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What Your DNA Can Tell You About How You Are Related to Others

What Your DNA Can Tell You About How You Are Related to Others

A group of people walking down a street in Kolkata, July 2017. Photo: Koushik Das/Unsplash

Trigger warning: Sexual assault, rape

Each cell in our body has a nucleus that contains two copies of the 23 chromosomes that make up our genome. One member of each pair is inherited from the mother and the other is inherited from the father. These chromosomes are numbered from 1 to 23.

Even if, say, the paternally-derived chromosome 6 carries a mutation that makes a gene non-functional, all will be well if that function is provided by the non-mutant gene copy on the maternally-derived chromosome 6. Only if both copies of chromosome 6 carry the mutation would an individual suffer the consequence of the gene’s absence. Such people are said to be homozygous for that mutation. Sadly, the consequences can be serious and represent the burden of a genetic disorder.

If the mutation is on the maternally-derived chromosome 23, also known as the X chromosome, then the normal gene on the paternal X can provide the missing function. But this happens only in biological females – because only they receive the paternal X chromosome; biological males receive the Y chromosome. The X and Y are called the sex chromosomes.

All is well for the females, but the males that inherit the mutant X suffer the loss of gene function. This is why sex-linked disorders like colour-blindness occur largely among biological males. The mutations for sex-linked disorders are in genes on the X. The other chromosomes (numbered 1 to 22) are called autosomes.

A consanguineous union is a union of related individuals. They are the major causes of homozygosis of autosomal mutations. Offspring from an uncle-niece marriage inherit the same mutation from both parents with a 1/8 probability – while those from a first cousin-marriage do so with a 1/16 probability.

Most genetic diseases are not yet amenable to simple cures. But there are a few happy exceptions. Ashwin Dalal, a medical geneticist in the Centre for DNA Fingerprinting and Diagnostics (CDFD), Hyderabad, evaluated a patient with a history of seriously debilitating involuntary movements. He found the patient carried a mutation in a gene that governs the response to a chemical called adenosine by increasing a second chemical called cAMP. This, in turn, led to increased neurotransmitter production that triggered the involuntary movements. Dalal read a report that caffeine antagonised the effect of adenosine, so he explored whether coffee might reduce the involuntary movements. It did and the patient’s condition improved markedly.

Next door to Dalal is the Laboratory of DNA Profiling Services. It provides DNA-based services to investigative arms of the government and to hospitals that offer organ transplant procedures. A small fraction of their cases have investigated crimes of incest. Incest is a mating between individuals related to the first degree (like father-daughter or brother-sister). Offspring from an incestuous mating have a 1/4 probability of becoming homozygous on the same autosomal mutation. In other words, 1/4 of their genome is homozygous. High levels of homozygosity are also seen in endogamous populations – with the custom of marrying only among themselves.

Recently, CDFD investigated a case of sexual assault of a minor girl. She later terminated her pregnancy. The scientists examined the DNA from her blood samples, the aborted foetus and one male suspect. The DNA profiles clearly showed that the victim and the foetus shared a mother-child relationship. However, the suspect was not the father of the foetus.

The scientists found that more than 1/4 of the foetal genome appeared to be homozygous. One has to be familiar with consanguineous unions to pick up the significance of this finding. It raised an unanticipated ethical question: should we formally report merely that the suspect was not the father, and ignore a possibly ominous red flag? Or should we formally recommend that the police also investigate the victim’s father and brother? Or should such a recommendation be conveyed informally, by phone?


Any two people who aren’t parent-child, grandparent-grandchild, siblings, first-cousins, or second-cousins still share an nth-degree cousinship. (In cases of incest, n = 1.) The value of n between cousins isn’t very large.

The arrest in 2018 of Joseph James DeAngelo, Jr., a serial rapist and murderer, exemplifies this fact. The DNA profile of the semen recovered from a rape kit was compared with millions of profiles in databases of companies offering DNA-based genealogy services to customers. The names and addresses of those whose profiles matched with that of the sample provided the basis to gradually close in on DeAngelo, Jr., and confirm that the semen had indeed come from him. The matches were with DeAngelo, Jr.’s second and third cousins.

Simulation studies have found that any two persons today likely share a common ancestor who lived as recently as around 3,000 years ago. And of those who lived a few more thousand earlier, each person was either an ancestor to all or to none of the people alive today. Exceptions can occur if one of them was, say, from North Sentinel Island of the Andaman and Nicobar. (The Sentinelese are thought to have remained genetically isolated from everyone else since at least 48,000 years ago, when sea levels were much lower.)

This astounding possibility inspired scientists to raise the standard of prose in a paper to a level seldom seen in the scientific literature, in 2004:

“No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu.”

D.P. Kasbekar is a retired scientist.

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