It's said that no one knows everything. In other words, there's always a surprise around the corner. Science strides to know everything about nature and the universe we live in. But can science really reach that point? This remains to be seen. What we do know, is that we have only scrathed the surfaced of genetics. There is still much left to discover in genetics. In this section of Genetics R US, we are going to look at the new modern discoveries made in genetics. So lets get started!!!!
If you remember, the now famous Gregor Mendel did experiments on flowers. He studied certain traits such as petal color. Today, we know that petal color is due to a protein that is deposited in the flower's petal. This protein is encoded by a gene, a small piece of the entire DNA inside a petal cell.
However, something should bother you when you start to answer this next question. What causes the difference in color? Then you'd probably start to wonder if both a white and pink flower even have the same gene. The answer to that second question is yes. Both the white and pink flower have the same gene. This is because both proteins in each flower perform the same job or function. That function is to give a flower its color. When two proteins perform the same job, then the genes that encode those proteins are the same. But what about the difference in color? What can possibly explain two different colors in two flowers that are of the same type or species?
This is where the word allele comes in. The difference in color is due to the fact that a gene can come in many different forms, or alleles. An allele is simply different forms of the same gene. Two or more individuals can have the same gene, but have different alleles. Many people find this too confusing to be true.
If you keep in mind that when two proteins perform the same function, such as give a flower color, then the genes that encoded those proteins are identical. Same protein function, same gene. However, what you must understand is that a variation on the actual protein itself is what causes an allele to exist. The variation can be in a protein's amino acid sequence, to the very shape of a protein. This explains why apples come in different colors. You have red, green, and yellow apples. Why?? All apples have the same gene that gives the fruit its color, but that same gene comes in different alleles.
So, when we review Mendel's work on plants, then new things become apparent. Mendel predicted that a plant must have two genes. One gene from each parent. This is true, but a plant also has two alleles. To make things easier, let's call this plant petal color gene, C.
This gene, C, comes in two alleles, C(w) and C(p). C(w) makes a protein that would give a flower white petals. C(p) encodes a protein that would give a flower pink petals. Because a flower can hold two genes or two alleles, we say that a flower is diploid. Because a flower is diploid, a flower could have two alleles that are identical, such as C(w)C(w). In this case, the flower would be white. Similarily, a flower could have two pink alleles, C(p)C(p), would which produce a pink colored flower. We say that an organism that has two identical alleles for a given trait is homozygous.
By the same token, a flower could hold two different alleles. In other words, a flower might have one copy of a white allele C(w), and one copy of a pink allele, C(p). In this case, we say that an organism that has two different alleles is heterozygous for a trait. With two different alleles, what color will the flower be?
The flower's color will depend on which of the two alleles is the wild-type allele. In our case of the flowers, the pink allele is the wild type allelle. The term wild-type is used to refer to the allele that nature has selected to best suit the survival of the flower species. For example, apples come in three basic colors, red, green, and yellow. When you think of apples, the color red probably comes to your mind. This is because apples in the wild are mostly red. Nature has selected the color red, (for some unknown reason), because the color red best helps apples survive in the wild. The allele that is not the wild type allele, (in our case of the flowers, the white allele), is referred to as the mutant allele. When two heterzygous parents mate, something interesting happens. Let's check it out.
If you remember, sperm and egg cells carry half a set of genes. In our case of the flowers, this means that a male or female heterozygote flower produces different types of egg and sperm cells. For example, a male flower with a pink allele and white allele can only produce two types of sperm cells. One that contains either a single pink or single white allele.
If both a male and female flower that are heterozygous for a given trait (flower color) mate, then a number possibilites arise when considering how the children will look. These possibilites can be fully seen if we use a tool called a punnett's square. The way it works is cool. You simply line up the different sperm and egg cells on each side of a square. You then mate each sperm cell with a single egg cell to see what the kid would look like. Each smaller square is a possibilty or an event that would yield a future child
Here's a pulmett's square. If you remember with our flower, a heterzygous flower with two different alleles can produce only two types of sperm or egg cells. We simply line each sperm and egg cell on both sides of the pulmett square and unite a sperm cell with an egg cell to see what the child would look like. Each square is a possibility which, in our case, there are four.
From the example shown right above you, you can see that there are a total of four possibilities, (four small boxes). Notice that out of these four possibilites, a white colored flower appears in one of our 4 possibilites. In other words, from this chart, it appears that within the children, 1/4 or 25 percent of the children, would come out as white.
If you remember, Mendel noticed that a trait would skip a generation and reappear in about 25 percent of the flowers. Now you understand why! Whenever two parents that have different alleles mate, it can be predicted how a trait will appear within the children. The punnet's square is a powerful tool because it yields all the possibilties that could exist. Then, percentages from the square can be easily calcuated.
Geneticists, people who study and work with genes, have other powerful tools besides the punnet's square. The geneotype is one of them. The geneotype is simply all the possible combinations that two or more allele can be written in.
With that in mind, let's now end our discussion on alleles. With this new knowledge, you have much more information than the next joe does. So the next time you open up the newspaper and see the term gene or allele, just remember Genetics R Us was glad enough to provide you the information.
For example, the flower color gene, which comes in two alleles, has three geneotypes: C(w)C(w), C(p)C(p), and C(p)C(w). The phenotype is the outward physical expression that the alleles are responsible for. In our case, the alleles, C(w) & C(p), gives a flower its color. The color comes in two ways, pink and white. These two colors are the phenotypes.
Unfortunately, the newspaper, and this web site, will have to wait for new information. The reason is that there are things about genetics we still don't understand. In the next section, we are going to look at the areas in genetics that are still being ironed out. Let's check it out!!!!
Geneticists work every day with genes and alleles.
Alleles and genes are all in the news
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