A gene that follows its own drum beat

THE PASSAGE of genes from one generation to another is a matter of chance. A given gene from a parent has a 50 per cent chance of turning up in the offspring, said Gregor John Mendel, often called the father of genetics. But now a Russian team, headed by Sergei Agulnik, says they have found in a species of Siberian mice a gene that doesn't leave its manifestation to chance.

An organism's genes are lined up in a certain order along threadlike structures called chromosomes, which are found in a cell nucleus. Chromosomes occur in pairs, one from each parent. Agulnik and his colleagues found the Siberian mice sometimes carry a gene called In-gene (from Inversion-gene), which upends part of the chromosome it is situated on, thereby reversing the order of genes. If the mouse inherits In-genes from both parents, it is likely to die.

The scientists found that if they mated In-gene-free male mice with females who carried the gene only on one of the chromosomes, 85 per cent of the progeny inherited the inversion, instead of the expected 50 per cent. On the other hand, when a male that carried two In-genes was mated with a female carrying a single In-gene, only 35 per cent of the offspring were found to possess two In-genes. In such a scenario, Mendelian genetics predicts a figure of 50 per cent.

The scientists explain that sperm cells contain only one set of chromosomes, but eggs -- at least in mice -- hang on to two sets of chromosomes until they are fertilised, after which they throw one set away. They found that when an In-gene-free sperm cell dips into an egg that carries one In-gene, the egg usually disowns its normal copy -- resulting in an offspring with one In-gene and one normal gene. But if the sperm is carrying an In-gene, the female chromosome set carrying the In-gene sacrifices itself.