Gregor Mendel's foundational work laid the groundwork for understanding how traits are passed from parents to offspring. This module delves into three key patterns of inheritance: dominance, incomplete dominance, and codominance, which illustrate the complex ways alleles interact to determine phenotype.

Genes are segments of DNA that code for specific traits. Alleles are different versions of the same gene. For example, the gene for pea flower color might have an allele for purple (P) and an allele for white (p). The combination of alleles an individual possesses (genotype) determines their observable characteristics (phenotype).

In simple dominance, one allele (the dominant allele) can completely mask the expression of another allele (the recessive allele) when both are present in a heterozygous individual. The dominant allele is typically represented by an uppercase letter, and the recessive allele by a lowercase letter. For instance, if purple flower color (P) is dominant over white flower color (p), a pea plant with genotype PP or Pp will have purple flowers, while only a plant with genotype pp will have white flowers.

In incomplete dominance, neither allele is completely dominant over the other. The heterozygous phenotype is an intermediate or blended expression of the two homozygous phenotypes. A classic example is the snapdragon flower color. If red (CR) and white (CW) alleles are present, a homozygous red plant (CRCR) has red flowers, a homozygous white plant (CWCW) has white flowers, and a heterozygous plant (CRCW) has pink flowers.

Codominance occurs when both alleles in a heterozygous individual are fully and simultaneously expressed. Neither allele masks the other, and both traits are visible in the phenotype. A prime example is the ABO blood group system in humans, where individuals with genotype IAIB have blood type AB, expressing both A and B antigens on their red blood cells. Another example is the roan coat color in cattle, where red and white hairs are both present.

Understanding these patterns is crucial for predicting inheritance. Dominance involves one allele overpowering another. Incomplete dominance leads to a blended intermediate phenotype. Codominance results in the simultaneous expression of both alleles. These variations highlight the complexity and diversity of genetic inheritance beyond simple Mendelian ratios.