Like a DJ, life mixes the genomic records from our parents and the controls on the mixing desk get reconfigured. But mum and dad don’t always agree on volume settings. Just like the way parents play different roles in your life as you grow up their genes play different roles during your development in utero. Some genes, when they come from mum, are always silent; the active copy comes from your dad. However, we get silent genes from dad too, and then mum’s copy gets to play at maximum volume. A number of these genes facilitate the way that babies grow and develop. Curiously dad seems to switch off genes that would restrict our growth, whereas mum compensates by switching off genes that promote growth.

Around 80 such genes have now been described in mammals, and although they constitute less than 1% of the human genome, their effects are strongly felt. Take for example the IGF2 gene, which makes a protein involved in embryonic development. This growth-promoting factor is silenced by females when they make eggs; you always get a quiet copy from mum. Experiments with mice have demonstrated that babies are born much bigger if mum’s IGF2 gene is not silenced. Very high doses of the protein kill developing embryos in the womb. In humans failure to inherit a silent copy of IGF2 causes Beckwith-Wiedemann syndrome. Babies with this disorder are over twice the normal weight. This poses a risk of miscarriage, although modern medical techniques mean that most babies can survive with the condition. Like ligers, however, these individuals have a reduced longevity. If mum’s IGF2 gene escapes silencing, certain kinds of cancer may ensue. This is one of the greatest risks for individuals with Beckwith-Wiedemann syndrome.