A chromosome is a long molecule of coiled DNA in the nucleus that carries many genes. A gene is a section of DNA that codes for a protein, and different versions of a gene are called alleles. A dominant allele is shown in the phenotype even if only one copy is present. A recessive allele is only shown when two copies are present. We usually write a dominant allele as a capital letter, such as B, and a recessive allele as a small letter, such as b.
An organism with two identical alleles of a gene is homozygous, such as BB or bb. An organism with two different alleles is heterozygous, such as Bb. The combination of alleles an organism has is its genotype. The characteristic that can be seen is its phenotype. For example, a plant with the genotype bb has the recessive phenotype.
A gamete is a sex cell that carries one allele of each gene. When a male gamete and a female gamete fuse at fertilisation they form a zygote, which has two alleles of each gene, one from each parent.