Environment & Gene Expression: Examples

The theory is done. Now the named examples the IB actually asks about — polluted air changing methyl tags, ligers that outgrow both parents, identical twins who drift apart, and a bacterium that only makes an enzyme when there is something to digest. Learn these as stories and the marks follow.

Air pollution and gene expression

Polluted air is a particular problem in cities and industrial areas. Some of the chemicals in it directly damage lung tissue, which can lead to conditions such as asthma or chronic obstructive pulmonary disease, and it also has a negative effect on cardiovascular health.

The epigenetic part is the interesting bit. Exposure to air pollution can change the methyl tags on DNA and on histone proteins. That alters the pattern of gene expression in the cell, and one result is a rise in inflammation in the body, which brings a higher risk of cardiovascular disease and lung conditions.

Follow the chain in the lungs and you can see how a phenotype changes: inflammation causes scarring, scarring leads to a thickening of the tissue, and thicker tissue means a longer diffusion distance, so the rate of diffusion of oxygen into the blood decreases. Exercise and a diet high in B vitamins have been suggested as ways of reducing the impact.

Erasing the tags: ova, sperm and imprinting

If every methyl tag you picked up from your lifestyle were passed to your children, the epigenome would get messier with every generation. So during egg and sperm development in mammals, most epigenetic tags are removed. This wipes out methylation patterns caused by environmental influences and stops them being handed on.

But not all of them go. Some tags are kept, and others are deliberately added to the DNA of sperm and egg cells, in a process called imprinting.

Here is what imprinting does. Normally you inherit two working copies of a gene, one from each parent. With an imprinted gene, only one copy is expressed — the other is silenced by epigenetic tags. Which copy gets silenced depends on which parent it came from: in sperm development, maternal genes are silenced, while paternal genes are silenced during egg development.

Ligers and tigons

Lions and tigers can be crossbred, and the result depends on which way round the cross goes. A male lion × female tiger gives a liger, which is typically larger than both lions and tigers. A male tiger × female lion gives a tigon, which is about the same size as its parents or even smaller.

Same two species, same genes for growth — so why the difference? Genetic imprinting. Lions and tigers have different reproductive habits, and this affects the expression of the gene responsible for growth. Male lions pass on genes that encourage growth, while female lions have imprinted genes that discourage it. Tigers of either sex do not pass down genes that discourage growth.

Put those together. In a liger, the growth-promoting genes from the male lion arrive alongside a female tiger’s genes, which carry no growth-limiting imprint — so nothing holds growth back and the cub grows much larger than its parents. In a tigon, the female lion contributes the growth-discouraging imprint, so the cub stays around parental size.

Monozygotic twin studies

Variation between members of the same family can come from genetic factors, environmental factors such as a change in skin colour after sun exposure, or a combination of both, as with height and weight. Twins give biologists a way to tell these apart.

Because monozygotic twins are genetically identical, any measurable difference in a trait between them points towards the environment. If separated twins still show similar traits, there is a good chance genes are responsible; big differences suggest environmental factors had an influence.

Comparing the two types sharpens this further. If monozygotic and dizygotic twins show similar traits to each other, exposure to the same environment may be the cause. If monozygotic twins are much more alike than dizygotic twins, the extra similarity is likely to be genetic.

And the epigenetic twist: even with the same genetic information, monozygotic twins still show some variation, and that can be put down to epigenetic changes between them — methylation of DNA or acetylation of histone tails picked up over a lifetime of different experiences.

Hormones as external signals

Steroid hormones such as oestrogen (also called oestradiol), progesterone and testosterone act as ligands that affect gene expression. They are small and lipid-soluble, so they can pass into the cell and bind intracellular receptors, which then influence which genes are transcribed. That mechanism is covered in detail under intracellular receptors.

The lac operon: lactose switching genes on

Bacteria give the neatest example of the environment controlling a gene. Regulatory genes control structural genes and the amount of protein they produce, often several at once. In prokaryotes, structural genes can be grouped into an operon: a cluster of genes controlled by the same promoter.

The best known is the lac operon. It controls production of the enzyme lactase (also called beta-galactosidase) plus two other structural proteins. Lactase breaks down lactose so it can be used as an energy source. It is an inducible enzyme — only synthesised when lactose is present — which stops the bacterium wasting energy and materials making an enzyme with nothing to digest.

Structure of the lac operon

Reading along the DNA, the operon contains a promoter for the structural genes, an operator, then lacZ (codes for lactase), lacY (permease, which lets lactose into the cell) and lacA (transacetylase). Upstream of the operon sits the promoter for the regulatory gene and the regulatory gene lacI, which codes for the lac repressor protein.

The repressor protein has two binding sites. One binds the operator; the other binds lactose, the effector molecule. Everything that follows comes from which of those two sites is occupied.

When lactose is absent

The regulatory gene is transcribed and translated to make the lac repressor protein. The repressor binds to the operator region upstream of lacZ. With the repressor sitting there, RNA polymerase cannot bind to the promoter, so the structural genes are not transcribed and no lactase is made. The bacterium saves its resources.

When lactose is present

Lactose is taken up by the bacterium and binds to the second binding site on the repressor. That distorts the repressor’s shape so it can no longer bind to the operator. The operator is now free, RNA polymerase binds to the promoter, and transcription goes ahead. The mRNA from all three structural genes is translated, lactase is produced, and the lactose is broken down for energy.

Worked examples