Heritability is one of the most misunderstood words in genetics. Instinctively it seems to relate to the genetic variation we each as individuals inherit from our parents (my father’s blue eyes, my mother’s big nose), but this isn’t the case. Heritability is about population based differences that arise from genetic differences in that population. For example, when we say that height is 90% heritable, it means that 90% of the differences between us is down to genetics, and 10% of the difference is down to environments. Interestingly, the more equal environments become (for example when there is a national curriculum at school) then the more the differences we see between people is accounted for by genetic differences. This makes sense as unequal environments must explain more variation than equal environments. This is partly why we get different heritability estimates in trates when looking at different populations.
Relates to the process that specifically relates to the genetic makeup of individuals (I got my father’s blue eyes and my mother’s big nose). Whilst this is interesting to study, especially when we think about our own traits, it is not as useful as considering how genetics works across entire populations.
To understand this you must first understand the two types of genetics research, quantitative and molecular. Quantitative genetics looks primarily at families (especially twin and adoption studies). Within families we know, on average, how genetically related everyone is (100% for identical twins, 50% for siblings and parents, 25% for cousins etc). Because we know this, we can measure traits and then work out from these relations just how important genetics are for that trait. Molecular genetics is a much more recent branch of genetics and allows us to look directly at the human genome to see if any particular genetic variations are associated with any particular trait variations. Molecular genetics studies typically account for much less genetic variation (only 1% or 2%) compared to quantitative genetics (anything from 20%-90%) and this difference has been dubbed as missing heritability. In truth, the heritability isn’t really missing, it’s just not been found yet. Not only is molecular genetics a very new science, but the human genome is enormous (about 3 billion base pairs long). In psychology a lot of the measures we use have a lot of error in them (you can’t measure personality with a ruler or scales), and the complicated statistics used in molecular genetics require a lot of control so that we are as certain as we can be before making any claims. All of these factors lead to the the different estimates between quantitative and molecular genetics. However, as these two branches become more interrelated with ever increasing sophistication, we are finding a lot more of this ‘misplaced’ heritability.
Sometimes in science the problem with defining a word is that there is no agreed on definition. This is arguably the case with the term ‘gene’ as different people will tell you different things. Essentially however, the term ‘gene’ relates to regions of DNA that hold instructions for making proteins, and proteins are the building blocks of all living things. Humans are thought to have between 20,000 and 25,000 genes, which is a lot fewer than were expected. About 2% of the whole human genome codes for proteins. We are still discovering what the other 98% is up to.
Traditionally, Junk DNA related to the 98% of DNA in humans that does not code for proteins (see above). As we discover more about DNA we have found plenty that goes on within this 98% as parts of it provide scaffolding and help with DNA reading and translation. However, we still aren’t sure what a lot of this DNA is doing or why we still have it (evolution tends to favour efficiency). If you’d like to know more about “Junk” DNA try http://www.nessacarey.co.uk/
Pleiotropy describes a phenomenon we see all the time in genetics: when one gene affects many traits. People who have albinism often only have one genetic variation, but have various different characteristics including lack of skin pigment and visual difficulties. In the behavioural sciences it is easy to imagine how one gene might affect several traits. For example, a genetic variation associated with poor attention may also be associated with behavioural difficulties and underperformance at school. In the behavioural sciences almost all traits are pleiotropic and polygenic (see polygenicity).
Polygenicity describes one trait that is affected by many genes. In the behavioural sciences almost all traits are polygenic. For example, reading is built up of so many components (clear vision, phonics, reading accuracy, comprehension, recall etc.) that it simply has to be built up by the functioning of a number of different genes. In the behavioural sciences almost all traits are polygenic and pleiotropic (see pleiotropy).