What is each gene made up of?

Each gene is made up of two alleles. Alleles are different versions of a gene that arise due to mutations and may result in different traits or characteristics in an organism. An individual typically inherits one allele from each parent, which contributes to the organism's genotype. This concept is fundamental in genetics, as the combination of these alleles can lead to variations in phenotypes, particularly for traits governed by Mendelian inheritance.

While it is worth noting that genes are composed of sequences of nucleotides (which are the building blocks of DNA), the concept of a gene being made up of alleles focuses on the genetic variation aspect. Alleles represent the functional expression of those nucleotide sequences, leading to observable traits in organisms. Understanding alleles helps explain genetic diversity within populations and the inheritance patterns observed in different species.