Source: gene: public domain; http://en.wikipedia.org/wiki/File:Gene.png eye color: public domain; http://commons.wikimedia.org/wiki/File:Eye_color.jpg
Hello, class. In today's lesson, we're going to be taking a look at the biological factors that influence the development of people. In particular, we're going to be looking at an area of biology called genetics. So before we get started, it's important to recognize our two key terms in genetics.
The first one is heredity, which is to say, the transmission of physical and psychological characteristics from a parent to offspring, or sometimes we call this inheritance. And DNA, which is the mechanism that allows us to pass that information from parent to offspring. DNA stands for deoxyribose nucleic acid. So let's take a look at some of the other factors that influence our heredity and our passing of different psychological characteristics.
So how do we understand all of this information that's going on in our DNA? Well, DNA inside of ourselves is organized into these x-shaped structures that we call chromosomes. And inside each one of our cells, we have 46 of these chromosomes, which are these bundles of DNA that organize all of our genetic information. Now, out of the 46 chromosomes that we have, half of them-- or 23-- come from one of our parents, and half of them come from the other. So essentially, we have a sort of back-up of each one of those chromosomes.
Now, these chromosomes are sort of big messes of DNA, and the DNA themselves are long strands. So it's kind of difficult to understand exactly what's going on there. So to make things a bit easier, we group the DNA into useful units that we call genes. A gene is a specific area on the DNA that carries certain hereditary information. It codes for specific processes or characteristics within our bodies. For example, there's a gene that we look at that influences our eye color.
Now, since we have two of each of these genes, it's hard to tell when one of them is going to be expressed or not. In other words, it's hard to tell when one shows up and the other one sits back, since they might be different. To help out with this, we organize genes into dominant or recessive characters.
A dominant gene is a gene where it will express itself each time it's presence. Essentially, it's the more powerful of the two, if you want to think of it that way. Although that's not necessarily the case. Or recessive, which is a gene that will only be expressed when it's paired with another recessive gene. In other words, when it doesn't have a dominant gene paired with it.
This is a little bit tricky to understand, so to help out with this, a lot of geneticists use what's called a Punnett square. A Punnett square sort of organizes the information so you can see when certain characteristics are going to be expressed and when not.
Let's use this as an example. Let's say that each of the parents-- one parent and the other-- have brown eyes, that they each have a dominant gene for brown, which is the big X, and a recessive gene for blue, which is the small x. So if we look, any time there's a large X, then the offspring-- or their children-- will have brown eyes.
So in the first one, there are two big X's, so that child will have brown eyes. And then the second and the third ones, there is one big X and one small x. And since the big X is dominant, then that child will also have brown eyes. However, in the fourth child who has two recessive genes, two small x's, this child will have blue eyes, because they don't have a dominant gene to express for brown eyes. So this is another sort of helpful way of trying to make sense of all this genetic information that we're coded.
Now as you may guess, the way that our genes actually work is generally not as simple as the Punnett square that we saw here. Most characteristics in genetics are polygenetic, which is to say that there are lots of genes that are working together to express that characteristic. But it's important to remember that genes express not only physical characteristics like our eye color, but also psychological characteristics. And this can help us to understand why certain psychological occurrences-- or even psychological mental disorders-- occur.
For example, some mental disorders, like bipolar disorder, schizophrenia, and autism, have a strong genetic basis, which is to say, it's much more likely for people to inherit those diseases from their parents. So if your parent has schizophrenia, you're 80% more likely than somebody whose parent doesn't to get schizophrenia in your life. So an understanding of genetics helps us to understand how some of these psychological characteristics get passed on.
The Human Genome Project is a project which in 2003 was able to sequence all of the three billion different DNA and genes types that exist within the human body. So now that we understand what the code looks like, we're a lot closer to understanding the biologic basis for the characteristics, as well the behaviors and mental states, that exists within human beings.