When a blonde baby is born to dark-haired parents, members of the family cast their minds back to see which grandparents or great-grandparents had blonde hair, as if to justify this difference which has 'skipped' a generation or two. In fact, it is not unusual for this to happen. It is often the case that one child in a family 'takes after' their father or mother and another doesn't seem to resemble anybody on either side. Actually, they do, but rather than a physical similarity it might be in the way they walk, their skill at solving maths problems, being a born leader or quite the opposite.
"Physical similarities are obviously determined by genes, so if your parents have blue eyes it is very probable that yours will be blue as well. And the same applies if your parents are very short, have a high IQ or if your father loses his hair before he is 30.
"In terms of character it is more difficult to determine how much is inherited and how much is learned, but the basic personality traits like extroversion, introversion or an aggressive temperament do have a high percentage of heritability," says Dr Juan José Tellería, secretary of the Spanish Human Genetics Association (AEGH) and lecturer at Valladolid University.
To a large extent, then, we are all like our parents, but who we are is also the result of environmental factors. For example: "If you separate twins and take them to live in different countries, you will see how their physical similarities gradually diminish because of those different environments. The food in each country, sports habits, whether or not we smoke, the infections we suffer during our lives, all those mould our physical appearance. It will not change the shape of our nose or ears, but it will affect weight, the risk of contracting vascular illnesses, the early appearance of wrinkles, etc," he explains.
The 'lottery' of eye colour'
It is the amount of melanin that determines eye colour, and that depends on genetics. "If the father and mother have blue eyes it is highly probable that their daughter will as well, but if one parent's eyes are a different colour, the child might take after either of them," says Dr Tellería.
In fact, the blue and green colour in eyes is actually the result of a lack of tonality in the iris. "This blue colour is the result of how light is diffracted in the eye. It is like the so-called blue sea. The sea isn't blue, it just looks as if it is," he says.
Returning to the subject of inherited traits, although it comes as a surprise if two parents with brown eyes have a blue-eyed child, there is a biological explanation.
"Eye colour is determined by a dozen genes, so the final result will depend on how those genes are combined. Your father received genetic load from your grandfather and grandmother, and you receive it from your father and your mother. If your paternal grandmother had blue eyes and those genes carry more weight in you, you will inherit the colour of your grandmother's eyes. It's as if that genetic load had been hidden, because it was different in your father and he has brown eyes, but the blue tone reappears in the granddaughter".
Snub nose, sign of identity
"Facial morphology such as the shape of the nose, the ears and the chin is determined by genes, although that doesn't necessarily mean you are going to have the same ears as your parents. Just like the eyes or any other physical trait, the genes involved in determining the shape of the ears may have combined in such a way that this aspect has not replicated. But in the case of identical twins, if one has a snub nose, for example, the other will as well, because they share 100 per cent of their DNA. While in those who come from different ova there is no reason for them to resemble each other much because they only share 50 per cent of their genome," says Dr Tellería.
Even the way we walk!
Every physical aspect can be inherited, and this includes even more than people imagine. The amount of hair we have and how likely it is to fall out, the strength of our nails, length of eyelashes and fingers, moles on our skin... even our way of walking. The way we walk is not a gesture, or not just a gesture, but "has to do with bone structure, and that is something strongly influenced by the genes", says Dr Tellería.
Height has nothing to do with the food we eat
Is my daughter going to be tall? Many parents keenly await paediatric check-ups, to see if their little one is going to be taller than average. But spurts in growth, beyond those which are normal at each age, depend a great deal on what a child has inherited from his or her parents.
"Height is extremely inheritable and in developed countries where there are no nutritional problems it is almost exclusively down to genes," says Dr Tellería.
