One of the basic principles of genetics is that traits are inherited from each parent through genes, or units of heredity. Some traits are dominant and some are recessive. The terms “dominant” and “recessive” refer to how the trait is displayed in the phenotype, or outward appearance, of an individual who inherits a copy of a gene from each parent.
What Are Dominant and Recessive Traits?
A dominant trait only needs one copy of the gene for the trait to be expressed. This means if an individual inherits one copy of the dominant gene for a trait from one parent, the trait will be displayed in the offspring. A recessive trait requires two copies of the gene for the trait to be expressed. This means both parents must pass on a copy of the recessive gene in order for the trait to be displayed in the offspring. If the offspring inherits only one copy of the recessive gene, the trait will not be displayed but they will be a carrier who can pass the gene on.
The difference between dominant and recessive traits comes down to how the gene product acts in determining the phenotype. A dominant allele produces a gene product that determines the organism’s phenotype even if only one copy is present. A recessive allele is masked in the phenotype if a dominant allele is present. Only when two copies of the recessive allele are present will the recessive trait be expressed.
Examples of Dominant and Recessive Traits
There are many examples of dominant and recessive traits that follow these basic inheritance patterns. Here are some common ones:
- Widow’s peak hairline – Dominant trait. Only one dominant gene needed to exhibit widow’s peak.
- Straight hairline – Recessive trait. Two copies of recessive gene needed to exhibit straight hairline.
- Unattached earlobes – Dominant trait. Only one dominant gene needed to have unattached earlobes.
- Attached earlobes – Recessive trait. Two copies of recessive gene needed to have attached earlobes.
- Dimples – Dominant trait. Only one dominant gene needed to have dimples.
- No dimples – Recessive trait. Two copies of recessive gene needed to not have dimples.
- Dark hair – Dominant trait. Only one dominant gene needed to have dark hair.
- Blonde hair – Recessive trait. Two copies of recessive gene needed to have blonde hair.
As you can see from these examples, an individual only needs one dominant allele to exhibit the dominant trait, while two recessive alleles are needed to exhibit the recessive trait. The recessive trait is masked if one dominant allele is present.
How Dominant and Recessive Genes Interact
Individuals have two copies of each gene, one inherited from each parent. The combinations of the different forms of genes, called alleles, determine the observable traits. There are three potential allele combinations:
- Homozygous dominant – Two dominant alleles. Dominant trait will be expressed.
- Heterozygous – One dominant, one recessive allele. Dominant trait will be expressed.
- Homozygous recessive – Two recessive alleles. Recessive trait will be expressed.
For example, imagine dark hair is a dominant trait and blonde hair is recessive. If two individuals with dark hair have children, the possible allele combinations are:
| Parent 1 Alleles | Parent 2 Alleles | Offspring Genotype | Offspring Phenotype |
|---|---|---|---|
| DD | DD | DD | Dark hair |
| DD | Dd | DD or Dd | Dark hair |
| Dd | Dd | DD, Dd, or dd | Dark or blonde hair |
As you can see, the only way for the recessive trait (blonde hair) to be expressed is if both parents pass on a recessive allele. Otherwise, the dominant trait (dark hair) will be expressed.
Complete Dominance vs Incomplete Dominance
In complete dominance, one allele completely dominates over the other. The classic example is the inheritance of the ABO blood types. The A and B alleles are dominant over O. The O allele is completely recessive.
In incomplete dominance, the phenotype displays a blend of the traits from both alleles. A classic example is snapdragon flower color. Red flowers and white flowers breeding together produce pink flowers in the offspring, displaying a mix of the two colors.
| Complete Dominance | Incomplete Dominance |
|---|---|
| One allele completely dominates over the other | Phenotype displays a blend of both alleles |
| Examples: ABO blood types, widow’s peak | Examples: Snapdragon flowers, human blood type AB |
| Offspring shows only dominant trait | Offspring shows a mix of traits |
The Usefulness of Understanding Dominance
Understanding dominant and recessive trait inheritance has many useful applications including:
- Predicting the likelihood of inherited genetic diseases and disorders
- Breeding plants and animals to select for desired traits
- DNA testing for presence of mutated genes
- Determining compatibility for organ transplants
- Establishing paternity and solving crimes with DNA evidence
- Tracing ancestry and genealogy through genetics
For example, some genetic disorders like Huntington’s disease are caused by dominant alleles. Knowing whether a disease allele is dominant or recessive helps predict the pattern of inheritance and risk within families. It also informs genetic counseling and family planning.
In breeding, crossing two organisms with desired dominant traits can reliably produce offspring with those traits expressed. This technique is used for crop improvement in agriculture and selective breeding in animals.
Overall, the concepts of dominance and recessiveness are foundational principles in genetics critical for understanding trait inheritance.
Summary of Key Points
- Dominant traits are expressed when only one allele is present while recessive traits require two copies of the allele.
- Individuals inherit one copy of each gene from each parent.
- The combination of alleles determines if dominant or recessive traits are expressed.
- Complete dominance means one allele completely dominates. In incomplete dominance, a blend of traits is seen.
- Understanding dominance has applications in medicine, agriculture, forensics and more.
Conclusion
The differences between dominant and recessive trait inheritance patterns provides key insights into genetics and heredity. While dominant traits require just one allele, recessive traits are only seen when two alleles are present. Knowing which traits are dominant versus recessive has important implications for understanding the risk of genetic diseases, animal and crop breeding, and DNA analysis. Mastering these fundamental genetic principles opens the door to a wide range of practical applications and scientific advancement.