Genetic inheritance or gene inheritance refers to how genetic information is passed from parents to their children, and this typically results in inheritable genetic variation.
It’s not just your eye color or hair color that can be passed down from your parents. Certain health conditions, diseases, and many characteristics or traits can also be passed down to you, genetically speaking.
Every human being has two copies of almost every gene, because one copy of each genome is from your biological mother, and one copy is from your biological father.
The combination of both genomes occurs when the egg and sperm, referred to as germ cells, fertilize in conception to create an embryo that will eventually develop into a human being.
The union of the two germ cells creates many unique, hereditary variations in individuals. The two unique germ cells match up and create thousands of different DNA sequences that comprise the genetic code of an offspring. These genes act as a blueprint, with parents passing on physical traits, characteristics, talents, strengths, weaknesses, and even health conditions to their children through what’s known as genetic inheritance.
Gene Inheritance from Parents
Inheritable genetic variations coming from the parents’ genomes explain why some siblings have certain similarities, such as a pointy nose or deep-set eyes, as well as many differences. These genes are responsible for the different traits siblings have from one another such as the youngest child having dimples, or the oldest sibling having a cleft-chin.
Blood types are also excellent genetic inheritance examples since siblings often have varying blood types even if they have the same set of parents. For example, if a woman with heterozygous blood type A reproduced with her husband who has heterozygous blood type B, they could have children with a blood type of A, B, AB, or O.
Moreover, gene inheritance also plays a key role in your family’s health history, which is why some diseases like certain types of cancer, cardiovascular disease, or diabetes, can run in families. Genetic inheritance from parents to offspring is a complex, fascinating topic and researchers today continue to explore the mysteries behind the human genome and genetic inheritance.
How Genetic Inheritance Works
Research affirms that your genes play a part in what makes you truly unique. You can think of genes as instructions inside all your cells. The body is composed of billions of cells working together. Inside each cell is your unique DNA sequence or genetic code which determines things like what you look like, what you’re gifted at, and how well your body works. This genetic code tells your cells how to develop, replicate, grow, and function. Genes are arranged in complex structures called chromosomes found in all the cells. Each individual has 23 pairs of chromosomes, coming from the sperm and egg. In other words, one pair of your chromosomes is from your mother, and the other pair is from your father.
Every individual person on this planet, even if they are siblings who have the same mother and father, will always have a different set of genetic instructions. Gene inheritance (the passing of genetic materials from the parents to a child) plays a huge part in inheritable genetic variations. When you examine the physical attributes of the same family, it is evident that some traits of the offspring are either from the mother or father. At the same time, there are also many differences between siblings.
Some genetic inheritance examples are that some members of the same family (for example the grandparent and the grandchild, or the mother and daughter) share the same eye color, hair color, height, curly hair, or even the same nose shape. Certain non-physical attributes such as a natural talent for music composition often appear to be rooted in family history, and evidence suggests that creativity and musical talents are traits with heavy genetic influences.
However, there are also numerous differences between family members from physical traits to types of personalities. Notably, studies indicate that even identical twins that come from the same egg and sperm could have slight differences in their individual DNA. Although identical twins look alike and share the majority of their genetic code, they also have inheritable genetic variations and don’t share 100% of their DNA.
Furthermore, genetic mutations could also be passed from the parent to the offspring, which is why certain diseases such as heart disease, osteoporosis, and certain types of cancer can be strongly influenced by your genetics.
Dominant vs Recessive Genes
When you hear the terms ‘dominant gene’ and ‘recessive gene’, this generally refers to how likely it is for a genetic inheritance to occur. Recessive genes are less likely to be expressed or manifest as physical traits, but it can happen if an offspring receives two copies of a recessive gene. This happens when both of their parents are recessive carriers of the same gene.
Remember that genes are contained in your chromosomes. Your cells carry two copies of each chromosome, and they have two versions of each gene. The different versions of a gene are called alleles, and alleles can be either dominant or recessive.
Human beings receive two versions of each gene (known as alleles) from each of their parents. If the alleles of a gene are different, one allele will be expressed or ‘observable’. This is often the ‘dominant’ gene that is being expressed.
For a recessive allele to be expressed, the offspring would have received two copies of the recessive allele, one from each parent.
