Final answer:
A version of a gene is called an allele, which forms part of an organism's genotype and can be either homozygous or heterozygous. The expression of alleles leads to an organism's phenotype, the observable traits.
Step-by-step explanation:
A version of a gene is called a(n) allele. When gametes, which are the reproductive cells, unite during fertilization, they form a zygote with a complete set of genes. These genes come in different versions known as alleles. The combination of alleles an individual inherits is known as their genotype, which can result in the organism being either homozygous, having two identical alleles, or heterozygous, having two different alleles. The actual expression of the genotype, through physical characteristics and observable traits, is termed the phenotype.
Mendel's work on inheritance used the ratios of phenotypes in the offspring to determine how alleles of a gene interact. When only one type of allele, the dominant allele, is expressed in the phenotype, the organism is either homozygous dominant or heterozygous. The recessive allele is only expressed in the phenotype when an organism is homozygous recessive (possesses two recessive alleles).