Phenotype Definition

Phenotype is a description of your physical characteristics. It includes both your visible traits (like hair or eye color) and your measurable traits (like height or weight). Phenotype also refers to characteristics related to your development and behavior, like elements of your personality.

The set of genes you carry (known as your genotype) is what determines your phenotype. It is also influenced by environmental factors like gender and temperature.

Phenotype vs. Genotype

While your phenotype describes your physical characteristics, your genotype is the complete set of genes you carry. It also can refer to the combination of different versions (or alleles) of the same gene you carry.

This distinction of genotype vs phenotype doesn’t just apply to humans: genotype refers to an organism’s genetic information, whereas phenotype refers to the observable characteristics, including traits, that result from that genetic information. So grandma’s red hair, your dog’s blue eyes, and a rose’s beautiful smell are all examples of phenotypes that come from each organism’s genotype.

Humans are “diploid” organisms, which means they have two copies of each chromosome—one from each parent. The exception to this is in biological males who have a single X and a single Y chromosome. At a given position in the DNA—or genetic locus—the pair of alleles from the two chromosomes makes up the genotype at that position. In other words, which trait an organism shows depends upon the combination of particular alleles it has.

For example, in humans the genetic locus for earwax type has two alleles: one for wet earwax (usually denoted with a W) and one for dry earwax (usually denoted with a w). An organism’s phenotype results from its combination of alleles.

In the earwax example, people with two wet alleles (WW), or one wet and one dry allele (Ww) have wet earwax. Only people with two dry earwax alleles (ww) have dry earwax. If only one copy of an allele is needed for a trait, the allele is dominant. If two copies are needed, the allele is recessive. So for earwax, the wet allele is dominant over the recessive dry allele.

Not every allele is recessive or dominant like earwax type. Alleles can also be codominant, which essentially means they combine to make a trait. One example is blood type, where someone with an AB blood type has a genotype comprised of an A allele and a B allele.

Or alleles can be incompletely dominant. One example is snapdragons, where a pink flower is the phenotype that results from a genotype consisting of one red allele and one white allele.

Phenotype Examples

Phenotype examples related to appearance were among the first to be studied and are commonly referred to in textbooks. The groundbreaking 1860s research of Gregor Mendel, often referred to as the father of modern genetics, for example, was based on the physical appearance of pea plants. He studied pea plant traits such as white or purple petals, short or long stems, and round or wrinkled seeds. In humans, phenotype examples include earwax type, height, blood type, eye color, freckles, and hair color.

And phenotypes aren’t just physical traits. Behavior is also considered a phenotype. An example of inherited behavior is provided by Border Collies, a dog breed that was bred to herd sheep. Domesticated Border Collies, even if they have never been around sheep their entire lives, will still display herding behavior, such as gathering one type of object around the house.

Interesting Facts About Phenotype and Gene Expression

Members of the same species can have different phenotypes based on geographic location. Human skin color is a great example. Sunlight is a two-edged sword--too much and you can get burned, too little and you might not have enough Vitamin D.

If the climate tends to be more sunny, for example in areas closer to the equator, then skin color needs to be darker to protect people from the sun. But in northern climates with relatively little sunlight, skin color needs to be lighter so more sunlight can be absorbed. This helps to explain why skin color becomes lighter the further north you go.

Two phenotypes can be correlated. In humans, one example is hair color and amount of hair: brunettes tend to have less hair than blondes (approximately 100,000 hairs compared to 120,000 hairs for blondes). This may have its roots in evolution. Hair developed in part to protect the skin from UV rays. The darker the hair, the less hair that was needed.  

Some phenotypes that are common today were almost unheard of thousands of years ago. For example, it used to be that almost no human adults could drink milk without feeling sick to their stomachs. But because it became important in places like Northern Europe and parts of Africa to be able to drink milk to survive, this phenotype is much more common now: almost ⅓ of adults today are able to drink milk.

It is important to note that the need for milk did not cause this mutation, which allows the gene for digesting milk to continue to be expressed into adulthood. Instead, those lucky few who already had the mutation survived and prospered better than their lactose intolerant neighbors. Over time, being lactose intolerant became much less common.

Certain human phenotypes are more common in certain regions. Red hair is a fascinating example. Red hair is most commonly found in the western and northern regions of Europe. Countries like Ireland and regions like the Volga have some of the highest concentrations of redheads in the world.

But just because you have red hair doesn’t automatically mean your family recently came from one of those areas. Red hair does occur in regions and peoples of the world perhaps less commonly associated with red hair, such as in Berber populations found in Morocco and Algeria and among Ashkenazi Jewish populations.

Similarly, freckles tend to be associated with people who have red hair and fair skin. However, gene variations that cause freckles are found worldwide, including in Asia, Africa, and the Americas.