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FAQ: Genetics

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One well studied example is parental imprinting. Certain genes are marked with chemical tags via a process called methylation while they are still in a sperm or egg, meaning that only the maternal or paternal copy is active in the offspring. As a result, certain traits are inherited exclusively from one side of the family.

There is also some evidence that environment can influence epigenetic factors. For example, Dutch women who were pregnant during the famines of the World War II gave birth to small babies. But, surprisingly, the next generation also spawned small babies even though they ate well, as if they "inherited" their mother's experience.

7. Do genes control everything about an organism, or is environment important?

The debate over the relative importance of nature and nurture would fill several encyclopedias, but modern genetics predicts that both should play a role. That should come as no surprise to anyone who views genes as a piece of cellular machinery. Dangerous chemicals, such as cigarette smoke, can jam that machinery or interfere in its workings.

Equally, a therapeutic environment can compensate for a faulty gene. For example, babies who are born with the disease phenylketonuria (PKU) lack an enzyme that metabolises the amino acid phenylalanine. It therefore builds up to toxic levels causing mental retardation. But babies are now screened for the defect at birth and those with two copies of the defective gene are given special diets low in phenylalanine. As a result, they develop normally.

8. How genetically similar are we to primates and other organisms?

Chimpanzee genes differ, on average, by roughly just 1% from human genes. Other apes' genes are 95% to 98% identical to ours, too. Rodent genes are 88% identical and chickens come in at 75% identical.

Once you leave the animal kingdom, wholesale comparisons between human genes and those of other species becomes trickier. About one-third of the genome of the fruit fly Drosophila melanogaster, for example, contains genes that are only shared by other arthropods, and one-quarter of human genes are shared only by vertebrates.

The function of some genes in flies, plants or worms appear close enough to their human counterparts that these animals can serve as models to study human biology and disease.

9. Most traits are controlled by a complex array of genes. But which human features depend entirely on single genes?

You already know some of your single gene traits like the back of your hand. Specific versions of different single genes cause: hair growth on the middle segments of the fingers, the top of the little finger to bend dramatically to the ring finger, and determine whether the left thumb crosses over the right - or vice versa - when fingers are interlocked.

The result of other single gene traits are as plain as the eyes, ears, and hair on your face. People with blue eyes, non-dangly ear lobes or a straight hairline have inherited specific gene varieties. The ability to roll your tongue into a tube and also to taste certain bitter chemicals is also conferred by certain types of single genes.

Defective versions of single genes can also cause disease such as cystic fibrosis, sickle cell anaemia and Huntington's disease.

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Computer graphic of part of the DNA molecule, showing its twisted double helix structure (Image: Dr A Lesk, Laboratory of Molecular Biology / SPL)



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