Sickle Cell Anemia Genetics
To better understand sickle cell anemia, it helps to understand a little about genetics.
DNA is the genetic material that largely determines our health. It is present in every cell in the body.
DNA is stored in string-like structures, known as chromosomes, inside the nucleus of cells. Each human has 23 pairs of chromosomes, for a total of 46 chromosomes. The majority of them are identified by the numbers, 1 through 22. The first 22 pairs of chromosomes are called autosomes: they do not determine our gender.
The letters X and Y represents the last pair of chromosomes. The last two chromosomes are called sex chromosomes, because they determine our gender. Females have two X chromosomes in every cell, while males have one X chromosome and one Y chromosome. The X and Y chromosomes contain portions that do not match.
Chromosomes contain thousands of genes. Genes act as codes, or templates, that determine the structure of the body's essential building blocks: enzymes, proteins, hormones, etc.
An abnormality in any one chromosome, or a single gene, can result in disease. Tiny abnormalities in a single gene (or collection of genes) can cause an illness, or increase our risk for a certain disease. These abnormalities are referred to as mutations.
If the mutation is on the X-chromosome, it is an X-linked disorder. Most X-linked disorders only affect males, because the unmatched genes on the X-chromosome are expressed without competition from the Y-chromosome. A female can have an x-linked disorder only when both copies of the x chromosome contain a defective gene.
Each of the chromosomes in a pair, carry codes for the same body trait or protein. For example, each of the number 19 chromosomes has codes for eye color, among other things; however, only one of the genes from the pair will be used for eye color. The gene that is used is called the "dominant" gene. In reality, the actual expression, or utilization, of genes is much more complicated.
Genetic diseases are transmitted by one of two main patterns of inheritance:
- Autosomal dominant transmission: this means that it only takes one mutant gene from either parent to cause the disease.
- Autosomal recessive transmission: this means that it takes two mutant genes, one from each parent, to cause the disease.
Genetic diseases are either autosomal dominant or autosomal recessive---they cannot be both.
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