Genetics Lesson

Big thanks to Louise, high school science teacher extraordinaire, for helping me better understand chromosomes and genes and their relationship to one another:

Every cell in your body contains 23 pairs of chromosomes, except for your sex cells (sperm and egg only have 23 chromosomes, not 23 pairs). Chromosomes come in an X shape and each side of the X is identical to the other (the l and the l stuck together in the middle by a centromere make an X). The X's will split when a cell divides so that the same information will go into each new cell. At that point, it will replicate (make a copy) so that it is a full X again.

Each X (chromosome) has a matching X.
xx xx xx xx etc (23 pairs)

The karyotype is interesting because it shows that each chromosome is the right size and has a matching partner. The only one that should not match up is if you are a boy, the 23rd set will have two different sized chromosomes (X and Y that make them a boy). Certain genetic disorders do not have the 23 matching pairs. Trisomy 21 is when there are three of the 21st chromosome which presents itself as Down's Syndrome. Trisomy 18, three of chromosome 18=Edward's Syndrome. The reason the karyotype does not show X's is because they are showing just half of the X which appears as an l because both sides are identical.

A good way to understand the relationship between chromosomes, genes and DNA is the following:
Chromosomes are made up of genes which are made up of DNA in the same way that.....
Sentences are made up of words which are made up of letters. If there is a one mistake in one of the words (one letter off), you can still read it most of the time. If there is a change in an entire word, it's can be hard to understand the sentence, or will change the entire meaning of the sentence, which will have much larger effects. Genes are what determine various characteristics and how proteins are made in our body. A gene is a section of that X shaped chromosome.

Taken from : http://ghr.nlm.nih.gov/condition=noonansyndrome
(Noonan's Syndrome has....)
Mutations in the PTPN11 gene, SOS1 gene, KRAS gene, or RAF1 gene cause the resulting protein to be continuously active, rather than switching on and off in response to signals that control growth and development. This constant activation disrupts the regulation of systems that control cell growth and division, leading to the characteristic features of Noonan syndrome.

The chromosome can also be thought of as a car. If a part (gene) of the car isn't working properly it may have certain affects on the car. Depending on the part, that will determine the way your car functions. Obviously brakes are more `important' than washer fluid, they affect the functioning of the car to a greater magnitude.