What is the term for our complex blend of hereditary characteristics inherited from our biological parents which constitutes our genetic makeup?

If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Alleles are copies of genes that influence hereditary characteristics. Each person inherits at least two alleles for a particular gene—one allele from each parent. They are also called allelomorphs.

A good example of how alleles are expressed is eye color; whether we have blue or brown eyes depends on the alleles that are passed down from our parents. Because they help determine what our bodies look like and how they're structured, alleles are considered an important part of the blueprint for all living organisms.

Alleles play a big role in determining our inherited traits, along with DNA and genes.

Deoxyribonucleic acid (DNA) is the hereditary material that humans and other living organisms get from each parent. It's technically a molecule that's responsible for carrying all of the necessary genetic information in the body’s cells. Half of a person's DNA comes from their mother, and the other half comes from their father.

Your DNA is organized into small parts called genes. Genes act as coded instructions to control how our bodies are built and influence what we look like. Experts estimate that humans have about 20,000 to 25,000 genes.

For most genes, one copy is inherited from the biological mother and one copy is inherited from the biological father (which we will refer to as simply the "mother" and "father" throughout).

The version of each gene that a parent passes down to their child is known as an allele. Alleles are located on chromosomes, which are the structures that hold our genes. Specifically, alleles influence the way our body’s cells work, determining traits and characteristics like skin pigmentation, hair and eye color, height, blood type, and much more. 

The traits we end up inheriting from our parents depend on how the alleles interact with each other. The specific way that alleles are paired together are known as inheritance patterns, which make up all the variations in a person’s genetic traits.

Because alleles provide at least two sets of instructions for each gene, the body has to figure out which “roadmap” to follow, or in other words, which trait needs to be expressed.

Take eye color, for instance. A person’s eye color is a result of the alleles that were passed down from parent to child. Different combinations of alleles produce brown, blue, green, or hazel eye colors, though the last two are more unique than brown or blue eyes. Here's are two common scenarios that might occur:

• If both parents contribute identical alleles for the eye color gene, they're known as homozygous. That means the instructions the alleles provide will be the same, so that eye color will appear. Homo- means same, and -zygous refers to the zygote that forms when a sperm fertilizes an egg.
• If the parents each contribute different alleles for a gene, they're known as heterozygous, meaning the instructions won't match up, and the body will have to go in the direction of the stronger (or dominant) allele. Hetero- means different.

Here's where it can get a little tricky. An allele can be dominant or recessive. Dominant alleles express a trait, even if there is only one copy. Recessive alleles can only express themselves if there are two copies—one from each parent. And you've probably figured out by now that dominant alleles overrule recessive alleles.

For example, a trait like blue eyes is considered recessive, so it generally only appears when the blue eye alleles are the same from both parents. Brown eyes are considered dominant, so you only need that brown eye allele from one parent in order to have brown eyes.

Brown eye color is a dominant trait, while blue eye color is a recessive trait. Green eye color is a mix of both and is dominant to blue but recessive to brown.

While two alleles make up the genotype, some traits, like eye color, have several alleles that influence the trait. This also includes blood type and hair color. New alleles arise in populations via mutation, and natural selection can also be an influence, deferring to some alleles over others

In fact, some biologists consider alleles to be so crucial to how humans have evolved that they define evolution as a change in allele frequencies within a population over time.

Alleles help decide almost everything about a living being. But even with a solid understanding of how alleles determine our traits and characteristics, genetics is still a complex field that scientists and researchers are learning more about every day.

It's worth mentioning that while it's possible to make fairly accurate predictions about what color eyes or hair your baby may have based on a combination of alleles, you can't always predict with absolute certainty which traits will appear.

Keep in mind that genetic combinations also depend on the "hidden" or recessive alleles that each parent may have.

Scientists originally thought that a single, simple inheritance pattern produced a person's eye color. But we now know that even dominant traits like brown eyes can be the result of multiple different allele combinations, and they can also disappear in one generation only to reemerge in a later generation.

In other words, because the way that alleles help determine eye color and many other traits is complex, genetic variations can sometimes produce unexpected results.

While traits like eye color or hair color typically do not have any serious health conditions attached to them, if you have any questions about the way alleles can influence certain genetic diseases, you should feel comfortable addressing these concerns with your healthcare provider.

Armed with some background information about your family tree and medical history, a healthcare professional should be able to help you determine whether a specific genetic condition may run in your family and what it means for you and your loved ones.

Any organism is a by-product of both its genetic makeup and the environment. To understand this in detail, we must first appreciate some basic genetic vocabulary and concepts.  Here, we provide definitions for the terms genotype and phenotype, discuss their relationship and take a look at why and how we might choose to study them. 

