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Observable traits are characteristics of an organism that you can see or measure. These might include eye color, height, or even something like the shape of a leaf for plants. When we talk about observable traits, we are focusing on what is visible or measurable from the outside. This means we aren't just talking about what's happening inside the organism at the microscopic level; instead, we are looking at the features that make each organism unique in appearance or function. These traits are influenced by both genetic factors, which come from an organism's genotype, and environmental factors, like the availability of nutrients or exposure to sunlight. The interaction between these factors can make the same genes result in different observable traits in different conditions.

The term genotype refers to the genetic makeup of an organism. This includes all the genes and alleles present within an organism's DNA. The genotype is like a blueprint that carries the instructions for building and maintaining the organism. These instructions determine potential traits, even if they are not always visible. For instance, someone might carry a gene for blue eyes (their genotype), but their eyes appear brown due to dominant genetics (their phenotype).

It’s important to note that the genotype includes every single gene, not just those that manifest as observable traits. Some genes might be recessive or masked by other more dominant ones. Additionally, the environment can influence whether certain genes are expressed, meaning two organisms with the same genotype might have different phenotypes if they are in different environments.

Alleles are different versions of a gene found at the same place on a chromosome. Think of them as variations in the genetic code that can lead to different outcomes for a specific trait. For example, the gene for eye color might have an allele for blue eyes and another allele for brown eyes.

Every organism typically has two alleles for each gene—one inherited from each parent. These alleles can be the same (homozygous) or different (heterozygous). If the alleles are different, one allele might be dominant and the other recessive. The dominant allele is the one that usually determines the observable trait (phenotype), while the recessive allele can be masked.

Understanding how alleles work helps explain why siblings can look different from each other even though they share the same parents—they inherit different combinations of alleles.