Heredity vs Non-Heredity: Understanding the Difference

Heredity is the study of measurable genetic changes that can be passed on to offspring/progeny.

Is everything ‘genetic’ hereditary?

No. Not everything that impacts our genes is hereditary. Only gene mutations within our germline that we are born with are inherited. Our ‘genetic code’ within our germline is present in all our cell types including our reproductive cells (egg or sperm), thus they can be passed on to offspring.

If you have a pathogenic (disease-causing) germline mutation for a hereditary thrombocytopenia syndrome for instance, you did not cause it and could not have prevented it. Not even with diet and supplemental changes. 

Changes to our DNA that are acquired are due to non-inherited influences, such as aging and even outside influences such as prolonged exposure to damaging UV radiation for instance. Acquired changes in our DNA usually only impact certain cells in our body and we refer to these as somatic gene variants. These types of variants cause most non-inherited cancers. They cannot be passed on to offspring because they are not present in the egg or sperm cells and were not inherited from a parent.

Genetic testing for somatic mutations is often done to determine the best course of treatment for targeted therapy in some conditions, such as cancer. Somatic mutations are also important in the diversity of the antibodies, T cell receptors, and B cell receptors. Somatic testing is not the same as germline testing for inherited (not acquired) germline mutations that can cause disease. It is important to be aware of the differences between germline and somatic mutations.

The environment can also impact genes by affecting gene expression. This is known as epigenetics. Epigenetic changes affect the way genes are expressed but do not alter the genetic code itself. Epigenetics is often used to study variability in disorders. Researchers are now looking at whether outside influences, such as nutrition, affect gene expression and regulation.

Testing for epigenetic signatures can be done through specialized testing centers but is not available at this time for every disorder. These changes are not inherited and not usually passed on.

Basic Modes of Inheritance

How are gene variants inherited?

Dominant inheritance – If the mutation identified is ‘dominant’ that means only one allele (one copy of a particular gene, which come in pairs; one from each parent) needs to carry a pathogenic variant in order for the disease to manifest. Dominant variants are primarily inherited from one parent, but some can be de novo, meaning they can happen by chance. There is a 1 in 2 risk (or 50% chance) to pass on dominant variants to offspring.

Heredity Figure 1

Recessive inheritance
– If the genetic disorder is ‘recessive’ that means in order for the disease to manifest both copies of the gene (or allele) from each parent need to have a pathogenic variant for the same condition. If only one copy of the gene from one parent has a variant, then that person is referred to as a carrier, and is not usually affected by the disease. If both biological parents are carriers for the same recessive condition, there is a 25% risk (1 in 4 chance) to have a child affected with that particular condition.

Heredity Figure 2

Sex-linked inheritance – If the variant identified is ‘sex-linked’ that means the variant is on the X or Y chromosome, usually the X chromosome (also called x-linked). Sex-linked conditions can be inherited in either a dominant or recessive fashion.

Heredity Figure 3

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