Genetics studies how traits are inherited through DNA. DNA (deoxyribonucleic acid) is a double-helix molecule containing four bases: adenine (A), thymine (T), guanine (G), and cytosine (C). Base pairs always match: A-T and G-C.
Genes are segments of DNA that code for specific proteins. The process of reading DNA to make proteins involves transcription (DNA → mRNA in the nucleus) and translation (mRNA → protein at ribosomes). The sequence of bases determines the amino acid sequence of the protein.
Mendelian genetics established the laws of inheritance. The law of segregation states that each organism carries two alleles for each trait, and these separate during gamete formation. The law of independent assortment states that alleles for different traits are inherited independently.
Dominant alleles mask recessive alleles when both are present. A dominant trait appears in the phenotype with just one copy (homozygous dominant or heterozygous). A recessive trait only appears when both alleles are recessive (homozygous recessive).