Genetics of Huntington's Disease

Genetics at a Glance
Whether or not a person develops Huntington's Disease depends entirely on genetics. If you need to brush up on your genetics, here is a fairly quick overview.

In the nucleus of each cell in the human body, there are 46 chromosomes. Chromosomes are made up of very long, randomly ordered chains of four nitrogenous bases: Thymine (T), Adenine (A), Cytosine (C), and Guanine (G). Sequences of a few nitrogenous bases in certain orders code for amino acids. There are 20 different amino acids from which proteins are composed. Chromosomes contain our DNA or genetic makeup. An easy way to understand DNA is to think of it as the blueprints of our body. DNA is divided into chunks called genes. Different genes contain the blueprints for different organs, bones, blood vessels, etc. When humans reproduce, they combine half of each of their DNA. In other words, the sperm and the ovum each contain 23 chromosomes. Twenty-two of these chromosomes are autosomal, or non-sex related, and one is a sex chromosome (X or Y). The sperm and the ovum, known as gametes, are also called haploid cells because they contain only half of the genetic material required to make a person. The sperm and the ovum combine to form a diploid cell (all 46 chromosomes), called the zygote. The zygote undergoes many divisions and transformations to eventually grow into a person.

Every person inherits an entire set of blueprints (23 chromosomes) from each parent. Therefore, you'll inherit two chromosomes, each with a gene determining what your eyes, hair, or any characteristic will be like. The chromosomes from each parent encoding corresponding body parts line up and combine to give you the traits you have. Corresponding chromosomes are called homologous.

To give you an example of how traits are determined, pretend that there are two types of gene encoding for hair color, one for brown hair (B) and one for blond hair (b). The different versions of a gene for any trait are called alleles. The combining of alleles follows the law of segregation discovered by a monk named Gregor Mendel. In any instance of combining alleles, one of the alleles is going to be dominant. You will soon see how this is important. If the allele for brown hair is dominant, the allele for blond hair is then recessive. If a dominant allele is passed on, no matter what the other allele is, the trait that the dominant allele encodes for will be expressed. Two recessive alleles must be passed on in order for the recessive trait to be expressed. Pretend your father was a pure brown haired man and your mother has blond hair. Your father would have two brown hair alleles (BB), and your mother would have two blond hair alleles (bb). You will inherit one allele from each parent. The only combination you could have is (Bb). Since brown hair, (B), is dominant, you will have brown hair. If you mate with a blond haired person, you could pass on either your brown (B) or blond (b) allele, while your mate will pass on a blond allele (b). The offspring will have one of four possible combinations: one dominant brown hair combination (Bb), or three blond hair combinations 3x(bb). If you mated with a brown haired person, with genes like yourself, (Bb), your offspring will inherit one of these four combinations: one pure brown hair (BB), two dominant brown hair 2x(Bb), or one blond hair (bb).

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