What you need to know...
- Comparison of discrete and continuous variation.
- Most features of an individual phenotype are polygenic and show continuous variation.
- Identification of phenotype and
genotype, dominant and recessive
characteristics and homozygous and
Something everyone knows but takes for granted is that we're all different. Each individual in a species is different from the others, and species are different from each other. The word we use to describe the differences between living things is variation. In this topic we're interested in the causes of variation and the different forms of variation. If you think about it, you already know what makes us different from each other. There are two major causes of variation...your genes and your environment. Apart from identical siblings, we're born with a different combination of genetic information. This is the benefit of sexual reproduction to species - it creates variation within the species. But as you know, the environment plays a part in our appearance too. For example, our nutrition can play a major role in our appearance. In this topic we're focusing on inherited variation. How do those genes we inherit bring about the differences between us?
Discrete and Continuous Variation
Variation between individuals of a species can be grouped into two large categories: discrete and continuous. Discrete variation is either/or and is often caused by the presence or absence of a small number if genes. For example, our ability to "roll" our tongues is determined by just one gene. We can all either roll our tongue or not - there's nothing in between.
If you were to do a survey of all the students in your class to find out what the variation was in terms of their ability to roll their tongues, there would only be two options: can roll or can't. If you were to draw a graph of the results, because of the discrete nature of this variation, you would draw a bar graph which might look a little like this.
In reality however, most examples of the ways in which members of a species vary from each other is more complicated than this. Most of the features of an organism are the result of a complex interaction between a number of different genes, often with the environment playing a part also. The involvement of a variety of genes in the determination of an organism's features is known as polygenic inheritance. This more complex cause brings about what is known as continuous variation. Height is a good example of continuous variation.
There are a wide range of different heights and if you look around your class it is unlikely that any two individuals are exactly the same height. It wouldn't make sense to draw a graph like the one above for heights as you could have a bar for each individual in the class! So, when we draw a graph of continuous variation we categorise the individuals and draw bars to show the frequency in each of the categories. This type of graph is called a histogram and is likely to look something like this:
If your sample is size is large enough, a graph of continuous variation of an organism will almost always have a shape like the one above. This is known as normal distribution.
If you're searching the web for variation resources you might quite often encounter the word discontinuous instead of discrete, such as on these BBC pages, the two are equivalent and so these resources are still useful. You just need to make sure you use the word discrete in your assessments.
As we've already mentioned, our genes play a crucial role in our features. We've known for some time now that our genes influence our characteristics, but we're still learning more about this today, as you can tell from this Google search. The ways in which our genes impact on our appearance is often complicated as mentioned above, but not always. In this course you need to know about the ways genes are inherited and expressed, but only in the most basic form. We're going to focus on single-gene inheritance, but as mentioned above most of our features are as a result of the complex interactions between many genes (polygenic). We'll save that for a future course!
Before we begin we need to revise some terms and introduce some new ones. Firstly, I'm assuming you remember what a gene is...if you're not sure stop reading now and go back and refresh your understanding of DNA, genes and chromosomes. We also need to introduce the terms phenotype and genotype. The phenotype of an organism is its physical appearance. For example, in the examples above this would be can or can not roll your tongue or height. The genotype is the genetic make up of the organism. Two organisms with the same phenotype will not necessarily have the same genotype as we'll soon see...
Take our ability to roll our tongues as an example. As mentioned above, this is controlled by just one gene. However, as we each receive two full sets of the 23 different human chromosomes at ferilisation, we each have two copies of the gene for tongue rolling. Something we've not mentioned yet, is that our genes can come in different forms. So, the gene for tongue rolling for example comes in two different versions: can roll and can't roll. For the sake of easiness we can summarise these two versions of the tongue rolling gene with letters. We'll use the same letter for them both as they're both versions of the same gene, so we'll just use a capital letter and a lowercase letter. For example we can summarise the 'can roll' version of the gene as 'R' and the 'can't roll' version of the same gene 'r'. There is a reason why I used the capital letter for the 'can roll' version. The 'can roll' version of the gene is the dominant version of the gene and 'can't roll' is recessive. What this means is that if you have both copies of the gene, you can roll your tongue. That version of the gene overrides the other, which is why we describe it as dominant.
