Genetic Disorders

Genetic information is packaged into chromosomes that are found in thecells nucleus, or DNA containing organelle. Virtually every human cell has 46 chromosomes, except for the sperm and egg (reproductive cells) which each have 23 chromosomes. Fertilization of the egg by the sperm results in a newly formed cell called the zygote. Each zygote receives 23 chromosomes from the egg and 23 chromosomes from the sperm. All but one of the 23 chromosomes are called autosomes. The remaining chromosome is the sex chromosome and it is either an X or a Y. After these sex cells unite, gender is determined. Females have two X's (XX), and males have one of each (XY). Females can only pass an X to their offspring, and males can pass either an X or a Y. Therefore, the male sperm is responsible for gender selection.

The duplicated set of 22 autosomes, numbered 1 through 22, are called homologous pairs in that there are two chromosome number 1 with similar genes and genetic material. There are approximately 50,000 genes on all the chromosomes. Individual genes are made of deoxyribonucleic acid (DNA), which makes up the genetic code or alphabet that produces specific proteins. Proteins can play important structural and functional roles in the body. Each gene has a set locus, or position, on a particular chromosome. Identical genes that are located on the same locus on corresponding chromosomes are called alleles.

A persons genotype represents the genes that they inherited. If is an autosomally inherited disease, the genotypes are written as a lower or an upper case letter A (such as AA, aa or Aa to represent both alleles known as the genotype), where capital letters define dominant genes and lower case letters define recessive genes. Genotypes are either homozygous or heterozygous. Having two identical alleles, such as AA or aa, makes the genotype homozygous for that locus. Having different alleles (for example, Aa) at a locus represents a heterozygous genotype. The observable features that characterize an individual are collectively called the phenotype. A phenotype can also extend to observable characteristics that can only be visualized with the help of a microscope or other equipment.