Ever wondered why you look somewhat like your mom or dad or siblings? Or why do some people have blue eyes while others have brown? The answers are hidden in your genes, which are like tiny instruction manuals for your body’s looks. Genes help decide the color of your hair, eyes, your height, and many more aspects of your looks. We call these characteristics ‘traits.’
Learning how genes decide our physical traits matters a lot. It helps us understand more about our past, our health, and our uniqueness. Moreover, it helps us appreciate how diverse life is and the common traits we share with other living beings.
In this article, we’ll break down some genetic basics and how they shape our physical traits. So, let’s get started!
Basics of Genetics
Genetics is the field of science that explores genes and how traits are passed from parents to children through changes in the DNA sequence. The human genome is the full set of genetic instructions in a person’s DNA. It includes all the genes and other elements that control the activity of those genes. Scientists estimate that humans have about 20,000 genes.
The genome of an organism is found in nearly all of its cells. The genome is stored in a structure called the nucleus in humans, plants, and animals. The human genome is mostly the same in all people, with just a few small differences.
Here are the simplified definitions of the basic terms used throughout genetics.
DNA is a molecule carrying instructions to build other molecules that help our body function. DNA is shaped like a twisted ladder, called a double helix.
The ladder’s sides are backbones, and the steps are pairs of four building blocks (adenine, thymine, guanine, and cytosine) called bases. The sequences of these bases act as the instructions to build molecules, mostly proteins.
A gene is a part of the DNA with instructions to build a particular protein. Proteins are molecules that perform various tasks in our body, such as starting chemical reactions, moving stuff around, fighting diseases, and controlling cell growth.
Different proteins affect different traits. For example, the protein called melanin decides the color of our skin, hair, and eyes.
A chromosome is a structure where proteins pack up the DNA tightly so it fits into a small space in a cell called the nucleus, where all genetic information of a cell is stored. Humans usually have 23 pairs of chromosomes in our cells.
The two chromosomes in each pair carry the same genes but may have different versions because we get one chromosome in each pair from our mom and the other from our dad.
An allele is a specific version of a gene. For instance, there are two alleles for eye color: one for blue eyes and one for brown eyes. Each person has two alleles for each gene: one from each parent. The combination of alleles we get from both parents is called the genotype, which decides a person’s actual traits and phenotype.
We get genes from our parents through a process called gene inheritance. This involves the creation and joining of reproductive cells – eggs and sperm – that randomly receive one chromosome from each of the 23 sets instead of both so that a fertilized egg will contain the 23 pairs needed for usual development. This process makes sure that each person gets half of their genes from their mom and half from their dad. It also introduces variation among siblings by shuffling and recombining the chromosomes from both parents.
How Genes Affect Physical Traits
The way we look, such as our height, skin color, or if we have dimples, is what we call physical traits. Genes, which are portions of DNA containing instructions for making specific proteins, are responsible for these traits. We get these genes from our parents, which can be found on chromosomes, structures that hold our genetic info in our cells.
The mix of genes we get from both our parents creates what is known as the genotype, which decides what traits we have, called the phenotype. Things around us, like where we live or what we eat, can also affect how our genes work and the traits that result.
Single gene traits with two alleles (Mendelian)
Some traits are decided by a single gene that can exist in two forms, known as alleles. We call these Mendelian traits after the scientist Gregor Mendel, who discovered inheritance rules using pea plants. For these traits, one allele is usually dominant or stronger than the other allele.
This means that the dominant allele can cover up the effect of the other, weaker allele on the trait. An organism can have two copies of the same allele (homozygous) or two different alleles (heterozygous) for such a trait.
Dominant and recessive alleles
We use capital letters to represent dominant alleles (like A), and lowercase letters for recessive alleles (like a). If an organism has two dominant alleles (AA) or one dominant and one recessive allele (Aa) for a trait, it will show the trait of the dominant allele. If it has two recessive alleles (aa) for a trait, it will show the trait of the recessive allele.
Examples of Mendelian traits
In humans, some traits follow Mendel’s rules:
- Earlobes: Earlobes that hang free are dominant over earlobes that are attached to the head.
- Tongue rolling: Being able to roll the tongue into a U-shape is dominant over not being able to do so.
