Thursday, January 27, 2011

Genetics: It Makes the World Go Round

Over the past week or so, I've become somewhat fascinated by genetics. Perhaps my background in mathematics is the cause, but I'm quite fascinated by the idea of simply building every idea out of another idea. 

To really understand genetics, we have to start out by going down to the molecular level. Every cell contains a nucleus, and within that nucleus, there's DNA. DNA is incredibly important to the body. It carries the information used to build an organism. But what makes it up?

DNA is composed of four different chemicals: guanine, adenine, thymine, and cytosine, which are typically abbreviated as their first letters. Within the strand itself, there are some rules these chemicals have to follow. A will always pair with T, and G will always pair with C. (Pair, by the way, means that they have been hydrogen bonded together. Picture a twisted ladder, and that's the shape of DNA. The rungs on the ladder can be thought of as hydrogen bonds between the two molecules. The sides of the ladder are actually made of sugar and phosphate bonded together.) 

Now, I read an excellent metaphor at the University of Utah website. It said that the letters of the strand can be thought of as letters of the alphabet. The letters come together to form words, and the words come together to form sentences. In the same way, different series of letters (for example, A T G T C A) can be thought of as coming together to form genes. 

Now, genes tell the cell to make certain proteins. Proteins, as we know, can give cells certain functions and abilities--for example, within a cell of the inner ear, they can allow the cell to work with other cells to hear sounds. Genes are composed of DNA, although there are many genes along a single strand of DNA. (There are approximately 25,000 genes within the human body!)

Of course, DNA isn't simply laying around the nucleus of the cell. It's packaged into units known as chromosomes. Chromosomes are simply big chunks of DNA with protein wrapped around it. Every human cell contains 23 pairs of chromosomes--46 in all. Each chromosome carries different DNA with different genes, which means that each one controls different traits. For example, the 23rd chromosome contains either an X and a Y chromosome or two X chromosomes. Whichever one of the pairs actually occurs defines the sex of the person.

This brings us into our next big topic, which is heredity. If you need a refresher, see my post on mitosis and meiosis before reading on. 

Because genes carry certain traits, and because each parent gives one set of 23 chromosomes to the child, a child will inherit certain traits from each parent. (Fans of the Harry Potter series will recognize that Harry inherited his father's hair and general appearance but his mother's eyes.) Because of this, each child has a different genotype (genetic makeup) and phenotype (physical appearance). When these children have children, they will pass on some genes from their mother and some genes from their father. This is how traits can pass through multiple generations.

Now, let's mix heredity and genes together. Genes are made up of what are called alleles. An allele can be either recessive or dominant. If it is dominant, its presence will be apparent in the child's phenotype regardless of whether another gene is present. If it is recessive, however, it will only be visible if it is paired with another recessive gene. Basically, dominant alleles are just that--dominant. They mask recessive alleles. 

Of course, it's possible for some interesting combinations to occur. If a person has two dominant alleles or two recessive alleles, they are known as homozygous. If they have a combination of  dominant and recessive alleles, they are heterozygous. Now, here is where inheritance becomes interesting. If two people, one who is homozygous dominant for a trait and another who is heterozygous for the same trait have a child, their child's phenotype will display the dominant allele. However, if they receive the recessive allele from their heterozygous parent, and they have a child with someone who also has a recessive allele for the same trait, then it is possible for their child to show the recessive trait! This is how traits can skip generations.

Well, I think that's all for now! Let me know if I mangled anything! 

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