Now that we have these letters to summarise the two versions of the gene, we can begin to think about the genotypes of individuals of different phenotypes. So, if an individual's phenotype is that they can not roll their tongue, what would their genotype be? That one's easy. It would have to be 'rr'. Everyone has two copies of the gene, so there has to be two letters. Neither of them can be 'R' as that is dominant and so that would mean that they would be able to roll their tongue!
What about people who's phenotype is that they can roll their tongue then? What's their genotype? That's not so straightforward. Their genotype can either be 'RR' or 'Rr'. Remember, because 'R' is dominant it only takes one copy of that version of the gene to be able to roll your tongue, but we wouldn't be able to determine which of the two genotypes a tongue roller is without further tests.
We have two more words to introduce here before we can move on: heterozygous and homozygous. These words are used as a way to describe the different genotypes of individuals. Heterozygous means that they have two different forms of the gene, whereas homozygous means their two versions of the gene are the same. In our tongue rolling example we would describe each of the genotypes as follows:
|RR||Homozygous Dominant||Can Roll Tongue|
|Rr||Heterozygous||Can Roll Tongue|
|rr||Homozygous Recessive||Can't Roll Tongue|
We can use this information to predict what we would expect the phenotypes of offspring from sexual reproduction to be. The diagrams below show what we would expect the phenotypes of the children from parents with different genotypes for tongue rolling.
Would the children of parents who are both homozygous, one dominant and one recessive, be able to roll their tongue?
This example shows that the children of a person who is homozygous dominant and a person who is homozygous recessive would all be heterozygous and so would all be able to roll their tongue. Remember, egg and sperm cells are gametes, which means they only have one set of the species' chromosomes. That's why they only have one letter in each.
Would the children of two heterozygous parents be able to roll their tongues?
This example is more complicated. Here we have two people who can roll their tongues but are both heterozygous. This means their gametes could get either form of the gene. So, the little table above helps us work out what proportion of genotypes and phenotypes we would expect in their children. In this example we would expect 25% of their children to be homozygous dominant (RR), 50% to be heterozygous (Rr) and 25% to be homozygous recessive (Rr). Because both RR and Rr have the same phenotype, the expected phenotypes would be 75% 'can roll' and 25% 'can't roll'. However, to actually see these expected ratios the couple would have to have a lot of children! So most of our genetic experiments are done with species which produce large numbers of offspring quickly, such as plants, fruit flies and mice.
The basic sort of experiment you could do to investigate the inheritance of seed surface would be to cross two homozygous plants with different phenotypes. You could then cross two of the offspring plants together. We use a shorthand to indicate which of the generations we're discussing in such an experiment as you'll see in the following diagram. The parents we can refer to as 'P1', the offspring as 'F1' and the offspring of the offspring as 'F2'.
So what genotypes and phenotypes would we expect from this cross then?
As you can see from the diagram above, 100% of the F1 generation would have the genotype Rr and would have round seeds. The offspring of two F1 plants are shown in the F2 generation. We would expect 25% of the F2 generation to be RR, 50% to be Rr and 25% to be rr. Therefore, 75% of the F2 plants should be round and 25% should be wrinkled. The ratio of the two phenotypes is 'Round 3:1 Wrinkled'.
Experiments such as these were first carried out by an Austrian monk named Gregor Mendel in the 19th Century. The following video is great for explaining this. There are more resources related to this video here.
As it mentions at the end of the above video, our understanding of genes has helped with the diagnosis of inherited diseases. For example, pedigree charts such as the one below can be used to help determine the inheritance of genes which are related to genetic diseases.
Quite often, the version of the gene which results in the disease is recessive and so individuals which are heterozygous have a copy of the gene, but are unaffected. These individuals are normally described as 'carriers'. In the example above, individuals 1, 2, 3, 6, 7, 10, 11, 13, 15, 18 and 20 are carriers. Without genetic testing, most of these individuals would not know if they were carriers for the condition or not. This is where a pedigree chart can be useful. Before couple 13 and 14 had their children they could have worked out that 13 must be a carrier for the disease as his father (4) had the condition and therefore must have been homozygous recessive and could only have passed on the disease-causing version of the gene. Working this out would occur as part of the process of genetic counselling.
This process has recently featured heavily in the news due to Angelina Jolie choosing to have a double mastectomy as a result of her high risk of developing breast cancer. You can find out more about genetic counselling for cancer on Macmillan's website.