For example, you likely remember the dominant/recessive eye color model from high school biology classes. This is one of the most common genetic inheritance examples taught in school.
If both of your parents have brown eyes and you have blue eyes, it’s likely that both of your brown-eyed parents were recessive gene carriers for blue eyes.
In other words, recessive alleles are only expressed if two copies are present, and this means their genotype is homozygous recessive. This term, homozygous recessive, refers to the offspring receiving two copies of the recessive allele, one from each parent.
Genetic mutations that cause diseases can be passed down from parents to offspring, due to mutated alleles. If the allele is recessive, it’s more likely to cause disease in those who are homozygous for that mutated gene or compound heterozygous i.e., carrying two different genetic mutations in the same gene causing the offspring to have two copies of it. For example, every person with cystic fibrosis is homozygous or compound heterozygous for a genetic mutation.
Cystic fibrosis is a recessive disease, which is why a person with cystic fibrosis must have received two copies of the genetic mutation. This happens when both parents are ‘CF carriers’, meaning they are both carriers of the same genetic mutation that causes the disease.
Parents of a child with cystic fibrosis may not have been aware that they were CF carriers. When it comes to family planning, it’s important to find out (through a DNA test) if you and your partner are both carriers of the same disease-causing mutation. If you’re both CF carriers, for example, there is a 25% chance your child will be affected with the disease
Inheritable Genetic Variation from Parents
Your unique genetic makeup is called a genotype. When it comes to genetic inheritance, this genotype could refer to your entire genome or just an individual gene and its alleles (a variant). Your given genotype influences your phenotype, which is an expressed physical trait.
For instance, your eye phenotype could refer to any of the most popular eye colors such as blue, green, or brown. The genotype for eye color coming from the parents could create different variations of eye color in their children. The allele for brown eyes is dominant, that’s why the majority of people in the world have dark eyes, and the allele for blue eyes is recessive. To illustrate, take note of the following gene inheritance probabilities for an offspring to come out with blue eyes:
- Parent A with blue eyes and Parent B with blue eyes will have a 100% chance of having an offspring with blue eyes.
- Parent A with brown eyes and Parent B with blue eyes result in a 50% chance of an offspring with blue eyes if the brown-eyed parent is a recessive blue gene carrier. If not a carrier, the chance is 0% blue eyes.
- Parent A with brown eyes and Parent B with brown eyes have a 25% chance of having a blue-eyed child if both parents carry the recessive blue allele and the baby inherits both copies. Otherwise, the chance is 0%.
The above genetic inheritance examples are simplified explanations of how certain eye colors could develop in babies and be passed on from generation to generation. There are other permutations in gene inheritance based on the genes, and the whole process is a lot more complex, especially since there are other eye colors like hazel, violet, or gray.
Dissecting Genetic Inheritance Patterns
As noted above, genetic inheritance plays a huge part in genetic disorders and disease risks because these come about due to genetic mutations. Remember, each person has 23 pairs of chromosomes from the mother and father. 22 of these pairs of chromosomes are called autosomes, and one pair of chromosomes include the sex chromosome that determines your gender or sex.
The genetic mutations that could manifest as disease could be in the autosome, sex chromosome, or mitochondria (energy-giving part of cells which also contain genetic codes). You could potentially inherit genetic conditions or disorders and pass them to your children based on where the faulty or mutated gene is located. A genetic health disorder is typically determined by the following genetic inheritance patterns:
Autosomal Dominant
In autosomal dominant inheritance, one copy of the mutated gene could cause the disease. The gene mutation could come from either parent. The parent with the mutated gene has a 50% chance of passing the disease to an offspring and a 50% chance of having a normal child. The mutation is usually present in every generation such as what is seen in Huntington’s disease, neurofibromatosis, and polycystic kidney disease.
Autosomal Recessive
In this genetic inheritance pattern, two copies of the mutated gene from each parent will result in the disease manifesting in the child, like Sickle Cell Anemia or Tay-Sachs disease. The parents typically don’t have any symptoms, and they only become aware of being carriers when their sick child undergoes genetic testing.
If both parents are recessive gene carriers, there’s a 25% chance that an offspring will carry the disease, a 25% chance a child is totally unaffected, and a 50% chance the child will be an asymptomatic carrier that could pass the disease to future offspring. Recessive inheritance typically skips generations.
