Genes are distinct portions of an organism’s DNA that correspond with a specific trait, while alleles are different versions of a given gene. All alleles are genes, but not all genes occur as multiple alleles.
Different types of organisms organize their DNA in slightly different ways. Most of the time, the DNA is grouped into several different pieces called chromosomes. Humans, for example, have 46 chromosomes. Because humans reproduce sexually, 23 of the chromosomes come from the organism’s mother and 23 come from its father. Other organisms have different numbers of chromosomes, such as some crayfish that have 200 chromosomes or fruit flies, which have eight chromosomes.
Discrete parts of each chromosome — the genes — control different aspects of the animal or plant’s structure or function. The number of genes varies just as the number of chromosomes does. Humans are thought to have about 23,000 genes. The size of various genes varies greatly. The smaller genes consist of only a few thousand base-pairs of DNA, while the longest human genes may contain more than 2 million base pairs.
Sometimes, genes occur in different versions. For example, a gene may control an animal’s pigment. One allele produces animals with dark colors, while a different allele produces animals with light colors.