"In developing countries, on the other hand, environmental factors do have more of an influence. Imagine a poor country where one child is well-fed and another goes hungry. The first will certainly grow taller, and this height difference will be not so much due to genes as to environmental factors, in this case better nutrition. In Spain we do not have these differences, so if a child is short it is because his or her parents are as well," says Dr Tellería. That may be the case nowadays, but in times of hunger such as during the post-war period, the environmental influence did make a difference. The proof of this is that the later generations, who have not been through such hard times, are taller.
Tooth decay can also be 'inherited'
Tooth enamel, the propensity to suffer dental caries... these are aspects where genetics have an influence, although so does the environment. "If your parents have a sweet tooth you will grow up thinking it is normal to eat sweet things so you will be more likely to have tooth decay, not just because your parents do but because you have 'inherited' habits which favour them, in this case a taste for sugar. The same applies to dental hygiene. In homes where people clean their teeth regularly, children present fewer dental problems," explains this specialist.
It starts in the womb
Our family conditions the way we are, even the career we study for, which is why we have 'sagas' of doctors, lawyers, family businesses, etc. It is not easy to distinguish how much of that is inherited and how much is learned: "However a study carried out by American researcher Ned A. Dochtermann of the University of North Dakota says 52 per cent of character traits depend on genes," says Dr Tellería. He also says the influence is notable from the beginning: "What the foetus experiences inside the womb affects the way it develops. A woman who suffers ill-treatment produces a high level of stress hormones, cortisol, and it would not be exceptional for that baby to suffer neurological problems, personality disorders or attention deficit," he warns.
Extroversion is a given trait
We inherit our basic personality traits and they will determine whether someone is extrovert or introvert, passive or active, organised or disorganised, meditative, persistent, or reflective. "Aggressive temperaments are also inherited," says the doctor.
"It's curious, but if twins are separated and brought up in different environments and are then given an IQ test at the age of 20, the result will be practically the same. However, if the family in which they live has adopted a child and he or she and the biological child of that family are educated in exactly the same way, their results in the IQ test will differ because genes have a greater weight than their environment," says Dr Tellería.
However, he also says that intelligence will not become evident until adolescence. "In the pre-school stage the genetic influence on intelligence in children is no more than 30 per cent, but it rises to over 70 per cent when they reach the age of 18 to 20. That is when this genetic inheritance flourishes and the differences between people become more obvious. That sometimes goes against our wishes, but intelligence, just like being tall or short, seems to be a matter of genetics."
The environmental factor obviously does have an influence: an environment that favours studying will contribute to better academic results even though someone doesn't have a very high IQ.
"My parents were Physics and Maths teachers and I have the same way of processing information as they have, but if my family had not had the means to allow me to study or if I had simply not grown up in a suitable environment for study, I would never have been able to become a doctor," says Juan José Tellería, as an example.
Elite athlete parent, professional sportsman son
An elite athlete has to have a specific personality: they have to be persistent, be prepared for hard work and able to set an aim in the long term and work to achieve it, at a time when most young people tend to want immediate satisfaction. These are basic personality traits which are inherited, so it is not unusual that in a family where a parent is an elite athlete, the children will become the same.
I recognise that gesture...
"You move your head just like your father," or "You make the same gestures as your mother." People often say this to us but it has nothing to do with genetics. Gestures are learned. The language that accompanies gestures, however, the verbal one, is another matter. "That is tremendously heritable, because the ability to dominate a language is a superior ability of the brain and is strongly related to intelligence," says Dr Tellería.
Roles can be replicated, but that is not because of DNA
"Everybody occupies a place in society, has a type of role: one might be a leader, another a collaborator, someone always causes trouble and another is always ready to lend a hand, and that tends to happen in generations of families," says the doctor. "Something similar occurs with the social niche one occupies: if a family is led by parents who are leaders in their field or in their business, it is probable that these behaviours will be replicated in their children, and that happens whether your father is a police officer or shopkeeper. It doesn't have to do with genetics so much, but what you have seen and learned, and how comfortable you feel in that social environment. And you do feel comfortable because it is an environment you know, because you have lived with it at home."