Will bub have brown or blue eyes and hair like Mum or Dad? Will he or she inherit your musicality or your partner's sporting prowess? Could you have a left-handed or red-haired child?
As you wait for baby, you've probably wondered what – or who – your little one will look like. But contrary to what high school biology classes may have led you to believe, predicting the physical and personality traits of your baby during pregnancy is virtually impossible.
Scientists estimate that we have between 20,000 and 30,000 genes that come in pairs – half inherited from Mum and half passed down from Dad. One recent study provides a 'best guess' of 22,333 genes but concedes this number could easily shrink or grow by 1,000 genes in the near future. What's more, even if experts did agree on a complete list of human genes, many of us might be missing some of the genes on the list because duplication of genes is through to vary between people.
And just to make the human genome even more confusing, there isn't a red-haired gene or a blue-eyed gene. Nor are the principles of dominant and recessive genes enough to explain the nuances of genetically inherited traits. Instead, most traits follow a polygenic mode of inheritance.
"With new research we can say there is not one gene which determines, for example, eye colour," says Dr Himanshu Goel from Hunter Genetics. "The colour of eye is a continuum from very light blue eyes to darker brown eyes. There are hundreds of different eye colours and these eye colours can be determined by a number of genes."
Your baby will inherit thousands of genes that influence everything from hair colour and eye colour to the shape of their earlobe, size of their toes and length their legs. These genes are shuffled and the possibilities are best understood as a spectrum rather that a set of defined possibilities. So your baby's skin colour may be darker than your skin but lighter than your partner’s. Or your little one’s eyes might be a different shade of blue to one parent.
"It just depends on the shuffle you get when the baby is born – it's like shuffling a deck and there’s hundreds and hundreds of millions of possibilities," says Professor Ravi Savarirayan from the Murdoch Childrens Research Institute.
Generally speaking, your baby will look like a combination of Mum and Dad – but how that blend plays out is impossible to predict. "A lot of things like skin colour, hair colour, facial appearance and personality will be similar but because of the way genetics works in every generation there’s an infinite possibility for variation and mixtures as well," says Prof Savarirayan.
"Although children usually look like their parents it is possible for two blue-eyed parents to have a child with brown eyes, it is possible for two short parents to have a tall child and it is possible for two right-handed parents to have a child who is left-handed, perhaps because there’s not just a one-on-one genetic factor – there are lots of genetic factors contributing.
"It may well be that a tall child is more like their grandparents or uncles and aunts because the genes get mixed up and shuffled around every generation so there’s really infinite capacity for variations that make us all different."
And don't forget that bub's physical and personality traits are influenced by their environment just as much as their genetics. Body shape and size, how we react to stress and academic ability are good examples of how nature and nurture interact.
"Lots of traits have a genetic component but many of those are also influenced by our environment," says Carolyn Shalhoub, a senior genetic counsellor and spokesperson for the Human Genetics Society of Australasia. "Most traits are partly genetic and partly influenced by environmental factors.
"Anxiety, for example, is a common trait in the general public and we know we do see anxiety running in the family, so you’d have to imagine there are some genetic influences that might make someone susceptible to developing anxiety, but their environmental set-up will also influence them."