thank you for your considered response.

from your comments i see we largely agree.

your reference to schizophrenia …

“If i had a schizophrenic parent, I would have a 50% chance of developing schizophrenia. The environment surely has a say on whether i am pushed over the edge or not, but i am far more likely to develop the condition than the average american (1% chance).”

…is completely consistent with what i wrote:

“genes are not autonomous. They are ‘expressed,’ ‘turned-on’ or ‘off,’ in response to environmental stressors/stimuli…Predisposition is not a cause! Genetic predisposition always requires exposure to an environmental stressor – emotional, chemical or physical.”

I highly recommend the book “Blaming the Brain” by Elliot Valenstein. On pages 144-145:

“Consider the evidence that schizophrenia and depression, tend to run in families. This by itself is not evidence of a genetic cause, as poverty also runs in families.”

While there are higher incidences of schizphrenia in children of schizphrenics and, more strikingly, identical twins of schizophrenics, there are still many shared and influential environmental factors that, as you say, may “push (one) over the edge” to be schizophrenic.

Those environmental factors can occur before birth, in gestation, growing in the womb of a schizophrenic mom and after birth, growing up in a household with a schizophrenic parent.

“It is certainly reasonable to assume that individuals may inherit a predisposition to develop a particular mental disorder…a constitutional predisposition or tendency towards developing a particular disease, but a predisposition is only realized under certain conditions” (Blaming the Brain)

Disorders like Sickle Cell Anemia, Cystic Fybrosis, Down Syndrome, etc, follow the rules of single gene inheritance. They are expressions of one gene; some of these disorders are autosomal some are sex linked or sex dependant, some are dominant and some are recessive. for example, cystic fibrosis is autosomal recessive, if you have both the recessive mutant alleles, you will have CF. It is possible to have different degrees of severity, but it is a sure fact that that gene will be expressed. As for down syndrome, those who suffer from it, have 3 copies of the 21st chormosome… there is no gray area there, if you have 3 of them, you will surely have down syndrome.

Conditions like Manic depression, schyzophrenia, possibly alcholism, depression, etc, are like our skin tone, height, and weight, they are polygenic traits that depend on the expression of many different genes. Both single gene and polygenic traits are multifactorial, which means the environment also plays a key role in the expression of the gene.
that said, genes are still extremely influential on whether one will suffer from a certain condition or not. If i had a schyzophrenic parent, I would have a 50% chance of developing schyzophrenia. The environment surely has a say on whether i am pushed over the edge or not, but i am far more likely to develop the condition than the average american (1% chance).

I wish i had the knowledge to write more… this is what i remember from my genetics class last semester. it’s such an interesting field! But that, according to my professor, is why certain conditions are hard to predict and seem to just run in families and pop up randomly. Environment plays an important role, but genes are still key.

look forward to seeing you again in person!!

Love the blog so far! I will definitely be a loyal reader!