What are Chromosomes and Genes?

Chromosomes are the physical structures that contain the DNA and genes necessary for development. Chromosomes are located in the center of the cell, in the area called the nucleus.

A normal human cell should contain exactly 46 chromosomes. There are 23 pairs of chromosomes. The first 22 pairs are identified by number and organized by size. The 23rd chromosome pair is referred to as the sex chromosomes which determine gender. Females normally have 2 of the same sex chromosomes, called the X chromosome, while men normally have 2 different sex chromosomes, known as the X and Y chromosomes.

A normal set of chromosomes is 46,XX for a female and 46,XY for a male. In a normal conception, both the egg and sperm cells should contribute exactly 23 individual chromosomes, one of each of the 22 numbered pairs and one of the sex chromosome pair. When an egg with 23 chromosomes fuses with a sperm with 23 chromosomes, the correct chromosome number of 46 (23 pairs) is again present, and the fertilized embryo has the best possible chance of developing appropriately.

Occasionally, an egg or sperm cell will misdivide as it is developing, and the mature egg or sperm will then contain more or less than 23 chromosomes. Any embryo that results from this type of conception will have a chromosome disorder because there is not exactly 46 chromosomes.

Down syndrome is a common example of aneuploidy. Pregnancies or individuals with Down syndrome have three copies of chromosome 21 (instead of two). The majority of Down syndrome is due to Trisomy 21.

Approximately 30,000 genes are located on the chromosomes. Typically, there are two copies of every gene; one copy is inherited from the mother and one copy is inherited from the father. A common exception is males, who have only one copy of the genes on the X chromosome and one copy of the genes located on the Y chromosome. Genes are instructions for development and function. These instructions are spelled out using an alphabet of only four letters, A, T, C and G. Different combinations of these four letters spell out different instructions. Most genetic disorders are caused by a change in the spelling of a gene (mutation). Mutations can affect the ability of a gene to function normally which can lead to disease.

Every person carries some gene mutations. Most of these mutations do not affect health status because the second copy of the gene functions well enough to compensate for the mutated gene. For certain genes, one of the genes having a mutation is enough to cause a disease to occur. These conditions are referred to as dominant genetic diseases. An example of a dominant genetic disease is Huntington disease.

Many genetic diseases only occur if both genes carry a mutation. These diseases are referred to as recessive genetic diseases. An example of a recessive genetic disease is cystic fibrosis. People who have only one mutation of a recessive gene are unaffected, but are referred to as a carrier of that disease. Many recessive diseases occur more frequently in different ethnic populations.

Genes located on the X chromosome are referred to as X-linked genes. Examples of X-linked genetic diseases are Hemophilia and Duchenne Muscular Dystrophy. Mutations on genes located on the X chromosome typically cause disease in males, since there is no second copy of the gene to compensate for the mutation. Females with mutations on the X chromosome may or may not have disease depending on how the gene is inherited (dominant or recessive). Additionally, one copy of the X chromosome is automatically shut-off in every cell. This process is called X-inactivation and typically occurs at random in every cell. In some instances, however, there is skewed X-inactivation and the two X chromosomes are not turned off in equal proportion throughout the body. This may influence the degree to which a female is affected with an X-linked genetic disease.

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