How do we inherit our genes?

We inherit a lot from our parents. We inherit their money, hopefully, their mannerisms, love, and in some families, their genes. In this blog post, we are going to discuss the ways genetic conditions can be passed on or inherited in a family by covering autosomal recessive, autosomal dominant, and x-linked inheritance patterns.

What are genes?

Genes are the instruction manuals to our bodies, telling us how to grow and develop. Our genes instruct our bodies to grow hearts, eyes, determine our height, hair and eye color. So, we need our genes right at the moment we are conceived. We inherit our genes from the sperm that fertilizes the egg we came from. The sperm provides half of our genes while the egg provides the other half. For every gene we have, we have two copies of it. One copy of each gene comes from our mom or egg donor and the other from our dad or sperm donor. When we go on to have children, we pass on one copy of each of our genes or half of our genetics.

There can be changes to our genes that cause them to not be able to carry out their intended function. These changes are called pathogenic variants. Most genes we inherit from our parents do not have pathogenic variants, but every once in awhile, we do inherit pathogenic variants which can cause a genetic condition. A genetic condition is caused either by both gene copies having a pathogenic variant or just one gene copy. There are three main patterns of inheritance: autosomal recessive, autosomal dominant, and x-linked.

What is Autosomal Recessive Inheritance?

Autosomal recessive inheritance pattern is the most discussed type of inheritance, particularly in reproduction. The reason this is most often discussed is because people typically only find out about being a carrier of an autosomal recessive condition after having carrier screening performed. Conditions with autosomal recessive inheritance patterns include spinal muscular atrophy, cystic fibrosis, sickle cell disease, and many others.

When someone has an autosomal recessive condition, their parents usually are carriers of that condition. A carrier has one gene copy with the pathogenic variant and the other gene copy is “normal”. Given the person has the other gene copy, their bodies are able to carry out the intended function of that particular gene. A carrier usually doesn’t have a family history of the condition nor do they typically have any signs or symptoms of the condition.

If a couple are both carriers of the same condition, then the chance they both pass on the gene copy with the pathogenic variant is 25%. They would have a 25% each time they became pregnant to have a child with that genetic condition. They have a 75% chance to not have a child with that condition.

If a couple are identified as carriers, then they can consider alternative pregnancy options. For some individuals, they plan to test any future pregnancies for the condition. Other couples may choose to use assisted reproductive technologies. They may select a sperm or egg donor, who is not a carrier of the condition, or they may choose to test the embryos for the condition with preimplantation genetic testing (PGT). Other times, people wait until their baby is born to consider testing at that time.

Autosomal recessive inheritance pattern and possible chance to have offspring that is doesn’t have the condition, are also carriers, or has the condition

What is Autosomal Dominant Inheritance?

Autosomal dominant inheritance can be seen in multiple generations because as opposed to autosomal recessive inheritance, only one gene copy has to have the pathogenic variant in order to cause the condition. Despite having the other working gene copy (without the pathogenic variant), it is not enough to make the designated protein or to keep the condition from occurring. Each time someone with an autosomal dominant condition goes to have a child, there is a 50% chance the child will also have the condition. Sometimes, autosomal dominant conditions are not inherited. A NEW pathogenic variant occurs at the time the person was conceived. In these cases, the person with the autosomal dominant condition is the only one in the family, but they still have the 50% chance to pass the condition on to future kiddos. Some examples of autosomal dominant conditions include Marfan syndrome, Neurofibromatosis, and people with BRCA1 variants.

Someone with an autosomal dominant condition can also consider the above reproductive options as a couple who are considered carriers.

Autosomal dominant inheritance pattern with the chance that offspring would inherit the condition

What is X-Linked Inheritance?

The two inheritance patterns above have the word "autosomal" in them because the genes that cause those conditions are on the autosomal chromosomes. These are the first 22 pairs of our chromosomes. Our sex chromosomes, the last pair, are either two X chromosomes or an X and Y chromosome. The reason the last pair are referred to as the sex chromosomes is because the genes on these chromosomes, in general, tell the developing body to have male or female genitalia.

Some genes are found only on the X chromosomes, and they can also have pathogenic variants. If inherited, then this would be called X-linked Inheritance. Most of the time, a genetic female is a carrier of X-linked conditions because females have two X chromosomes. One gene copy has a pathogenic variant while the other gene copy does not have a variant. The genetic female usually is not expected to have the condition, but they are able to pass it on to their children. However, there are times that a female DOES have symptoms related to the condition and will need evaluations and monitoring.

For genetic males, they have only one X chromosome and a Y chromosome. Not all of the genes on the X and Y are the same. If a genetic male’s gene copy on their only X chromosome does not work, then they have the condition because they have no other gene copy.

X-linked conditions are also on the carrier screening. Both genetic males and females can be assessed for the conditions, yet usually only genetic females are evaluated for the condition. Some examples of x-linked conditions include Fragile X and Duchenne Muscular Dystrophy.

X-linked inheritance pattern and the chance that offspring could inherit the variant as well as the genetic sex of the offspring

There are other patterns of inheritance and considerations to make when it comes to how we inherit our genes, but the above are the most common inheritance patterns discussed for genetic conditions.

* This blog constitutes general information about genetic testing and medical screening. This blog does not offer or provide medical advice or diagnosis, and nothing in this blog should be construed as medical advice or diagnosis. Do not rely on the information in this blog/article to make medical management decisions. Please consult with a medical professional before making those decisions. Do not delay in seeking professional medical advice if you think you have a medical concern. Do not disregard professional medical advice based on any information received in this blog.