However, if one parent has the disease and the other is not a carrier, all children will only be carriers. If one parent has the disease and one parent is a carrier, there’s a 50% chance that the child is a carrier and a 50% chance that the child will acquire the disease.
Mitochondrial
The mitochondria in every cell of the body come with their own genetic codes. Mutations in the mitochondria could only be passed through maternal gene inheritance. The reason for this is the egg cells are the only ones that contribute to mitochondrial development. Both male and female offspring could be affected by this when the mother is a carrier or afflicted with a genetic condition, such as Leigh Syndrome or progressive loss of neurocognitive abilities and movement. These mitochondrial mutations that result in disease are very rare, occurring in fewer than 1 in 1,000 individuals.
X-Recessive
These disorders like Fabry syndrome, color blindness, and hemophilia are caused by recessive gene mutations on the X chromosome. Males only have one X chromosome and one Y chromosome. Females have two X chromosomes. In males, one copy of the altered gene on the X chromosome is enough to cause the disorder. For females, the gene mutation would have to occur in both X chromosomes to manifest. X-linked recessive genetic inheritance is highly unlikely to occur in females and more likely in males.
Notably, there is also no male-to-male gene inheritance because the X chromosome in males only comes from the mother. If the father is healthy and the mother is a carrier, each son has a 50% chance of acquiring the disease and each daughter has a 50% chance of being a carrier. However, if the father is affected by the disease and the mother is healthy, none of the sons will get the disease but all the daughters will become carriers.
X-Linked Dominant
This is less common than the recessive variant, but it could affect both males and females since only one gene copy on the X chromosome is needed for the disorder to manifest. Notably, females who get the disease are less severely affected than males. Moreover, affected males can only transmit the condition to the daughters and never the sons, while affected family members can pass the condition to sons and daughters. Examples of X-linked dominant disorders are vitamin D-resistant rickets and ornithine transcarbamylase deficiency.
Are You a Carrier of a Genetic Mutation? Make Educated Choices with the Right Knowledge
If you are contemplating having children with your partner, it is important to discuss your family medical history. Talking about potential genetic disorders that could develop in your future offspring must be a part of your family planning discussion. Should you discover that you have debilitating genetic anomalies such as Huntington’s disease or cystic fibrosis, you could undergo embryonic screening and IVF to protect your future children from genetic mutations, and safeguard your family’s overall health from physical to financial aspects, since illness also impacts finances.
If you’re serious about family planning with your partner, you could benefit from both taking a CircleDNA test. If you both take this comprehensive DNA test, you can find out if you and your partner are carriers of certain genetic mutations, and you can find out if you’re both carriers of the same mutation. This insightful at-home DNA testing kit provides hundreds of health reports based on your DNA, as well as information about other genetic traits you might have. If you find out about any genetic disease risks, you can include this information in your family planning discussion. Additionally, you could also share gene inheritance information with other biological family members of yours, so they become aware of any genetic risks of developing certain health conditions, and they can take preventative measures.
Resources:
- Understanding Genetics (Appendix E: Inheritance Patterns) https://www.ncbi.nlm.nih.gov/books/NBK115561/
- Molecular genetic analysis of ABO blood group variations reveals 29 novel ABO subgroup alleles (Xiahong Cai et.al.) https://pubmed.ncbi.nlm.nih.gov/23521133/
- A Guide to Genetics and Health (Genetic Alliance) https://www.ncbi.nlm.nih.gov/books/NBK115604/
- Genes, brain dynamics and art: the genetic underpinnings of creativity in dancing, musicality and visual arts (Marinos Sotiropoulos and Maria Anagnostouli) https://pubmed.ncbi.nlm.nih.gov/34997732/
- Many Identical Twins Actually Have Slightly Different DNA (Smithsonian Magazine) https://www.smithsonianmag.com/smart-news/identical-twins-can-have-slightly-different-dna-180976736/
- What are the odds my baby will have blue eyes based on genetics? (Family Education) https://www.familyeducation.com/what-are-the-chances-my-baby-will-have-blue-eyes-a-genetic-explanation
- Mitochondrial DNA Common Mutations Syndromes (CHOP) https://www.chop.edu/conditions-diseases/mitochondrial-dna-common-mutation-syndromes
- What Does It Mean to Be Homozygous? https://www.healthline.com/health/homozygous#examples
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