Genes dictate many aspects of an organism, from the color of a flower to the structure of a human hand. Heredity describes the process by which these characteristics are passed from parents to their offspring. Each parent contributes a set of genetic instructions to their progeny, influencing the traits that will develop. These instructions are contained within genes, which are segments of DNA located on chromosomes.
Genes exist in different forms called alleles. These alleles are responsible for the variations observed in a specific trait. For instance, a gene for eye color might have alleles for blue eyes or brown eyes. The combination of these alleles determines an individual’s observable characteristics.
What Makes a Trait Dominant?
A dominant trait manifests its characteristic when only one copy of its specific allele is present. An individual needs to inherit the dominant allele from just one parent for the trait to be expressed. This single dominant allele is sufficient to determine the observable outcome.
The dominant allele’s instruction is followed, effectively masking the presence of the other allele’s instruction. The trait linked to the dominant allele will always be visible if that allele is part of an organism’s genetic makeup. This explains why certain traits appear more frequently across generations.
Unveiling Recessive Traits
A recessive trait only appears when an individual inherits two copies of the specific recessive allele. If a dominant allele is also present, it will override the recessive allele’s instruction, preventing the recessive trait from being observed.
Individuals carrying one dominant and one recessive allele for a trait are known as carriers. They do not express the recessive trait themselves, but they can pass the recessive allele to their offspring. The recessive trait remains hidden until two carriers reproduce, creating an offspring who inherits two recessive alleles. This explains why certain traits can skip generations before reappearing.
Genetics in Action: How Traits Appear
The combination of alleles an individual possesses for a particular gene is known as their genotype. This genetic makeup directly influences the observable characteristic, or phenotype, that the individual displays. Understanding the relationship between genotype and phenotype is central to grasping how dominant and recessive traits manifest.
There are three primary genotype combinations that determine a trait’s appearance. An individual can be homozygous dominant, meaning they have two copies of the dominant allele for a specific gene. In this case, the phenotype will always be the dominant trait. For example, if ‘B’ represents the dominant allele for brown eyes, an individual with a ‘BB’ genotype will have brown eyes.
Alternatively, an individual can be homozygous recessive, possessing two copies of the recessive allele. Only when both alleles are recessive does the recessive trait appear as the phenotype. If ‘b’ represents the recessive allele for blue eyes, an individual with a ‘bb’ genotype will have blue eyes because no dominant allele is present to mask it.
The third possibility is a heterozygous genotype, where an individual carries one dominant allele and one recessive allele. Despite the presence of the recessive allele, the dominant allele’s instruction prevails, and the individual expresses the dominant trait. Therefore, an individual with a ‘Bb’ genotype for eye color will still have brown eyes, demonstrating the masking effect of the dominant allele.
Common Examples of Inheritance
Many human characteristics illustrate dominant and recessive inheritance patterns. The ability to roll one’s tongue is a dominant trait. This means that if an individual inherits at least one allele for tongue rolling, they will be able to perform the action.
Attached earlobes versus free earlobes serve as a common example. Free earlobes are dominant, while attached earlobes are recessive. An individual will have free earlobes if they inherit even one allele for this trait, whereas attached earlobes only appear if both inherited alleles are for the attached form.
Certain conditions also follow these patterns. Huntington’s disease, a neurological disorder, is an example of a dominant inherited condition. A person needs to inherit only one copy of the altered gene to develop the disease. Conversely, cystic fibrosis, a condition affecting the lungs and digestive system, is a recessively inherited disorder. An individual must inherit two copies of the altered gene, one from each parent, to be affected by cystic fibrosis.