We’ve all discussed genetic conditions around the dinner table and heard things like, “that disease skips generations” or “we don’t have that condition in our family so it’s not coming from me”. While it can sometimes appear that way, these comments are based on common misconceptions about inheritance. Let me explain…
Let’s start by reviewing how we inherit our genetic information. Our genetic material comes in 23 pairs of chromosomes – half from our mom and half from our dad. Twenty-two of the chromosomes are numbered 1 to 22 (called autosomes) and the twenty-third pair determines sex– XX for females and XY for males. Within our chromosomes we have genes, which tell our body how to work and develop. Sometimes there are changes, also known as mutations, in the genes which cause them not to work properly.
Ok, now that we have that covered, let me give you a refresher on the three most common inheritance patterns and how they work:
In this case, the gene mutation is on one of a pair of chromosomes and an individual only needs one mutation to have symptoms of the condition. In other words, the effects of a dominant mutation are significant enough that they cover up the fact that one normally-working gene is present.
Autosomal dominant conditions occur equally in males and females. These are the conditions that you will find in multiple generations of a family since each person that has an autosomal dominant condition has a 50% chance with each pregnancy to pass the dominant gene mutation to their children.
Examples of common autosomal dominant conditions include Marfan syndrome, achondroplasia, and neurofibromatosis.
When a condition follows autosomal recessive inheritance, a gene mutation is required on both copies of a chromosome pair for an individual to have symptoms of the condition. In other words, having one normal copy of a gene is enough to prevent a person from having the condition.
Autosomal recessive conditions also occur equally in males and females. These are the conditions that aren’t seen in every generation. That is because when both parents are carriers of mutations in the same gene, they have a 25% chance with each pregnancy to have a child with the disorder.
Carriers of autosomal recessive conditions usually do not have symptoms.
Conditions due to mutations on the X chromosome follow X-linked inheritance. Most of these conditions are recessive, so females do not usually have symptoms. Males with a mutation on their X chromosome will have the condition since they only have one X. Having a normal Y chromosome does not prevent symptoms of an X-linked condition.
Males with X-linked conditions will never pass the condition to their sons (since they give their sons their Y chromosome) and all of their daughters will be carriers of the condition (since they give their daughters their X chromosome).
Now let’s go back – why does it sometimes look like a condition skips a generation? Here is one scenario that serves as a possible explanation:
X-linked recessive inheritance – a male with an X-linked condition will not have any affected sons or daughters. However, each of his daughters is a carrier of this X-linked condition. Therefore, these daughters will have a 50% chance that their sons will have the disorder (and once again, none of her daughters will have the condition but 50% will be carriers).
What about genetic conditions that suddenly appear in a family?
Autosomal recessive inheritance – in fact, it’s estimated that everyone carries approximately 3-5 autosomal recessive gene changes. However, people don’t usually know this unless they have children with someone who carries a mutation in the same gene or an individual has genetic carrier testing.
So, the next time someone tries to pass on a common misconception about the inheritance of genetic diseases, you will know the facts!
*How to Solve Pedigree Analysis Problems