What is a genome, anyway? The myths, a timeline and links to the stars - Hindustan Times
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What is a genome, anyway? The myths, a timeline and links to the stars

Aug 12, 2023 03:58 PM IST

From the simple fruit fly to the Neanderthal and then the human race, see how the genome map evolved, spawning myths and providing missing links along the way.

Think of it as our complete genetic blueprint. The guide that identifies us as human at a genetic level. All living beings are made up of cells. In humans, almost every cell contains approximately 25,000 genes. And inside those genes is DNA, an acid made up of four chemicals (called nucleotides or bases): adenine (A), thymine (T), cytosine (C) and guanine (G).

 (Pixabay)
(Pixabay)

The order of these letters (or the DNA sequence) encodes the information that instructs each cell what to do and when to do it. So whether you have curly hair, dimples, are green/red colourblind, or are lactose intolerant, it’s because your body has followed the instructions set by your DNA.

Your entire sequence of genes and bases is called your genome. No two are alike, which is why no two humans, even identical twins, are alike. But even if you compared two people who have nothing in common, at the genome level, they are far more similar than they are different. On average, a human is 99.6% identical to someone else. What makes you distinctively you is the minuscule 0.4%.

These differences are called genomic variants. But they alone do not define who you are. A person’s diet, environment, lifestyle and social dynamics contribute to their uniqueness too.

Only 1% of our DNA encodes proteins, the building blocks of life. The rest of the 99% used to be called “junk DNA” or “dark DNA” because of how little it was understood and accessed. But scientists are now learning that it plays an important role in controlling genes and how they act. This emerging field is known as epigenetics.

More breakthroughs are yet to come. The mapped human genome is merely a template. We don’t yet know how it varies from person to person and population to population. Once that happens, we’ll be able to work out why certain people are predisposed to some diseases, and why others are innately protected.

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HOW WE GOT HERE: SEQUENCING THE MILESTONES

Scientists started small. They sequenced the genomes of small, simple organisms. As techniques improved it became possible to sequence the genomes of more complex organisms, such as humans. See how they did it.

2000. It started with the fruit fly: Why? The insect had already been well studied at that point. Scientists saw it as a model organism, with high-quality sequences with which to compare the assembly.

2000. Then they added mustard: Arabidopsis thaliana, a wild member of the mustard family was fully sequenced. It was the first plant to have its genome sequenced.

2001. A quest is reborn: Launched in 1990, the Human Genome Project had set out to obtain the ‘genetic blueprint’ of humans. It was declared complete in 2001 and the second phase of the project, a finished reference human genome, was launched, and completed two years later.

2002. The mouse clicked: The International Mouse Genome Sequencing Consortium generated the first draft sequence of the mouse genome. Recent studies suggest that 95% of the mouse genome is similar to our own, so studying it can help understand more about human health and disease.

2002. Grains of truth: Rice was the first sequenced crop genome, paving the way for the sequencing of more complicated crop genomes.

2005. Clues from a chimp: The first draft of the DNA sequence of a non-human primate, the chimpanzee, was published taking us closer to the answer: What makes us human?

2008. Healing powers: By sequencing the cancer genome, scientists showed that the technique can identify disease-associated mutations.

2010. Even pandas participated: An international team sequenced the genome of Jingjing, a three-year-old panda and mascot of the 2008 Beijing Olympics. It was found that the animal has the genes required to digest meat, but not its staple food, bamboo.

2010. History got a future: The publication of the first draft genome of a Neanderthal in 2010 marked a turning point in understanding our ancestors.

2020. X marked the spot: The human X chromosome is fully mapped, with no gaps.

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HARD CELL: WHAT WE GOT WRING ABOUT GENETICS

Genes and twins: It was long assumed that identical twins, because they are formed inside the same egg and receive the same genetic material from their parents, are also genetically identical. But a 2021 study published in the journal, Nature Genetics, suggests that identical twin pairs differ by 5.2 mutations on average. On a scale of 3 billion letters in our genome, this is insignificant, but a difference nonetheless.

Mutation’s bad rap: Not all mutations are bad. If our DNA didn’t differ from others’, all humans would look the same, with no variation in height, hair texture, eye colour or skin tone. All or none of us could digest lactose, and would be susceptible to the same illnesses. Some genetic mutations, often called variants, can cause problems, but many have generated incredible adaptations – skin colour or hair colour for instance.

Genes and race: You can’t study a person’s genome and tell what race they are. The science just doesn’t exist. But tracing biological ancestry (which is different from race), is possible using a genealogical DNA test.

Designer babies: Genome editing to artificially select the traits of your baby isn’t quite the reality movies and TV make it out to be. Ethical concerns aside, we have two challenges to solve first. You can’t change one trait without it affecting others – genes don’t exist in a vacuum. For a trait like intelligence, for instance, we don’t even know all the genes involved, let alone select them.

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BACKGROUND CHECK: HOW CELEBRITIES HAVE USED GENETIC TESTING

Ellen DeGeneres has been found to have an ancestor in common with Kate Middleton.
Ellen DeGeneres has been found to have an ancestor in common with Kate Middleton.

In an episode of her talk show in 2011, Ellen DeGeneres revealed that she had received a letter from the New England Genealogical Society, explaining that she and then-princess-to-be Kate Middleton are 15th cousins. Their common ancestor was a certain Sir Thomas Fairfax. DeGeneres is also Queen Elizabeth’s 19th cousin, twice removed, through King Edward III.

In a 2017 episode of the British genealogy documentary series, Who Do You Think You Are? actress Liv Tyler unravelled the mystery of her father Steven Tyler’s maternal family line. It turns out, the Aerosmith frontman descended from Robert Elliott, a drummer boy in 1812, whose son, George, played the drum in the American Civil War. It seems he was destined to be a musician.

The genealogy website Ancestry found that George Clooney is a descendant of Abraham Lincoln – the 16th President of the United States. The family bloodline for both notable figures links to Lincoln’s maternal grandmother Lucy Hanks. This makes Clooney Lincoln’s half-first cousin, five times removed.

Benedict Cumberbatch has played his own ancestor King Richard III. The Ancestry site also discovered that Cumberbatch is distantly related to Sir Arthur Conan Doyle, the author who created Sherlock Holmes. Doyle, who died in 1930, and Cumberbatch are 16th cousins, twice removed. Their common ancestor was John of Gaunt, duke of Lancaster, fourth son of King Edward III and father of Henry IV.

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