An allele is a specific version of a gene, which is a segment of DNA that provides instructions for a particular trait. Every person inherits two alleles for each gene, one from each parent. A recessive allele is a type of allele that will not influence an individual’s traits if only one copy is present. For the recessive trait to be expressed, it requires inheriting two copies of the allele, without a dominant allele to overpower it.
Dominant and Recessive Inheritance
The interplay between dominant and recessive alleles determines how a trait is expressed. An individual’s genetic makeup is called the genotype, while the observable physical characteristic is the phenotype. If an individual inherits one dominant and one recessive allele, a condition called heterozygous, the dominant allele will mask the effect of the recessive one. For a recessive trait to be visible in the phenotype, an individual must be homozygous recessive, meaning they have two identical copies of that allele.
A classic example is the inheritance of attached versus unattached earlobes. The allele for unattached earlobes is dominant, while the allele for attached earlobes is recessive. A person with two recessive alleles will have attached earlobes. A person with one allele for unattached earlobes and one for attached earlobes will display the dominant trait of unattached earlobes.
This relationship explains why a trait might seem to skip a generation. An individual can carry a recessive allele without showing the trait themselves. They can then pass this allele on to their children. If their partner also passes on a recessive allele for the same trait, their child will express the recessive phenotype.
Visualizing Inheritance with Punnett Squares
A Punnett square is a diagram used to predict the possible combinations of alleles that offspring can inherit from their parents. The setup involves a grid where the alleles from one parent are listed along the top and the alleles from the other parent are listed down the side. Each box within the grid represents a possible genotype for an offspring and helps visualize the probability of inheriting a specific trait.
Consider a scenario where both parents are heterozygous for a trait, meaning they each have one dominant and one recessive allele. For example, if ‘E’ represents the dominant allele for unattached earlobes and ‘e’ represents the recessive allele for attached earlobes, both parents would have the genotype ‘Ee’. They both have unattached earlobes but are considered “carriers” of the recessive allele.
The Punnett square shows a 25% chance the offspring will be homozygous dominant (EE), having unattached earlobes. There is a 50% chance the offspring will be heterozygous (Ee), also with unattached earlobes but being a carrier for the recessive trait. Finally, there is a 25% chance the offspring will be homozygous recessive (ee), expressing the trait of attached earlobes.
Examples of Recessive Traits
Many common human traits are determined by recessive alleles. For instance, having a straight hairline is a recessive trait, whereas a widow’s peak is dominant. Similarly, having type O blood is the result of inheriting two recessive alleles. The alleles for type A and type B blood are codominant, meaning if a person inherits both, they will have type AB blood; the O allele is recessive to both A and B.
Some significant genetic conditions are also caused by recessive alleles. Cystic fibrosis is an autosomal recessive disorder that affects the lungs and digestive system. Sickle cell anemia is another example, a condition that affects red blood cells. In both cases, individuals who inherit only one copy of the recessive allele are carriers but typically do not show symptoms of the disorder.