Heredity dictates how physical characteristics, or traits, are passed down from biological parents to offspring. Every organism inherits a complete set of instructions for building and maintaining itself, contained within its genetic material. These instructions are organized into functional segments called genes. Genes act as the fundamental blueprint, determining everything from eye color to blood type. Since offspring receive genetic material from both parents, the interaction between these inherited instructions determines which traits become visible.
Understanding Alleles and Genetic Traits
The instructions that make up a gene come in different versions, known as alleles. An individual inherits two alleles for every gene, one from each biological parent. The combination of these two alleles makes up the underlying genetic composition, or the genotype, of that trait. The observable expression of this genotype—the physical characteristic—is called the phenotype.
Alleles are categorized based on how they interact. A dominant allele expresses its associated trait even when paired with a different version of the gene. Conversely, a recessive allele’s trait is only expressed when paired with another identical recessive allele. The dominant version essentially masks the presence of the recessive version in the physical expression of the trait.
The Mechanism of Dominance
For a trait governed by simple dominance, only a single copy of the dominant allele is required for that trait to appear. This relationship determines how many copies are necessary to express a dominant trait. Using ‘A’ for the dominant allele and ‘a’ for the recessive allele, we can illustrate the three possible genotypes.
An individual with two dominant alleles (AA) is homozygous dominant and will display the dominant trait. An individual with one dominant and one recessive allele (Aa) is heterozygous; the dominant ‘A’ masks the recessive ‘a’, so this individual also displays the dominant trait. The recessive trait is only expressed if the individual is homozygous recessive (aa), meaning they inherited the recessive allele from both parents. The dominant allele often codes for a functional protein, so one working copy is enough to produce the required biological effect.
Predicting Inheritance Using Genotypes
Understanding the relationship between alleles allows for predictions about how traits will appear across generations. Geneticists use a simple tool to visualize the probability of offspring inheriting a specific genotype from their parents. This method maps out all the potential combinations of alleles that an offspring could receive.
Consider two parents who are both heterozygous for a trait (Aa), meaning they each possess one dominant and one recessive allele but express the dominant phenotype. Each parent has a 50% chance of passing on the dominant ‘A’ and a 50% chance of passing on the recessive ‘a’ to their child. When these possibilities are combined, there are four equally likely outcomes for the offspring’s genotype: AA, Aa, aA, and aa.
Three of these four combinations (AA, Aa, and aA) include at least one dominant allele. This results in a 75% probability that the offspring will inherit a genotype that expresses the dominant phenotype. The remaining combination (aa) is the only one that results in the recessive phenotype, giving it a 25% chance of appearing. This 3:1 ratio of dominant to recessive phenotypes is a predictable pattern that reinforces the power of a single dominant allele to determine the physical characteristic.
Exploring Non-Mendelian Patterns
The straightforward rules of simple dominance do not apply to all inherited traits. Many characteristics follow more complex patterns of inheritance that deviate from this basic model. These exceptions show that the relationship between alleles is not always one of complete masking.
In some cases, neither allele is completely dominant, leading to a blending of traits known as incomplete dominance. For example, crossing a plant with red flowers and one with white flowers might result in offspring with pink flowers. Another variation is codominance, where both alleles are fully and separately expressed simultaneously. The human AB blood type is an example of codominance, where both A and B antigens are present on the red blood cells.