Genetics

__//**DNA**//__
Genetics is the study of DNA and how traits are transmitted from parents to offspring.

We will start with DNA and its structure.

Here is an example of DNA:



http://www.scq.ubc.ca/wp-content/dna.gif

The structure of DNA looks like a twisted ladder and is called a DOUBLE HELIX. The sides of the ladder are made up of a sugar, called deoxyribose, and a phosphate group. The rungs of the ladder are made up of four kinds of nitrogenous bases. The four bases in DNA are: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The rungs are pairs of nitrogenous bases, Adenine always and only binds with Thymine, and Cytosine always and only binds with Guanine.

The bases code for genes that control what proteins are made. This is how genes are expressed and we inherit our traits from our parents.

__//**DNA Replication**//__
DNA replication is the process by which DNA makes an exact copy of itself. The reason it does this is so that all cells have the exact same genetic information. http://weloveteaching.com/2011/organics/DNA_REP.GIF

In DNA replication an enzyme called Helicase opens up the DNA strand. The place where the DNA opens up is called the replication fork.

DNA polymerase then comes in and pairs the exposed nucleotides with the compliment. For example, if there is a C exposed on the DNA strand, polymerase will pair it with a G.

Because the original DNA is used as a template, and because A always bids with T and C always binds with G, the end result is two identical DNA molecules.

__//**Transcription and Translation**//__
Transcription and translation is how the genes are expressed. Transcription is when the DNA, which does not leave the nucleus, writes a message to be read by the cell. The message is written using RNA. There are three kinds of RNA, the RNA that writes the message from DNA is called messenger RNA or mRNA.



http://room114.wikispaces.com/file/view/transcription_sidways.gif/32044187/transcription_sidways.gif

__//**How traits are inherited**//__
There are many ways in which traits are inherited. The first is dominant and recessive. Dominant and recessive traits have two phenotypes (what the organism looks like) and three genotypes (homozygous dominant, heterozygous and homozygous recessive) In homozygous dominant or heterozygous, the organism displays the dominant trait. In homozygous recessive, the organism displays the recessive trait.

Example of dominant/recessive punnett square:

In the above case P means the flower is purple and p means the flower is white. PP is homozygous dominant Pp is heterozygous pp is homozygous recessive

Another means of inheritance is incomplete dominant. In this mode of inheritance, there are three phenotypes and three genotypes. Homozygous dominant, heterozygous and homozygous recessive. The homozygous dominant genotype produces the dominant trait, the homozygous recessive genotype produce a recessive trait, and the heterozygous genotype produces an intermediate blend of both traits.

Example of a incomplete dominant punnett square:



Codominant inheritance is where there are three genotypes and three phenotypes. Homozygous for one trait produces that trait, homozygous for the other trait, produces that other trait, and heterozygous produces a combination of the two, where both traits are shown equally, instead of a blend of the two.

Codomiant punnett square looks like this...



http://www.rpdp.net/sciencetips_v2/images/l12a5/abobloodsystem.jpg

__//**Mendel's Laws of Inheritance**//__
Gregor Mendel was a monk who studied pea plants and genetics. While some of his techniques may be questionable by today's standards, he did come up with two important laws for inheritance. The first law is the **__Law of Segregation.__** The Law of Segregation states that when making a gamete, the alleles for that gene separate, or segregate, during meiosis. This means that one gamete will get one allele and one will get the other.



http://course1.winona.edu/sberg/ILLUST/metphsIb.gif

In the above example, the original genotype of the parent cell is Gg, after meiosis, each gamete has either G or g.

The second of Mendel's laws is the **__Law of Independent Assortment.__** This law states, that chromosomes line up independent of each other in meiosis 1 and are therefore assorted separately from one another. In other words, if you have an initial genotype in a parent cell that is GgYy, the possible gamete combinations can be GY, Gy, gY, gy. G doesn't always have to go with Y and g with y.

http://preuniversity.grkraj.org/html/9_GENETICS_files/image004.gif

Genotype is what alleles are present Phenotype is what an organism looks like
 * Punnett Squares:** A chart that shows possible genotypic and phenotypic outcomes for offspring.



http://blog.savcds.org/swanson/files/2011/02/Punnett_square_PSF.png