DNA, Genes and Heredity
How DNA stores the instructions for life, how genes encode traits, and how those traits are passed from parent to offspring — the foundations of genetics.
Primary sources: NCBI Bookshelf: Mendel's Experiments · NIH NHGRI DNA Fact Sheet
01 · DNA structure
The double helix stores the code.
DNA (deoxyribonucleic acid) is a double helix: two strands wound around each other, held together by complementary base pairs. Each strand is a polymer of nucleotides, each consisting of a deoxyribose sugar, a phosphate group and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G) and cytosine (C).
Base pairing is specific: A pairs with T (two hydrogen bonds); G pairs with C (three hydrogen bonds). This complementarity is what makes DNA replication and transcription possible. Watson, Crick, Franklin and Wilkins established the double helix model in 1953.
In eukaryotes, DNA is wound around histone proteins to form nucleosomes, which condense further into chromosomes. Humans have 46 chromosomes (23 pairs) containing roughly 3.2 billion base pairs — the human genome. See our DNA glossary entry for more on structure and our cells and organelles guide for where chromosomes are stored.
02 · Genes and alleles
Genes encode traits; alleles are variants.
A gene is a specific sequence of DNA bases that codes for a functional product — usually a protein, which then determines a trait (e.g. eye colour, blood group). The human genome contains approximately 20,000–25,000 protein-coding genes, which make up only about 2% of the total DNA; the rest includes regulatory sequences, introns and non-coding RNA genes.
An allele is one of two or more alternative sequences at a gene locus. Because humans are diploid, every individual carries two alleles per gene — one on each homologous chromosome. An organism with two identical alleles (AA or aa) is homozygous; two different alleles (Aa) is heterozygous.
The genotype is the combination of alleles; the phenotype is the observable trait. Dominant alleles (written as capitals) mask recessive ones in heterozygotes. For example, the allele for brown eyes (B) is dominant over blue (b): BB and Bb individuals both have brown eyes; only bb gives blue eyes.
03 · Mendel's laws
Three rules govern inheritance.
Gregor Mendel studied pea plants in the 1860s and formulated three laws that still underpin modern genetics.
01
Law of Segregation
Each organism carries two alleles per gene. During gamete formation the two alleles separate, so each gamete carries only one.
02
Law of Independent Assortment
Genes on different chromosomes are inherited independently of each other. (Linked genes on the same chromosome are an exception.)
03
Law of Dominance
When an organism is heterozygous (carries two different alleles), the dominant allele determines the phenotype; the recessive allele is masked.
Mendel's laws are powerful but not universal. Incomplete dominance, codominance, sex-linkage, polygenic inheritance and gene-environment interactions all add complexity beyond the basic model.
04 · Heredity in practice
Punnett squares and pedigrees.
A Punnett square predicts the probability of each genotype in offspring from two known parents. Cross two heterozygous brown-eyed parents (Bb × Bb) and the expected ratio is 1 BB : 2 Bb : 1 bb — meaning 75% brown eyes, 25% blue.
A pedigree traces a trait across multiple generations of a family, allowing geneticists to determine whether a trait is dominant or recessive, autosomal or sex-linked. Pedigrees are the first tool used to investigate inherited conditions in clinical genetics.
Modern genomics uses whole-genome sequencing to find disease-causing variants directly. CRISPR-Cas9 can now edit specific alleles — read our CRISPR news article for how this is being applied to inherited diseases, and our DNA glossary entry for the molecular details of replication.
Questions about DNA, genes and heredity
- A gene is a defined region of DNA that codes for a functional product (usually a protein). An allele is a particular variant of that gene. Humans have two alleles of most genes — one inherited from each parent. See the NHGRI Genetics Glossary for formal definitions.
- Genotype refers to the specific alleles an organism carries (e.g. Bb for eye colour). Phenotype is the observable result — what you can measure or see. Phenotype depends on genotype but can also be influenced by the environment (e.g. height is genetic but also affected by nutrition). See NCBI Bookshelf: Mendel's Experiments.
- DNA stands for deoxyribonucleic acid. In eukaryotic cells most DNA is in the nucleus, coiled around histone proteins to form nucleosomes and packaged into 46 chromosomes (humans). A separate circular DNA molecule is found in mitochondria (and chloroplasts in plants). See our DNA glossary entry for the structural details.
- Gametes are formed by meiosis and carry one allele per gene. Fertilisation combines two gametes, restoring the diploid chromosome number. The offspring inherits one allele from each parent. Dominant alleles mask the effect of recessive ones in heterozygotes. See NCBI Bookshelf on Mendel and our DNA glossary entry.
See DNA in the cell
DNA is packaged inside the nucleus. Our cells and organelles guide explains the structure of every compartment in the cell.