What Are Traits?

Portrait of mother and daughter with red hair close up
Red Hair Is a Recessive Gene. Uwe Krejci / Getty Images

Have you ever wondered why your eyes are just like your mother's? Or why your hair color is similar to your grandfather's? Or why you and your siblings share features? These physical characteristics are known as traits; they are inherited from parents and expressed externally.

Key Takeaways: Traits

  • Traits are inherited characteristics from our parents that are expressed externally in our phenotype.
  • For any given trait, one gene variation (allele) is received from the father and one from the mother.
  • The expression of these alleles determines the phenotype, whether dominant or recessive.

In biology and genetics, this external expression (or physical characteristics) is called a phenotype. The phenotype is what is visible, while the genotype is the underlying gene combination in our DNA that actually determines what is expressed physically in the phenotype.

How Are Traits Determined?

Traits are determined by an individual's genotype, the summation of the genes in our DNA. A gene is a portion of a chromosome. A chromosome is composed of DNA and contains the genetic material for an organism. Humans have twenty-three pairs of chromosomes. Twenty-two of the pairs are called autosomes. Autosomes are typically very similar in males and females. The last pair, the twenty-third pair, is the sex chromosome set. Those are very different in males and females. A female has two X chromosomes, while a male has one X and one Y chromosome.

How Are Traits Inherited?

How are traits passed from one generation to the next? This happens when gametes unite. When an egg is fertilized by a sperm, for each chromosome pair, we receive one chromosome from our father and one from our mother.

For a particular trait, we receive what is known as an allele from our father and one allele from our mother. An allele is a different form of a gene. When a given gene controls a characteristic that is expressed in the phenotype, the different forms of a gene show as the different characteristics that are observed in the phenotype.

In simple genetics, alleles can be homozygous or heterozygous. Homozygous refers to having two copies of the same allele, while heterozygous refers to having different alleles.

Dominant Traits vs. Recessive Traits

When alleles are expressed via simple dominant versus recessive traits, the specific alleles inherited determine how the phenotype is expressed. When an individual has two dominant alleles, the phenotype is the dominant trait. Likewise, when an individual has one dominant allele and one recessive allele, the phenotype is still the dominant trait.

While dominant and recessive traits may seem straightforward, note that not all traits have this simple inheritance pattern. Other types of genetic inheritance patterns include incomplete dominance, co-dominance, and polygenic inheritance. Due to the complexity of how genes are inherited, specific patterns can be somewhat unpredictable.

How Do Recessive Traits Occur?

When an individual has two recessive alleles, the phenotype is the recessive trait. For example, let's suppose that there are two versions of a gene, or alleles, that determine whether or not a person can roll their tongue. One allele, the dominant one, is symbolized by a big 'T'. The other allele, the recessive one, is symbolized by a little 't'. Let's suppose two tongue rollers get married, each of whom is heterozygous (has two different alleles)for the trait. This would be represented as (Tt) for each. 

Traits
Traits are inherited characteristics that are expressed externally in our phenotype. Copyright Evelyn Bailey

When a person inherits one (t) from the father and then one (t) from the mother, the recessive alleles (tt) are inherited and the person can not roll their tongue. As can be seen in the Punnett square above, this would happen approximately twenty-five percent of the time. (Note that this tongue rolling is just for the sake of providing an example of recessive inheritance. Current thinking around tongue rolling indicates the involvement of more than just a single gene, and is not as simple as was once thought).

Other Examples of Weird Inherited Traits

A longer second toe and attached earlobes are often cited as examples of a "weird trait" that follows the two dominant/recessive alleles forms of one gene inheritance. Again, however, evidence suggests that both attached earlobe and longer second toe inheritance are quite complex.

Sources

  • “Attached Earlobe: The Myth.” Myths of Human Genetics, udel.edu/~mcdonald/mythearlobe.html.
  • “Observable Human Characteristics.” Nutrition & the Epigenome, learn.genetics.utah.edu/content/basics/observable/.