What is the definition of a genotype? 

In biology, a gene is a section of DNA that encodes a trait. The precise arrangement of nucleotides (each composed of a phosphate group, sugar and a base) in a gene can differ between copies of the same gene. Therefore, a gene can exist in different forms across organisms. These different forms are known as alleles. The exact fixed position on the chromosome that contains a particular gene is known as a locus.A diploid organism either inherits two copies of the same allele or one copy of two different alleles from their parents. If an individual inherits two identical alleles, their genotype is said to be homozygous at that locus.However, if they possess two different alleles, their genotype is classed as heterozygous for that locus. Alleles of the same gene are either autosomal dominant or recessive. An autosomal dominant allele will always be preferentially expressed over a recessive allele.

The subsequent combination of alleles that an individual possesses for a specific gene is their genotype.  

Genotype examples

• A gene encodes eye color.
• In this example, the allele is either brown, or blue, with one inherited from the mother, and the other inherited from the father.
• The brown allele is dominant (B), and the blue allele is recessive (b). If the child inherits two different alleles (heterozygous) then they will have brown eyes. For the child to have blue eyes, they must be homozygous for the blue eye allele. 

Figure 1: Inheritance chart detailing how an individual may inherit blue or brown eyes depending on the alleles carried by their parents, with the brown eye color allele being dominant and the blue eye color allele being recessive. Other examples of  genotype include:

• Hair color
• Height
• Shoe size

The sum of an organism’s observable characteristics is their phenotype. A key difference between phenotype and genotype is that, whilst genotype is inherited from an organism’s parents, the phenotype is not.

Whilst a phenotype is influenced the genotype, genotype does not equal phenotype. The phenotype is influenced by the genotype and factors including:

• Epigenetic modifications
• Environmental and lifestyle factors 

Figure 2: Flamingos are naturally white in color, it is only the pigments in the organisms that they eat that cause them to turn vibrantly pink.

Environmental factors that may influence the phenotype include nutrition, temperature, humidity and stress. Flamingos are a classic example of how the environment influences the phenotype. Whilst renowned for being vibrantly pink, their natural color is white – the pink color is caused by pigments in the organisms in their diet. A second example is an individual's skin color. Our genes control the amount and type of melanin that we produce, however, exposure to UV light in sunny climates causes the darkening of existing melanin and encourages increased melanogenesis and thus darker skin. Observing the phenotype is simple – we take a look at an organism’s outward features and characteristics, and form conclusions about them. Observing the genotype, however, is a little more complex. Genotyping is the process by which differences in the genotype of an individual are analyzed using biological assays. The data obtained can then be compared against either a second individual’s sequence, or a database of sequences.

Previously, genotyping would enable only partial sequences to be obtained. Now, thanks to major technological advances in recent years, state-of-the-art whole genome sequencing.

Figure 3: A workflow depicting the various steps of whole genome sequencing (WGS).

(WGS) allows entire sequences to be obtained. An efficient process that is increasingly affordable, WGS involves using high-throughput sequencing techniques such as single-molecule real-time (SMRT) sequencing to identify the raw sequence of nucleotides constituting an organism’s DNA. 

WGS is not the only way to analyze an organism’s genome - a variety of methods are available.

Understanding the relationship between a genotype and phenotype can be extremely useful in a variety of research areas.

A particularly interesting area is pharmacogenomics. Genetic variations can occur in liver enzymes required for drug metabolism, such as CYP450. Therefore, an individual’s phenotype, i.e. their ability to metabolize a specific drug, may vary depending on which form of the enzyme-encoding gene they possess. For pharmaceutical companies and physicians, this knowledge is key for determining recommended drug dosages across populations.

Making use of genotyping and phenotyping techniques in tandem appear to be better than using genotype tests alone. In a comparative clinical pharmacogenomics study, a multiplexing approach identified greater differences in drug metabolism capacity than was predicted by genotyping alone. This has important implications for personalized medicine and highlights the need to be cautious when exclusively relying on genotyping.

Using animal models such as mice, scientists can genetically modify an organism so that it no longer expresses a specific gene – known as knockout mice. By comparing the phenotype of this animal to the wild type phenotype (i.e. the phenotype that exists when the gene has not been removed), we can study the role of certain genes in delivering certain phenotypes.

The Mouse Genome Informatics (MGI) initiative has compiled a database of thousands of phenotypes that can be created and studied, and the genes that must be knocked out to produce each specific phenotype. 

Senior Science Writer