Genes are very small structures inside almost every cell of the body. Genes are the instructions, or blueprints, that tell our body how to grow and develop, build necessary proteins, and, thus, determine an individual’s characteristics, such as eye color and blood type. It is estimated there are 30,000-40,000 genes, each of which is an instruction guiding the cells of the body to grow and survive. Genes come in pairs and are made of strands of genetic material called deoxyribonucleic acid, or DNA. Genes line up similar to beads on a string to form larger structures called chromosomes. Genetic disorders are caused by change(s) in the instruction code of a particular gene or genes, preventing the gene(s) from performing their proper function.
Just as genes come in pairs, chromosomes also come in pairs. Each cell in our body has 23 pairs of chromosomes (for a total of 46); one member of each pair is inherited from the mother and the other from the father. The first 22 pairs (numbered 1 though 22) are called autosomes and they determine most of our features. The last pair are the sex chromosomes and they determine if we are male or female. Females have two X chromosomes and males have one X chromosome and one Y chromosome.
Some disorders are determined by changes in more than one gene. These disorders, known as complex disorders, do not follow a predicted pattern of inheritance as is seen in other rarer genetic disorders caused by a change (mutation) in only one gene, such as cystic fibrosis, sickle cell anemia, or hemophilia. Sometimes changes in the
genes that contribute to complex disorders must be in combination with certain environmental factors, such as exposure to certain chemicals, medications, or maybe even diet. This type of inheritance is often referred to as multifactorial, or “complex,” because many different factors, genetic and/or environmental, are involved. A person will have a complex disorder if he or she has the right combination of changed genes and environmental exposures. Sometimes the genes that contribute to complex disorders are called susceptibility genes because they make a person susceptible to developing the disorder after exposure to specific environmental factors, but they do not cause the disorder alone. The close relatives of someone with a complex disorder have a higher chance of developing the disorder than the close relatives of someone who does not have the disorder. Diabetes, heart disease, autism, Alzheimer disease, neural tube defects, Parkinson disease, and many cancer syndromes are examples of disorders that can be caused by multifactorial or complex inheritance.
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