Genetics

What this part of the map covers

Genetics is the study of individual genes and their roles in inheritance, the passing of DNA from parents to children.^{[10, 9]} It draws on MedlinePlus Genetics (US National Library of Medicine), the CDC (US), the US Department of Energy (DOE) and an open textbook by Gerald Bergtrom of the University of Wisconsin-Milwaukee. It explains; it gives no medical advice.

DNA: the molecule of heredity

DNA (deoxyribonucleic acid) is the hereditary material in humans and almost all other organisms.^[1] Its information is stored as a code of four chemical bases: adenine (A), guanine (G), cytosine (C) and thymine (T).^[1] The order of those bases carries the instructions for building and maintaining an organism, much as the order of letters spells out words and sentences.^[1]

A base, a sugar and a phosphate together make a nucleotide, and nucleotides line up in two long strands that wind into a spiral called a double helix.^{[1, 9]} The bases pair up, A with T and C with G, forming base pairs.^[1] Picture a twisted ladder: the base pairs are the rungs, and the sugars and phosphates are the sides.^{[1, 9]}

Griffith’s “transforming principle” experiments with bacteria led to studies that proved DNA was the stuff of genes.^[11] Later, from the finding that every species has equal amounts of A and T, and of G and C, Watson and Crick inferred how the bases pair.^[12]

Genes and chromosomes

A gene is the basic physical and functional unit of heredity, and genes are made of DNA.^[2] MedlinePlus Genetics estimates that the human genome holds about 19,900 genes used to make proteins; many other genes make no protein and instead help control other genes.^[2] People typically have two copies of each gene, one from each parent, and versions of the same gene that differ slightly in their DNA are called alleles.^[2]

Inside the nucleus, DNA is packed into thread-like chromosomes, coiled many times around supporting proteins called histones.^{[3, 13]}

Copying and repairing DNA

Each strand of the double helix can act as a pattern for copying the sequence of bases.^{[1, 14]} One new strand grows continuously, while the other is built in short pieces called Okazaki fragments.^[14]

Copying is prone to errors, and some copying enzymes proofread and correct many of them.^{[15, 14]} When proofreading misses a wrong base, a backup system called mismatch repair can step in.^[15] When the BRCA repair proteins work poorly, damaged DNA is not fixed efficiently, the likely basis of a raised chance of breast cancer.^[15]

From gene to protein

Making a protein from a gene takes two steps, transcription and translation, together called gene expression.^[4] In transcription, an enzyme called RNA polymerase binds at a site called the promoter and copies the gene’s information into RNA, a similar molecule that uses uracil (U) in place of thymine.^{[16, 4]} In translation, a ribosome reads the RNA three bases at a time; each three-base codon usually stands for one amino acid, and a “stop” codon ends the chain.^{[4, 18]}

Proteins are long chains built from 20 kinds of amino acids, and their order sets each protein’s shape and job.^[5] This flow of information from DNA to RNA to protein is sometimes called the central dogma of molecular biology.^[4] With a few exceptions, such as mitochondria and some prokaryotes, the genetic code is the same from bacteria to humans.^[17]

Switching genes on and off

Short DNA stretches called enhancers bind proteins called transcription factors that speed up a gene’s transcription, and they can lie thousands of base pairs away from that gene.^[19] Epigenetics is defined by the DOE as the study of how gene expression can change without the genes themselves changing.^[10] Bergtrom describes it as heritable changes in chromatin and gene expression that dividing cells pass on, which helps explain why tissues differ.^[20]

Gene variants

A gene variant is a permanent change in a gene’s DNA sequence.^[6] It used to be called a mutation, and the two words are still used for the same thing, but because DNA changes do not always cause disease, “variant” is thought to be more accurate.^{[6, 7]} A variant may swap one base for another, change one amino acid, create an early stop signal, or add or remove bases and shift every later codon (a frameshift).^[7]

Inherited variants pass from parent to child and are present in virtually every cell.^[6] Others arise during life, from things like the sun’s ultraviolet radiation or copying errors, and cannot be passed on.^[6]

How traits are inherited

Some conditions come from a variant in a single gene.^[8] In autosomal dominant inheritance, one altered copy is enough to cause the condition; in autosomal recessive inheritance, both copies are altered.^[8] In X-linked inheritance, fathers cannot pass the trait to their sons.^[8]

Genomes and genomics

A genome is an organism’s complete set of DNA, including all its genes.^{[10, 9]} The US Department of Energy describes the Human Genome Project, a ten-year effort led by the US government, as culminating in the first complete sequencing of a human genome in 2000.^[10] Human DNA has about 3 billion bases, and more than 99 percent of them are the same in everyone.^[1]

Common misconceptions

“A mutation means disease.” Most variants do not lead to disease.^[6]

“Every gene makes a protein.” Many genes help control other genes instead.^[2]

What this page does not cover

This overview does not cover Gregor Mendel or the wider history of genetics, incomplete dominance or codominance, Punnett squares or inheritance odds, the full table of codons, sex chromosomes beyond X-linked inheritance, how many chromosomes people have, genetic testing or gene therapy.

Going further

See Cells, Evolution, Medicine and Anatomy.