Q: What are genes and genetic disorders and how are genetic disorders inherited?

Genes are basic units of hereditary information that code for all of the body’s traits and functions. A change, known as a pathogenic variant, in a gene can cause that gene not to function properly. This can lead to disease. Genes are carried on larger structures called chromosomes. Most individuals have 46 chromosomes or 23 pairs (one chromosome from each pair is inherited from a person’s mother and the other from the person’s father). These pairs are numbered 1 through 22 and the 23rd pair (called the sex chromosomes) determines whether a person is male or female. Typically, a male has one X and one Y chromosome, and a female has two X chromosomes. When a male child is conceived he received the Y chromosome from his father and one of his mother’s X chromosomes (at random). When a female child is conceived she received the X chromosome from her father and one of her mother’s X chromosomes (at random). There are multiple ways by which a genetic disease can be inherited. Autosomal recessive conditions, like Cystic Fibrosis, occur when pathogenic variants are present in both the maternal and paternal copies of a gene. Each parent of an affected individual has one copy of a gene that works properly and one copy that does not. The parents are generally unaffected and are considered “carriers” of the disease. X-linked conditions, like Fragile X Syndrome, occur when there is a pathogenic variant in a gene on the X chromosome. “Carrier” females have a working copy of a gene on one X chromosome and a copy with a pathogenic variant on the other X chromosome. If the X chromosome with the pathogenic variant is passed on by the female, along with a Y chromosome from the male, the resulting son will have the X-linked condition. Males with an X-linked condition are generally affected and females are generally unaffected carriers (carrier females may be affected, to a variable degree, based on disease or pathogenic variant).
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