Genetics has a set of key terms. A chromosome is a long molecule of DNA that carries many genes. A gene is a section of DNA that codes for a protein, and an allele is a version of a gene. Humans have 23 pairs of chromosomes in body cells.
A dominant allele is always expressed if it is present, so one copy is enough to show the characteristic. A recessive allele is only expressed if there is no dominant allele present, so two copies are needed. Capital letters are used for dominant alleles and small letters for recessive ones, for example B and b.
If the two alleles of a gene are the same (BB or bb) the organism is homozygous. If they are different (Bb) the organism is heterozygous. The combination of alleles an organism has is its genotype, and the characteristic that results is its phenotype. For example, the genotype Bb gives the phenotype brown eyes if B is dominant.
A gamete is a sex cell, such as a sperm or egg, that carries one allele of each gene. A zygote is the cell formed when two gametes fuse at fertilisation.