An organism's phenotype is something that you can appreciate with your senses. If it's their hair color, you can see it with your eyes. If it's their vocal quality, you can hear it with your ears. Even if a phenotype is only present microscopically, like the red blood cells in sickle cell disease, its effects can be appreciated by the individual that suffers from it. Phenotypes can also be behavioral, which you may have noticed if you've ever adopted a pet breed described as "friendly," "brave," or "excitable."
Phenotype Definition
Phenotype is best understood as the observable characteristics of an organism.
Phenotype - The observable characteristics of an organism determined by its gene expression in a given environment.
Phenotype in Genetics
The term phenotype is used most often when studying genetics. In genetics, we are interested in an organism's genes (genotype), which genes get expressed, and how that expression looks (phenotype).
While an organism's phenotype certainly has a genetic component, it's important to remember that there can be a huge environmental component affecting phenotype as well (Fig. 1).
Genetic and Environmental Factors Can Both Decide Phenotype
A simple example of environment and genes determining the phenotype is your height. You get your height from your parents and there are over 50 genes that help determine how tall you will be. However, many environmental factors join the genes in determining your height. Most of these are quite obvious, such as sufficient nutrition, sleep, and good health. Still, other factors like stress, exercise, sun exposure, chronic disease, and even socioeconomic status influence height. All these environmental factors, plus your innate genes, work to determine your phenotype - how tall you are.
Some traits are decided 100% genetically. Often, genetic diseases like sickle cell anemia, maple-syrup urine disease, and cystic fibrosis, get their diseased phenotypes due to a mutated gene. If someone has the mutated gene, no amount of lifestyle changes can make the disease more or less likely to appear. Here, genotype decides phenotype.
An individual with cystic fibrosis has this disease because they have a mutated copy of the CFTR gene on both of their chromosomes 7. The CFTR gene normally codes for a chloride channel, so a mutated CFTR leads to absent or faulty channels, and the symptoms or phenotype of the disease - coughing, lung problems, salty sweat, and constipation - are entirely caused by this genetic defect.
On the other hand, some traits have environmental and genetic components. Many mental health disorders, like schizophrenia, bipolar disorder, and personality disorders, have both genetic and environmental factors influencing them. Other diseases like Alzheimer's, diabetes, and even cancer have both genetic and environmental components.
For example, smoking increases the risk of many types of cancers - this is an environmental factor. But even without smoking, one of the biggest risk factors for cancers such as breast cancer and colon cancer is someone in your close family having it before - a genetic component.
Phenotypic Characteristics and Identical Twins
Another classical example of the environment's influence on phenotype is in identical twins. Monozygotic (identical) twins have the same DNA sequences, hence the same genotype. They are not, however, phenotypically identical. They have phenotypic differences, in looks, behavior, voice, and more, that are observable.
Scientists have often studied identical twins to observe the environment's impact on genes. Their identical genomes make them excellent candidates to help us decipher what else is involved in determining phenotype.
Two typical twin studies compare the following groups:
- Monozygotic vs dizygotic twins
- Monozygotic twins raised together vs. monozygotic twins raised apart.
Monozygotic twins come from the same original egg and sperm cells, which later on in the development process split to form two clumps of cells which eventually lead to two fetuses.
Dizygotic twins are from two different eggs and are essentially two siblings born in the same pregnancy. Thus, they are referred to as fraternal twins. They typically share about 50% of the same genes, while monozygotic twins share 100%.
When comparing monozygotic twins to dizygotic twins, scientists are attempting to discover phenotypic factors that are more heavily influenced by genetics. If all sets of twins were raised together, then any trait shared more heavily by monozygotic twins is a trait that has higher genetic control over phenotype.
The same can be said when comparing monozygotic twins raised apart to those raised together. Suppose monozygotic twins raised apart share a trait at the same rate as monozygotic twins raised together. In that case, the similarity of genetics appears to play a stronger part than the vast variation in their environments.
Types of Phenotypes
What types of phenotypes do twin studies help us elucidate? Virtually any trait can be examined this way, although twin studies are often used to examine psychological or behavioral phenotypes. Two identical twins will have the same eye color or ear size. But do they respond identically, or even similarly, to certain behavioral stimuli? Did they make similar choices growing up, even if they grew up many miles apart, with different adoptive parents, having never met one another? How much of these phenotypic variations are due to their upbringing and environment, and how much is due to their genetic similarity?
Ultimately, the modern practice of twin studies has led to the development of three broad categories of phenotypes: those with a high amount of genetic control, those with a moderate amount, and those with more complex and nuanced inheritance patterns.
- High amount of genetic control - Height, eye color
- Moderate amount - Personality and behavior
- Complex inheritance pattern - Autism spectrum disorder
Difference Between Genotype and Phenotype
What are some instances in which genotype and phenotype may differ? "The Father of Genetics," Austrian monk Gregor Mendel, discovered the Law of Dominance (Fig. 2), which helped explain why the genotype and phenotype of an organism are not always intuitive.
Mendel's Law of Dominance - In a heterozygote organism, which is one with two different alleles for a particular gene, the dominant allele is observed exclusively.
If you were to see a green pea, for example, then its phenotype for color is green. Its phenotype is its observable characteristic. But would we necessarily know its genotype? Does the fact that it is green mean that both alleles that decide the color code for the "green" trait? Let's answer those questions one at a time.
1. Would we necessarily know the genotype of a green pea from seeing its color?
No. Let's say that, as Mendel discovered, peas can have two possible colors. Green and yellow. And let's say that we know that green color is the dominant trait (G) and yellow color is the recessive trait (g). So yes, a green pea can be homozygous for the green trait (GG), but according to The Law of Dominance, a pea with a heterozygote genotype (Gg) will also appear green.
Ultimately, we can't determine just from looking at a green pea if it's (Gg) or (GG), so we cannot know its genotype.
2. Does the fact that it is green mean both alleles which decide the color code for the green trait?
Again, no. Because green is the dominant trait, the plant only needs one green allele to appear green. It might have two, but it only needs one. If the plant were yellow, as yellow is the recessive allele, yes, the plant would need two yellow alleles to appear yellow, and then we would know its genotype - (gg).
A hint for exams: if you know an organism has a recessive phenotype, and the observed trait follows the principles of Mendelian Inheritance, you know its genotype as well! You must have two copies of the recessive allele to have a recessive phenotype, so its genotype is just two copies of the recessive allele.
Phenotype - Key Takeaways
- Phenotype is defined as an organism's observable and palpable characteristics due to how its genes interact with the environment.
- Sometimes phenotype is entirely due to genetics; other times, it is simply due to the environment. Often, the phenotype is due to a combination of the two.
- Twin studies examining mono- and dizygotic twins have been used to demonstrate genetic components of heritability in phenotype.
- We can determine the genotype of an organism with a recessive phenotype just by looking at it.
- Phenotype is not always obvious - things like talkativeness in a person or antibiotic resistance in bacteria are examples of phenotype!
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