What epigenetics is

Most genes follow the inheritance rules Mendel worked out, so you can predict the offspring of a cross from the parents. This chapter covers three exceptions. The first, epigenetics, changes how a gene is used without changing the gene itself.

Three rules and three exceptions

Inheritance that follows Mendel’s pattern, the kind Chapter 17 covered, obeys three rules:

1. Apart from rare mutations, genes pass unaltered from cell to cell and from parent to offspring.
2. Genes obey the law of segregation: a parent’s two alleles separate, one into each gamete.
3. Genes obey the law of independent assortment: alleles of different genes sort into gametes independently of one another.

Each pattern in this chapter breaks one of those rules, which is how you tell them apart.

Check yourselfTwo genes sit right next to each other on the same chromosome. Which of Mendel’s rules are they likely to break?

The law of independent assortment. Genes that close together tend to be inherited as a unit instead of sorting into gametes separately.

Changes that copy along with the cell

Every cell in your body carries the same genes, yet a muscle cell and a skin cell use different ones. Early in the embryo, cells that will become muscle mark the genes for actin and myosin, the proteins that make muscle contract, so those genes stay on. Other cells mark the same genes so they stay off. Each time a cell divides, its daughters inherit the marks, so adult muscle cells still make actin and myosin and most other cells don’t.

Chapter 15 called a change like this an epigenetic change. That kind of lasting, copyable switch is the subject of epigenetics: the study of changes in gene expression that are passed from cell to cell and can be reversed, but don’t alter the DNA sequence. A heritable change in gene expression of this kind is called an epimutation. The prefix epi- means “over”: the control sits on top of the sequence rather than in it.

Some epigenetic changes last a lifetime, like those on the muscle genes. Others are temporary. Many flowering plants, for example, must go through a cold winter before they can flower. The cold places epigenetic marks on flowering genes, the marks last through the flowering season, and then they’re erased.

Passed to daughter cells or to children

Some epigenetic changes travel from one generation to the next. Epigenetic inheritance is the passage of an epigenetic change from parent to offspring through sperm or egg. Genomic imprinting, in the next segment, is an example.

Many epigenetic changes never get that far. A chemical in cigarette smoke can cause an epigenetic change in a lung cell. The change passes to that cell’s descendants and may help cause lung cancer, but it doesn’t reach the smoker’s children.

Check yourselfA skin cell picks up an epigenetic mark that silences a gene. Will the cell’s daughter cells carry the mark? Will the person’s children?

The daughter cells will, because epigenetic changes pass from cell to cell. The children won’t, because only changes carried in sperm or eggs reach offspring.

Four ways to mark chromatin

Epigenetic effects come from the same chromatin changes Chapter 14 showed controlling transcription. The four most common are DNA methylation, chromatin remodeling (moving or evicting nucleosomes), covalent histone modification (tagging histone tails with chemical groups), and placing histone variants at particular sites.

The same four changes also handle short-term gene regulation inside a single cell. A change counts as epigenetic only when it persists through cell divisions.

Check yourselfA signal briefly triggers histone acetylation at a gene, and the mark is gone by the cell’s next division. Is this an epigenetic change?

No. The chromatin change is real, but an epigenetic change must be passed from cell to cell. A mark that doesn’t survive cell division is ordinary short-term gene regulation.