What is a key characteristic of heterozygous individuals?

Heterozygous individuals are defined by the presence of two different alleles for a particular trait. This is a fundamental concept in genetics, particularly in Mendelian inheritance. When an individual is heterozygous, for example, one allele may be dominant while the other is recessive. In many cases, the dominant allele will mask the expression of the recessive allele, resulting in a phenotype that reflects the dominant trait. However, the presence of two different alleles is what characterizes heterozygosity.

The other choices do not accurately reflect the nature of heterozygosity. Individuals with identical alleles for a trait are referred to as homozygous, which does not describe heterozygous individuals. The expression of both alleles equally is more characteristic of codominance or incomplete dominance rather than standard Mendelian dominance. Lastly, showing only one form of the trait generally pertains to homozygous organisms where only one type of allele is present and expressed in the phenotype. Thus, the defining trait of heterozygous individuals is indeed that they carry two different alleles for a given trait.