Introduction:
In genetics, alleles are alternative forms of a gene that occupy the same position (locus) on a chromosome. Each individual inherits two alleles for each gene, one from each parent. The presence of two different alleles at a particular locus results in heterozygosity.

Explanation:
Heterozygosity refers to the condition in which an individual has two different alleles at a specific gene locus. This means that the two alleles inherited from the parents are not identical. The presence of different alleles allows for genetic variation within a population.

Significance of Heterozygosity:
Heterozygosity is important because it can lead to the expression of different phenotypes. The combination of two different alleles can result in a dominant-recessive relationship, where one allele (dominant) masks the effect of the other allele (recessive). This allows for the expression of different traits and characteristics in individuals.

Examples of Heterozygosity:
1. Eye Color: The gene for eye color has multiple alleles, such as brown and blue. A person who inherits one brown allele and one blue allele will have heterozygous eye color, resulting in a specific eye color phenotype.
2. Blood Type: The ABO blood group system is determined by three alleles: A, B, and O. An individual who inherits allele A from one parent and allele B from the other parent will have heterozygous blood type AB.

Contrast with other terms:
1. Homozygosity: Homozygosity refers to the condition in which an individual has two identical alleles at a specific gene locus. For example, an individual with two alleles for blue eye color would be homozygous for that trait.
2. Hemizygosity: Hemizygosity refers to the presence of only one copy of a gene in a diploid organism. This typically occurs when there is a deletion or loss of one copy of a gene on one of the sex chromosomes.
3. Nullizygosity: Nullizygosity, also known as null allele, refers to the absence of a functional allele at a particular gene locus. This can occur due to mutations or deletions in the gene.

Conclusion:
The presence of two different alleles at a particular locus results in heterozygosity. Heterozygosity allows for genetic variation and can lead to the expression of different phenotypes. Understanding the concept of heterozygosity is crucial in the field of genetics to comprehend inheritance patterns and genetic diversity within populations.