An organism is considered homozygous for a particular gene when both inherited alleles are identical (e.g., AA or aa). In contrast, a heterozygous organism possesses two different alleles for the same gene (e.g., Aa). Homozygosity ensures that the organism will pass the same allele to all its offspring for that specific trait.

The wild type refers to the phenotype of the typical form of a species as it occurs in nature. It is distinguished from mutant phenotypes, which result from genetic mutations. In genetics, the wild type is often used as the standard of comparison for studying the effects of specific gene variations.

In genetics, a dominant gene is one that produces the same phenotype whether it is present in a homozygous or heterozygous state. It effectively masks the presence of a recessive allele in a heterozygote. This principle, established by Gregor Mendel, explains how certain traits are inherited and expressed over others in successive generations.

The phenotype represents the total set of observable traits of an organism, including morphology, development, and physiological properties. It is the result of the expression of an organism's genotype in conjunction with the environmental factors it encounters during its life. While the genotype provides the genetic blueprint, the phenotype is the actual physical manifestation observed.

An allele is one of two or more versions of a gene. Individuals inherit two alleles for each gene, one from each parent. If the alleles are the same, the individual is homozygous for that gene; if they are different, the individual is heterozygous. Alleles are responsible for the variations in inherited characteristics among individuals of a species.

In the ABO blood group system, the A and B alleles are codominant. When both alleles are present in an individual (genotype AB), both antigens are expressed simultaneously on the surface of red blood cells, demonstrating codominance rather than simple dominance.

Multiple alleles refer to the presence of three or more alternative forms of a gene at a single locus in a population. While an individual diploid organism can only carry two alleles, the population as a whole may exhibit many variations. A classic example is the ABO blood group system in humans, which is governed by three distinct alleles.

The genotype refers to the specific genetic makeup or the set of alleles present in the DNA of an organism. It determines the hereditary potential and limitations of an individual, which may be expressed as the phenotype depending on environmental and genetic interactions.

Codominance occurs when both alleles of a gene pair are fully expressed in the phenotype of a heterozygote. Neither allele is dominant or recessive, resulting in an organism that displays traits associated with both alleles simultaneously, such as AB blood type.

The fruit fly, Drosophila melanogaster, is a model organism in genetics. Its diploid somatic cells contain 8 chromosomes, consisting of four pairs: three pairs of autosomes and one pair of sex chromosomes. This small number of chromosomes makes it an ideal subject for studying inheritance patterns and genetic mutations.