Welcome to our exploration of alleles. Alleles are different versions of the same gene. Genes are segments of DNA that code for specific proteins or traits. For example, the gene for blood type has multiple alleles - A, B, and O. These different alleles are why people have different blood types. Alleles are responsible for the variation we see in traits like eye color, hair color, and many other characteristics.
Let's explore how alleles are inherited. Most organisms inherit two alleles for each gene, one from each parent. When both alleles are the same, like AA or aa, we call this homozygous. When the two alleles are different, like Aa, we call this heterozygous. In this example, both parents have the genotype Aa, meaning they each have one dominant A allele and one recessive a allele. When they have a child, the child inherits one allele from each parent. In this case, the child inherited an A allele from Parent 1 and an a allele from Parent 2, resulting in a heterozygous Aa genotype.
Now let's explore dominant and recessive alleles. Dominant alleles, often represented by capital letters like A, mask the effect of recessive alleles, represented by lowercase letters like a, when both are present in a heterozygous individual. The observable trait is called the phenotype, while the genetic makeup is called the genotype. We can use a Punnett square to predict the possible combinations of alleles in offspring. For example, in eye color, brown eyes are dominant over blue eyes. If both parents are heterozygous (Aa), their children have a 75% chance of having brown eyes (AA or Aa) and a 25% chance of having blue eyes (aa). This gives us a phenotype ratio of 3 brown to 1 blue. Other examples of dominant and recessive traits include free earlobes versus attached earlobes, and the ability to roll your tongue versus not being able to roll it.
Not all alleles follow the simple dominant-recessive pattern. Let's explore two alternative inheritance patterns. First is codominance, where both alleles are fully expressed in the heterozygous condition. The classic example is the AB blood type, where both A and B antigens appear on red blood cells. A person with type AB blood has both the A and B alleles, and both are expressed. Second is incomplete dominance, where the heterozygous phenotype is a blend or intermediate of the two homozygous phenotypes. A classic example is flower color in snapdragons. When a plant with red flowers (RR) is crossed with a plant with white flowers (rr), the offspring have pink flowers (Rr). The red and white traits blend to create pink. Finally, some genes have multiple alleles in a population. The ABO blood type system is a perfect example, with three alleles: A, B, and O. This creates the four blood types: A, B, AB, and O.
Let's summarize what we've learned about alleles. Alleles are different versions of the same gene that determine variations in traits like eye color, blood type, and many other characteristics. Most organisms inherit two alleles for each gene, one from each parent. When both alleles are the same, the individual is homozygous; when they're different, the individual is heterozygous. Dominant alleles mask the effect of recessive alleles when both are present in a heterozygous individual. However, not all alleles follow this simple pattern. Alternative inheritance patterns include codominance, where both alleles are fully expressed, and incomplete dominance, where the heterozygous phenotype is a blend of both homozygous phenotypes. The diversity of alleles in a population is what creates the wonderful genetic diversity we see within species. Understanding alleles helps us comprehend inheritance patterns, genetic disorders, and the mechanisms of evolution.