Mostly I am concerned about the mitochondria abnormality found in Drew. I have been doing a lot of research and believe that this is an X-Linked abnormality. I have learned the following:
X-linked inheritance refers to conditions or traits for which the gene is located on the X chromosome. Recall that females have two X chromosomes while males have only one X chromosome.
The majority of X-linked conditions are X-linked recessive, meaning that one normal (working) copy of the gene would compensate for a non-working copy. It is much more common for males to have X-linked recessive conditions than females, since males do not have a second copy of their X chromosome to compensate if their one copy has a mutation. There are some cases when females can be affected with an X-linked recessive condition, but this is much less common.
X-linked dominant inheritance is quite rare.
Females will only be affected with an X-linked recessive condition if both copies of the gene on their X chromosome have mutations (this is quite rare). If a female has a mutation in only one copy of the gene on their X chromosome, they will not have the condition. Males, on the other hand, will have the condition when the gene on their one X chromosome has a mutation, since they do not have a second copy to compensate
When a female is a carrier of an X-linked recessive condition (i.e. has a mutation in one copy of an X-linked gene), each of her offspring will have a 50% (1 in 2) chance of inheriting the working copy, and a 50% (1in 2) chance of inheriting the non-working copy. Thus, daughters of a female carrier have a 50% chance of being a carrier and a 50% chance of being a non-carrier, and sons have a 50% chance of being affected and a 50% chance of not being affected.
When a male has an X-linked condition, all of his daughters will be carriers (since all female offspring receive an X from their father), and none of his sons will be affected (since he will pass on his Y chromosome to all of his sons).
This information leaves me worried for my sister and for my daughter. My prayer is that my Mom passed this mutation only to me and I pray that I did not pass this mutation to my daughter. It is passed on to females 50% of the time, so I am praying that it was not passed to them. If it was passed to them, when they have children they will have a 50% chance of passing the mutation to thier daughter, and worse, they will have a 50% chance of having a son who is deaf.
After learning that this abnormality runs in my family, I have thanked God a lot today that my brother was not born deaf. He was very lucky to be born in the group of 50% of males that are not affected with the condition. It is still unbelievable to me that this runs in the family and yet we have no knowledge of anyone born with infant hearing loss.