The ABC's of Genetics & Children Who Are Deaf and Hard of Hearing
"How can a condition be genetic yet never seen before in a family?"
Nearly all of our genetic information comes in pairs packaged in GENES. Genes are made of DNA and determine our physical features and how our bodies' function. Genes are coiled into CHROMOSOMES. Humans typically have 46 chromosomes. Twenty-three chromosomes come from the mother, and 23 from the father. Females have two X chromosomes, and males have an X and a Y chromosome.
Some conditions require two non-working genes for the problem to occur (autosomal recessive). If a child receives two non-working genes, one from each parent, he or she will show the condition. As long as one gene of the pair is functioning, the parent has no idea that the non-working gene is present. The child's condition is "genetic" even though it has never been seen in the family.
The most common cause of severe to profound congenital deafness occurs when a child inherits two copies of an altered gene known as GJB2 which makes a protein called Connexin 26. This form of hearing loss is sometimes called DFNB1. Pendred syndrome and Usher syndrome are also inherited in this way.
Some conditions happen when only one gene is not working properly (autosomal dominant). Many times these conditions are passed from one parent to a child. However, on occasion these conditions can happen for the first time in a family. Our genetic material changes as cells grow and divide. Normally these changes called mutations do not cause any problems. However, if a change occurs in an egg or sperm, the change will be copied into all the cells of the baby's body causing a "genetic" condition that was not present in the family. Branchiootorenal (BOR) syndrome, Waardenburg syndrome and Stickler syndrome are examples of conditions inherited in a dominant manner.
Some conditions result when there is a change in a gene located on the X chromosome (X-linked recessive). Men and boys generally show the condition because they have only one X chromosome. Women and girls generally do not show an X-linked condition because their paired gene on the other X chromosome is able to compensate. A change (mutation) can occur in an egg to cause a condition. A non-working gene can be passed silently through several generations of women before it appears in a baby boy. There are only a handful of genes known to cause isolated hearing loss in this way. The vast majority of boys born with isolated hearing loss do not have an X-linked form.
There are other ways that genetic conditions can occur in families for the first time that are too detailed for the purposes of this article. Genetic research continues to discover more ways that genes interact to cause conditions for the first time in families.
If this article raises questions, feel free to contact Boys Town National Research Hospital’s Certified Genetic Counselor, Kristal Platt, at (531) 355-6365 or