Differences Between Dominant & Recessive Phenotypes

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

In this lesson, we're going to compare and contrast dominant and recessive phenotypes. Here, you'll understand differences in how genes create dominant versus recessive phenotypes, how they are inherited, and different types of dominant and recessive inheritance.

What Is a Phenotype?

Think about some of your traits. Are you tall or short? Brown hair or black hair? Is your nose wide or narrow? In biology, we call traits like these phenotypes. Your phenotype is your expressed traits, and can include traits you don't see, like intelligence, personality, and certain diseases.

What Makes Phenotypes Dominant or Recessive?

Phenotypes are controlled both by your genes, called your genotype, and the environment. Which genes you inherit from your mother and father, in combination with environmental exposure, determine your phenotype. You always get two versions of a gene, or alleles, one from mom and one from dad. Some alleles are considered dominant, meaning if you get one allele for the trait, that's going to be your phenotype. Other traits are recessive. They are masked by the dominant phenotype, and you need to get two copies of that trait to see it in your phenotype.

Discovery of Dominant and Recessive Phenotypes

The first person to describe the link between genotype and phenotype was Gregor Mendel. He looked at inheritance of lots of traits in pea plants. Luckily for him, pea plant inheritance is relatively simple. In pea pods, the phenotype of green pod color is dominant to yellow. Based on this idea, you would think mating two green pea plants would always make green offspring. However, what Mendel observed is when two green plants were mated, sometimes they produced a yellow offspring in a specific ratio each time.

How could a plant have a recessive phenotype from two dominant parents? Mendel figured out that the green pea pod color was dominant to yellow. A green pea plant could have one dominant gene for green, but still have a recessive gene for yellow that doesn't show up, since the green masks it. Thus, the plant looks green, but carries an allele for yellow phenotype. This type of genotype is called a heterozygote. Heterozygotes still have a recessive allele, but you only see the dominant phenotype.

Yellow color is recessive to green color in pea plants
yellow and green pea plants

Molecular Biology of Phenotypes

So, how does this happen at a cellular level? How can the product of one gene simply take over the other? Wouldn't it make more sense if the yellow color was expressed just as much as the green?

First, we need to review a little about molecular biology, or what goes on inside of cells. Genes code for proteins, and proteins make up all aspects of our traits. For example, our hair follicle cells produce a protein called keratin. The location at which keratin is added to hair along with other factors, determine if our phenotype for hair texture is curly or straight. The genes make proteins that ultimately controls our traits.

Recessive Genes

So, how can one gene hide another in a dominant versus recessive phenotype? There are a couple ways this can happen. First, the recessive gene might not make a protein at all. The gene would be essentially broken. So, all you see is the dominant protein being made, and thus the dominant phenotype.

An example of this is eye color in fruit flies. Normally, fruit flies have red eyes. Red eyes is a dominant phenotype compared to red eyes. The white gene in fruit flies produces red pigment, like how humans have various colored eyes. However, if the fruit fly inherits two recessive copies of the white gene, no red protein is made, and their eyes are white. The white recessive gene is broken. But, if the fly gets even one copy of the dominant white gene, then they will make some red pigment, making their eyes red.

The phenotype for red eyes is dominant to the phenotype for white eyes
eye color

Dominant Genes

Although we usually think of the recessive gene as being the 'bad' gene, sometimes you want two recessive alleles and a dominant protein causes the problem. One example is a neurodegenerative disorder called Huntington's disease. This disease typically has an onset later in life and is characterized by uncontrolled movements, emotional problems, and cognitive decline, meaning the person will have greater and greater trouble thinking.

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