- Widow’s peak: A hairline that forms a point in the center of the forehead is dominant over a straight hairline.
Traits decided by many genes
Some traits are decided by many genes that work together and are also influenced by environmental factors. We call these polygenic traits, which means “many genes.” These traits show a continuous variation, meaning that there are many possibilities rather than just two or three.
How multiple genes influence traits
For traits influenced by many genes, each gene may have two or more forms that contribute to the trait in different ways. The forms may have additive effects, meaning each one adds a bit of variation to the trait.
For example, a gene influencing height might have two forms, A and a, where A adds 5 cm and a adds 3 cm to the height. So, an organism with two A forms (AA) would be 10 cm taller than an organism with two a forms (aa).
The forms might also have interactive effects, meaning they affect each other’s influence on the trait. For instance, a gene influencing skin color could have two forms, B and b, where B produces more pigment and b less. Another gene for skin color could have two forms, C and c, where C enhances pigment production and c reduces it. So, an organism with a mix of forms (BbCc) would have lighter skin than an organism with BbCC forms.
Examples of traits decided by many genes
In humans, examples of traits influenced by many genes are:
- Height: Many genes affecting growth of bones, muscles, hormones, and nutrition influence our height.
- Skin color: Many genes affecting the production and distribution of melanin, a pigment protecting the skin from sun damage, determine our skin color.
- Eye color: Many genes influencing the amount and type of pigments in the iris, the colored part of the eye, determine our eye color.
Interaction of genes and environment in determining physical traits
Environment refers to the mix of physical and social factors that shape us as we grow. It includes factors like the food we eat, the schools we go to, our culture, our relationships, and our experiences. The environment can work with our genes to decide our physical traits.
Both our genes and environment influence our physical traits. Genes are the instructions we get from our parents that decide how our bodies are built. The environment is the external factors that affect how these instructions are carried out.
Take the color of our eyes as an example. Our genes decide it, but sunlight exposure can also play a role. Our genes also determine the color of our hair, but it can change as we grow older if we’re stressed or if we use hair dye. Our genes determine our height but can also be affected by what we eat, health issues, or hormone levels.
The nature vs. nurture debate
The nature vs. nurture debate is an age-old discussion about whether our genetics (nature) or our lived experiences and environment (nurture) impact our traits more. Some people believe that certain factors are naturally present in us regardless of environmental influences. This belief is called nativism or innatism. Others believe that everything about us is decided by our experiences and what we learn. This view is called empiricism or behaviorism.
This debate has implications for many aspects of psychology, such as intelligence, personality, mental health, and behavior. For example, some researchers believe that intelligence is mostly inherited and decided by genes, while others think that intelligence is mostly shaped by education and environment.
Examples of traits shaped by the environment
Many physical traits are to some extent shaped by environmental factors. Some examples include:
- Weight: Weight is influenced by our genes, but it can also be affected by our diet, how much we exercise, our lifestyle, stress levels, and health conditions.
- Muscle mass: Muscle mass is influenced by genes, but it can also be affected by how active we are, what we eat, our hormone levels, and how old we are.
- Skin color: Skin color is influenced by genes, but it can also be affected by exposure to sunlight, the climate we live in, pollution, and the skincare products we use.
How Genetic Variations Influence Our Physical Traits
Genetic variations are the differences in DNA sequences among individuals or groups. Genetic variations can come from variations (changes in DNA), recombination (mixing of DNA during sexual reproduction), or gene flow (movement of genes between groups). Genetic variations can influence physical traits by affecting how genes work or interact with each other.
Understanding genetic variations
Variations are changes in DNA that can happen randomly or due to environmental factors like radiation or chemicals. They can affect one or more nucleotides (the building blocks of DNA) or larger parts of DNA. Variations can affect genes differently depending on where they occur and how they change the DNA sequence.
Single nucleotide polymorphisms (SNPs) are a type of variation that affects only one nucleotide in a DNA sequence. SNPs are the most common form of genetic variation among humans, accounting for about 90% of all genetic variations. SNPs can affect how genes work or how proteins are made.
Genetic variations can shape physical traits by changing the structure or function of proteins that are involved in different biological processes. Proteins are the molecules that do most of the work in our cells and tissues. Proteins are made from amino acids that are coded by genes.
