Understanding Non-Mendelian Genetics (Patterns of Inheritance)

Health Line

a day ago

In Mendelian inheritance patterns, you receive one version of a gene, called an allele, from each parent. These alleles can be dominant or recessive. Non-Mendelian genetics don't completely follow these principles.
Genetics is an expansive field that focuses on the study of genes. Scientists who specialize in genetics are called geneticists. Geneticists study many different topics, including:
how genes are inherited from our parents
how DNA and genes vary between different people and populations
how genes interact with factors both inside and outside of the body
If you're looking into more information on genetics topics, you may come across two types of genetics: Mendelian and non-Mendelian genetics.
This article reviews both types of genetics, with a focus on non-Mendelian genetics. Continue reading to learn more.
What is Mendelian genetics?
It's possible that you may remember some concepts of Mendelian genetics from your high school biology class. If you've ever done a Punnett square, you've learned about Mendelian genetics.
The principles of Mendelian genetics were established by the Austrian monk Gregor Mendel in the mid-19th century based on his experiments with pea plants.
Through his experiments, Mendel pinpointed how certain traits (such as pea color) are passed down across generations. From this information, he developed the following three laws, which are the basis of Mendelian genetics:
Dominance. Some variants of a gene, called alleles, are dominant over others. Non-dominant alleles are referred to as recessive. If both a dominant and recessive allele are inherited, the dominant trait will be the one that shows.
Segregation. Offspring inherit one allele for a gene from each of their parents. These alleles are passed down randomly.
Independent assortment. Genetic traits are inherited independently of each other.
Pea color: An example of Mendelian genetics at work
To illustrate how Mendelian genetics works, let's use an example with pea plants, in which yellow pea color (Y) is dominant and green pea color (y) is recessive.
In this particular example, each parent pea plant is heterozygous, meaning it has a dominant and recessive allele, noted as Yy.
When these two plants are bred, noted as Yy x Yy, the following pattern of inheritance will be seen:
25% of offspring will be homozygous dominant (YY) and have yellow peas.
50% of offspring will be heterozygous (Yy) and have yellow peas.
25% of offspring will be homozygous recessive (yy) and have green peas.
What are examples of health conditions that follow Mendelian patterns of inheritance?
There are several health conditions that follow Mendelian patterns of inheritance.
Alleles for sickle cell anemia and cystic fibrosis are recessive. This means that you need two copies of the recessive allele, one from each parent, to have these conditions.
In contrast, the allele for Huntington's disease is dominant. That means that you only need a single copy of the allele (from one of your parents) to have it.
Sex-linked conditions
Some health conditions can be linked to genes in the sex chromosomes (X and Y). For example, hemophilia is X-linked recessive.
In those assigned male at birth, who have a single X chromosome, only one copy of the recessive allele is enough to have hemophilia. That's why hemophilia is more common in males.
Individuals assigned female at birth have two X chromosomes, meaning they need two copies of the recessive allele to have hemophilia.
What are non-Mendelian genetics?
Exceptions exist for every rule, and that's also true for genetics. Simply put, non-Mendelian genetics refers to inheritance patterns that don't follow Mendel's laws.
Here are some different types of non-Mendelian genetics:
Polygenic traits
Some traits are determined by two or more genes instead of just one. These are called polygenic traits and don't follow Mendelian inheritance patterns.
Examples of polygenic health conditions include:
hypertension
diabetes
certain cancers, such as breast and prostate cancer
Mitochondrial inheritance
Your mitochondria are the energy factories of your cells and also contain their own DNA, called mtDNA. While there are some exceptions, mtDNA is usually inherited from your mother.
You get your mtDNA from your mother because the mitochondria present in sperm typically degrade after fertilization. This leaves behind just the mitochondria in the egg.
Examples of Mitochondrial health conditions include Leber hereditary optic neuropathy (LHON) and mitochondrial encephalomyopathy.
Epigenetic inheritance
Epigenetics refers to how genes are expressed and regulated by factors outside of the DNA sequence. This includes things like DNA methylation, in which a chemical called a methyl group is added to a gene, turning it 'on' or 'off'.
Epigenetic factors can change as we get older and are exposed to different things in our environment. Sometimes, these changes can be passed down to the next generation, which is called epigenetic inheritance.
Certain cancers (such as breast, colorectal, and esophageal cancer) have been linked to epigenetic changes. Neurological disorders like Alzheimer's and metabolic diseases like Type 2 diabetes have also been associated with epigenetic inheritance.
Genetic imprinting
While we inherit two copies of a gene, one from each parent, in some cases, only one copy of the gene may be turned 'on' via DNA methylation. This is called imprinting, and it only affects a small percentage of our genes.
Which gene is turned 'on' can depend on where the gene came from. For example, some genes are only turned 'on' when they come from the egg, while others are only 'on' when they come from the sperm.
Examples of conditions associated with genetic imprinting include Beckwith-Wiedemann syndrome, Silver-Russell syndrome, and Transient Neonatal Diabetes Mellitus.
Gene conversion
Gene conversion can happen during meiosis, the type of cell division that helps make sperm and eggs. After meiosis, each sperm and egg contains one set of chromosomes and therefore one set of alleles to be passed down to offspring.
During meiosis, genetic information from one copy of an allele (the donor) may be transferred to the corresponding allele (the recipient). This results in a genetic change that effectively converts the recipient allele to the donor allele.
Genetic conditions influenced by gene conversions include hemophilia A, sickle cell disease, and congenital adrenal hyperplasia.
What are examples of health conditions that follow non-Mendelian patterns of inheritance?
Most health conditions we're familiar with don't follow Mendelian inheritance patterns. These conditions are often polygenic, meaning the effects of multiple genes contribute to them.
For example, cystic fibrosis is caused by inheriting two copies of a recessive allele of a specific gene. However, there's not an isolated 'heart disease' allele that we inherit that causes us to develop heart disease.
Mitochondrial disorders, which are caused by changes in mtDNA, are another type of health condition that follows non-Mendelian patterns of inheritance. This is because you typically inherit mtDNA from your mother.
Sometimes problems with genetic imprinting can lead to disorders. Prader-Willi syndrome and Beckwith-Wiedemann syndrome are two examples.
How do Mendelian and non-Mendelian genetics contribute to our understanding of genetic diseases in humans?
Understanding both Mendelian and non-Mendelian inheritance patterns is important in understanding how different genetic diseases may be passed down.
For example, if you have a certain genetic disease or you know that one runs in your family, you may have concerns about future children inheriting it.
In this situation, working with a medical professional, such as a genetic counselor, who understands a disease's inheritance patterns can help you get an understanding of the risk of future children having the disease.
Additionally, understanding genetic changes and inheritance can affect future therapies. This information can be important for developing gene therapies for a variety of genetic diseases.
Takeaway
Mendelian genetics focuses on the principles that there are dominant and recessive alleles and that we randomly inherit one copy of an allele from each parent.
Some health conditions follow basic Mendelian inheritance patterns. Examples include cystic fibrosis and Huntington's disease.
Non-Mendelian genetics don't follow the principles outlined by Mendel.
Many health conditions we're familiar with don't follow Mendelian inheritance patterns because they're polygenic, affect mtDNA, or are associated with imprinting.