In genetics, two core concepts explain how traits are inherited and expressed: genotype and phenotype. If you've ever wondered why certain traits, like eye color or hair texture, run in families or vary among individuals, understanding genotype and phenotype is the key.

This article breaks down these concepts, explains the roles of dominant and recessive alleles, and explores the complex relationship between genetic code and physical traits.

What Is Genotype?

The genotype is the genetic blueprint. It’s the specific combination of alleles—different forms of a gene—that an individual inherits from their parents. Humans are diploid organisms, which means we have two copies of each gene: one from each parent.

When both copies (alleles) are identical, we call the individual homozygous for that trait. When they differ, the individual is heterozygous.

For example, consider a gene responsible for eye color. Your genotype for this gene determines whether you have two copies of the allele for brown eyes, two for blue eyes, or one of each. The genotype is like an instruction manual that directs how traits will be expressed, though not all genotypes visibly show up as traits.

What Is Phenotype?

Phenotype is the observable expression of your genotype—your physical traits, like eye color, hair texture, or height. If genotype is the instruction manual, phenotype is the end result. Traits like brown or blue eyes are phenotypic expressions that result from your genetic makeup, plus any environmental influences that may play a role.

While the genotype provides the potential for certain traits, the phenotype is what you can actually observe.

Dominant vs. Recessive Alleles: Who Decides the Traits?

In many cases, the expression of traits depends on the interaction of dominant and recessive alleles. Here’s a quick breakdown:

• Dominant Alleles: These alleles will express their trait in the phenotype even if only one copy is present. For instance, if an individual has one dominant allele for brown eyes, they will have brown eyes.

• Recessive Alleles: Recessive alleles only express a trait when two copies are present. In the eye color example, someone with two recessive alleles for blue eyes will have blue eyes, but if they have one dominant allele for brown eyes, the brown will dominate.

Classical Dominance Genetic Notation

Geneticists use notation to track dominant and recessive traits. Uppercase letters represent dominant alleles, while lowercase letters represent recessive alleles.

If you have blue eyes, you know you will have the homozygous recessive genotype (bb). And if you have brown eyes, you do not know what genotype you have (BB or Bb) without performing a test cross.

Incomplete Dominance: Ex. Hair Texture

Let’s take a closer look at hair texture to see genotype and phenotype in action. Hair texture is influenced by a gene that has two alleles: one for curly hair (dominant) and one for straight hair (recessive).

• CC (Homozygous Dominant): Two curly alleles will result in curly hair.

• Cc (Heterozygous): One curly and one straight allele will still result in wavy hair

• cc (Homozygous Recessive): Two straight alleles will result in straight hair.

So, based on your hair type you can estimate the genotype.

Complex Traits - Polygenic Inheritance

While some traits, like hair texture, are controlled by a single gene and have a single expression, others are more complex and influenced by (often) dozens of genes—this is known as polygenic inheritance.

Traits like height, skin color result from the combined influence of many genes, each gene is inherited traditionally, but producing a wide range of possible phenotypes as a whole.

Polygenic traits don’t follow simple dominant-recessive rules.

Instead, they create a spectrum of phenotypes, adding complexity to genetic inheritance.

Genotype and Phenotype in Evolution and Adaptation

Genotypic and phenotypic diversity within populations isn’t just about physical appearance—it’s essential for evolution. Genetic variation provides a foundation for natural selection, allowing populations to adapt to their environments over time. For instance, a population of animals with fur color that blends well with the environment may have better survival chances, leading to more offspring with the advantageous trait.

Learn More at King of the Curve!

In summary, genotype and phenotype are fundamental concepts in genetics that explain how traits are inherited and expressed. While the genotype is the genetic code, the phenotype is the observable expression of those genes. Understanding dominant and recessive alleles helps in predicting how traits will manifest in offspring, while polygenic traits show us the complexity of genetics.

There’s a lot more to genetics, including two-gene traits, x-linked, y-linked, and epistasis (think albino)!

Genetics provides a fascinating window into the diversity of life and the science behind inheritance. For more deep dives into genetics and biology, explore our content on the King of the Curve app or at kingofthecurve.org!