a lot of its non-coding DNA so as to better protect itself.
    • In some rare cases people are born with one extra chromosome. Those born with three chromosome 21 have Down’s syndrome. Other possibilities is an extra X chromosome, leading to Klinefelter’s syndrome (XXY), XYY syndrome, Triple X syndrome, XXYY syndrome, 48, XXXX, or 49, XXXXX. An extra copy of any other chromosome normally results in miscarriages. Some very rare cases of autosomal trisomies can survive to birth, notably when it affects chromosomes 13 or 18, but result in seriously shortened life expectancy.
    • Humans, like most animals, are diploid, meaning that they have only two sets of chromosomes. However that is not the case of all life beings. Plants in particular are often polyploid. There are varieties of wheat that are tetraploid (four sets of chromosomes) and others that are hexaploid (six sets of chromosomes). Some strawberries can be decaploid (ten sets of chromosomes). Polyploid animals include the goldfish, salmons, and salamanders. Polyploidy occurs in some human tissues like muscles or the liver. When two or three spermatozoids fertilise an ovum at the same time, a human foetus will be triploid or tetraploid. However almost all such pregnancies end as miscarriage and those that do survive to term typically die shortly after birth.

    The Human Genome

    • The first complete human genome was only decoded in 2007. The two first individuals who got their full genome sequenced that year were Craig Venter and James D. Watson.
    • A human genome is identical at 98% to a chimpanzee’s genome. In comparison, two random human beings are in average 99.5% identical. Gorillas are 97% identical to either humans or chimps, meaning that humans are more chimp-like than gorillas.
    • Most of our genome is made of junk DNA. This junk is composed either of deactivated genes that were once useful for our non-human ancestors (like a tail), or parasitic DNA from virus that have entered our genome and replicated themselves hundreds or thousands of times over the generations, but serve no purpose. Genome size is therefore not related to the complexity of life. For example, the genome of the unicellular Amoeba dubia has been reported to contain more than 200 times the amount of DNA in humans.


    • Although autosomal DNA is inherited equally from each parent, a few genetic diseases seem to be worse when inherited from one’s father (e.g. Huntingdon’s disease), because mutations occur or repeat themselves at a higher rate in men, and increase with the father’s age. This is also why older fathers (over 40 years old) have higher chances of having children suffering from schizophrenia, depression or autism.
    • Some genes have different functions depending on whether they are inherited from one’s father or mother. These are called imprimted genes. For example, the maternal copy of a gene on chromosome 15 is known as UBE3A, while the paternal copy is SNRPN. Inheriting two paternal copies or missing the maternal copy causes Prader-Willi syndrome, whereas two maternal copies or a deletion of the paternal copy leads to the very different Angelman syndrome.
    • Rather than inheriting a homosexual gene, gay men tend to have several older brothers (including abortions and miscarriages). The reason is that the mother’s body accumulates antibodies against genes responsible for the masculinisation of the foetus’ brain at each pregnancy with a boy. The risk of male homosexuality therefore increases with the number of boy carried by a mother before. This does not apply to girls.


    • Neurotransmitters such as serotonin, dopamine, adrenaline and noradrenaline influence our mood and personality. Their levels is influenced by our environment, but the sensitivity of the brain to these neurotransmitters is genetically determined.
    • Low serotonin levels increase depression, anxiety, risk of suicide and violence. Carbohydrates and cholesterol both increase serotinin levels.
    • Excessive dopamine can lead to schizophrenia. Too low dopamine levels engender boredom and low activity, and in extreme cases Parkinson Disease. The long variants (7-repeat or more) of the dopamine receptor D4 (DRD4) causes dopamine to be consumed more quickly by the brain. People with this variant will usually have more novelty-seeking, thrill-seeking and adventurous personality than average to compensate for naturally lower dopamine levels.


    • Some people possess a deletion on the CCR5 gene, which makes them more resistant to smallpox, HIV, plague and other viruses (e.g. West Nile virus). This mutation is commonest in North-East Europe.
    • The ABO blood type is related to cholera resitance, with AB confering the strongest resistance, and O the weakest. On the other hand, the O blood group seems to be the most resistant against malaria and syphilis, and less susceptible to many kinds of cancers.
    • Many genetic diseases survived natural selection because they confer immunity against epidemic diseases. For instance, the CFTR mutation causing cystic fibrosis protects against the dysentry and fever of typhoid. Sickle-cell anaemia and thalassaemia are both protective against malaria. Genetic resistance to TB has for side-effect an increased susceptibility for osteoporosis. Tay-Sachs disease, mostly found among people of Ashkenazi Jewish ancestry, is also protective against TB.
    • Studies have shown that men and women are most attracted to the smell of people with the most different immune system from their own. This is also a way of Nature to prevent inbreeding. Differences in immune systems can be identified by comparing our HLA types, among other genes of the major histocompatibility complex (MHC).


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