Some genetic variations can change a protein’s sequence of amino acids, which can affect its shape or activity. For example, sickle cell anemia is a genetic disease caused by a mutation that changes one amino acid in the protein hemoglobin, which carries oxygen in red blood cells. This mutation causes hemoglobin to form unusual shapes, making red blood cells look like sickles and less good at carrying oxygen.
Other genetic variations can affect how much or when a protein is made or how it works with other proteins or molecules.
For example, lactose intolerance is a condition caused by a genetic variation that affects how much lactase enzyme is made in the small intestine. The lactase enzyme breaks down lactose (a sugar found in milk) into glucose and galactose. People with lactose intolerance have low lactase enzyme levels and can’t digest lactose properly, which causes stomach problems.
The role of genetic variations in disease and health
Genetic variations can affect disease and health by increasing or decreasing the risk of developing certain conditions or affecting the response to treatments or interventions. Some genetic variations can cause or contribute to diseases that are passed down in families, such as cystic fibrosis, Huntington’s disease, or breast cancer. Other genetic variations can work with environmental factors to influence the susceptibility or resistance to diseases that are influenced by both genes and the environment, such as diabetes, asthma, or heart disease.
Genetic variations can also affect how individuals respond to drugs or other therapies. Some genetic variations can affect how drugs are broken down in the body or how they bind to their targets in the body.
This can affect the effectiveness or safety of drugs for different people. For example, some people have genetic variations that make them break down certain drugs faster or slower than others, which can affect the best dosage or duration of treatment.
Other people have genetic variations that make them more or less sensitive to the effects of certain drugs, which can affect the risk of side effects or bad reactions.
Future of genomics in shaping health and physical traits
Genomics is the study of the entire genetic makeup of a living thing. This includes studying how these genes are structured, how they work, how they change over time, and how they interact with each other. Genomics has changed our understanding of how genes shape physical traits and health. It has also helped us to create new technologies that can make our lives better and healthier.
A main goal of genomics is to find out which changes in our genes can affect the risk, diagnosis, outcome, and treatment of different diseases.
For example, genomics can help us discover which genes are involved in diseases like cancer, diabetes, Alzheimer’s disease, infections, and rare diseases. By knowing the genetic reasons behind these diseases, we can create better medicines, vaccines, and treatments that are specifically made to suit each person’s genetic makeup.
Another goal of genomics is to understand how genes work with each other and the environment to make complex traits like height, weight, skin color, intelligence, and personality. These traits are influenced by many things, like gene activity, changes in gene functions, microbes in our bodies, and our lifestyles. Genomics can help us understand how these factors affect gene activity and function and how they contribute to the variety in humans and human evolution.
The future of genomics is full of hope and promise. With new technology for sequencing genes, analyzing data, and editing genes, we can expect to get more complete and accurate genetic data from people and groups all over the world. We can also expect to use this data in many areas like medicine, farming, biotechnology, crime investigation, and conservation. Furthermore, we can expect to make genetic services and products more affordable and accessible for everyone who needs them.
However, the future of genomics also has some challenges and risks. We need to make sure that genetic data is shared openly and responsibly and that the privacy and consent of the people involved are respected.
We need to make sure that genetic research is done ethically and fairly and that all groups and communities are represented and included. We need to make sure that knowledge about genetics is communicated clearly and openly and that the public is educated about the benefits and limitations of genomics.
Understanding how genes shape our physical traits in relation to health and disease is crucial for several reasons. It can help us figure out which genetic factors affect our risk and response to different diseases. Second, it can help us make more personalized and effective treatments for these diseases. It can help us appreciate the variety and complexity of human biology and behavior.
Genomics is a powerful tool that can improve our knowledge and our lives. By using its potential responsibly and cooperatively, we can create a better future for ourselves and others.
Dr. Sumeet is a seasoned geneticist turned wellness educator and successful financial blogger. GenesWellness.com, leverages his rich academic background and passion for sharing knowledge online to demystify the role of genetics in wellness. His work is globally published and he is quoted on top health platforms like Medical News Today, Healthline, MDLinx, Verywell Mind, NCOA, and more. Using his unique mix of genetics expertise and digital fluency, Dr. Sumeet inspires readers toward healthier, more informed lifestyles.