relationships between genes, the environment and the regulation of genes in producing variation in phenotype, including the role of epigenetic factors

What this dot point is asking

VCAA wants you to explain how the same genotype can produce different phenotypes depending on environment and epigenetic regulation, and how genes are switched on and off by mechanisms that do not change the DNA sequence itself.

The answer

Genotype to phenotype is not one-to-one

The simple rule (genotype produces phenotype) is incomplete. The same genotype can produce different phenotypes when:

1. The environment changes (temperature, nutrition, hormones, pH, stress).
2. Epigenetic modifications switch genes on or off.
3. Stochastic (random) variation in gene expression at the cellular level produces different outcomes.

Phenotype = genotype + environment + epigenetics + chance.

Environmental effects on phenotype: examples

Arctic foxes carry one genotype for coat colour but grow white fur in winter and brown fur in summer, triggered by photoperiod and temperature. The same genome; two phenotypes within one individual.

Hydrangea flower colour depends on soil pH. Acidic soils (pH < 6) make aluminium available, producing blue flowers. Alkaline soils make the same plant produce pink flowers. Genotype identical; environment dictates phenotype.

Himalayan rabbits are mostly white but have dark fur on the cool extremities (ears, nose, paws, tail). The melanin-producing enzyme is heat-sensitive: it folds correctly only at low temperatures. So pigment forms only where the body surface is cold. Genotype identical across the body.

Phenylketonuria (PKU). A baby with two recessive alleles for PKU cannot break down phenylalanine. Without dietary intervention, phenylalanine builds up and damages the brain. With a low-phenylalanine diet, the child develops normally. Same genotype; phenotype controlled by environment (diet).

Identical twins. Monozygotic twins start with identical genotypes but diverge phenotypically as they grow up, in disease risks, weight, behaviour, and even DNA methylation patterns. Differences in nutrition, exercise, stress, sleep and chance environmental exposures accumulate.

Epigenetics: regulation without changing the DNA sequence

Epigenetics is the study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence. The two main mechanisms:

1. DNA methylation. A methyl group (-CH3) is added to a cytosine base, almost always at CpG sites (cytosine followed by guanine). The enzymes are DNA methyltransferases (DNMTs).

• Heavy methylation in or near a gene's promoter typically silences the gene (blocks transcription factor and RNA polymerase binding).
• Removal of methyl groups (by passive dilution during DNA replication, or active demethylases) reactivates the gene.

DNA methylation is the dominant mechanism that turns whole genes off in particular cell types or developmental stages.

2. Histone modification. Histone proteins (the spools DNA wraps around) have tails sticking out that can be chemically modified.

• Acetylation of histone tails (adding acetyl groups, by histone acetyltransferases, HATs) loosens the chromatin, making the DNA more accessible to transcription factors: gene expression rises.
• Deacetylation (by histone deacetylases, HDACs) compacts the chromatin: gene expression falls.
• Methylation of histone tails can either activate or silence depending on which residue is methylated.

Together, DNA methylation and histone modification set the epigenetic state of each gene in each cell type.

3. Non-coding RNAs (such as microRNAs and long non-coding RNAs) also regulate gene expression, sometimes durably enough to count as epigenetic.

Why epigenetics matters

Cell differentiation

Every cell in your body has the same genome but they express very different genes. A liver cell has methylated, silenced muscle genes; a muscle cell has methylated, silenced liver genes. Differentiation is largely an epigenetic process that locks in cell identity.

X-inactivation

In female mammals, one of the two X chromosomes in each cell is largely silenced by heavy methylation and other epigenetic marks, producing a Barr body. This balances X-gene dosage between males (XY) and females (XX). The choice of which X is inactivated is random in each cell, producing the patchy phenotype of calico cats.

Imprinting

Some genes are expressed only from the maternal or paternal copy based on the epigenetic mark inherited from the parent. About 1% of human genes are imprinted; disruption causes diseases such as Prader-Willi and Angelman syndromes.

Disease

Aberrant methylation patterns are central to many cancers (silencing of tumour suppressor genes). Diet, smoking, stress and pollutants can alter the methylome.

Trans-generational effects

The Dutch Hunger Winter (1944 to 1945) caused severe famine for pregnant women. Children conceived during the famine had altered methylation at metabolic genes (such as IGF2) and increased risk of obesity, diabetes and cardiovascular disease decades later. Some of these epigenetic marks were detectable into the second generation. This suggests environmental exposures can leave an inheritable epigenetic signature, though the degree of trans-generational inheritance in humans is debated.

Comparing genetic and epigenetic variation

Implications for phenotype

A trait's phenotype reflects:

• The alleles at the gene loci involved (genotype).
• The environment the organism develops and lives in.
• The epigenetic state of those genes (which is partly set by environment, partly by developmental programme, partly by inheritance).
• Random variation in gene expression and developmental noise.

This explains why even identical twins differ; why a clone is not a perfect copy of its original; why heritability of traits like height or intelligence is high but never 100%; and why diet and lifestyle matter for disease risk regardless of genotype.

Examples in context

Example 1. Tasmanian devil DFTD and gene methylation at University of Tasmania. University of Tasmania researchers studying devil facial tumour disease (DFTD) found that the cancer evades the immune system by hypermethylating MHC class I gene promoters. The methylation silences MHC expression so the tumour cells become invisible to the devil's T cells. Crucially, the underlying DNA sequence is unchanged - the change is epigenetic. Drugs that demethylate DNA (such as 5-azacytidine) restore MHC expression and immune recognition. The Save the Tasmanian Devil Program now uses this approach in experimental treatments, illustrating how environmental selection on epigenetic marks can change a tumour's phenotype without any sequence mutation.

Example 2. Honeybee queen vs worker phenotype. Honeybees demonstrate that the same genome can give two radically different phenotypes through epigenetics. Larvae fed royal jelly become queens with full ovaries, while genetically identical larvae fed worker jelly become sterile workers. The difference is methylation of the DNMT3 gene and downstream developmental genes. Australian commercial honeybee breeders in Victoria, including those supplying the Royal Melbourne Show, harness this by selecting larvae and grafting them into queen cups. The example shows that phenotype is a function of (genes plus environment plus epigenetic regulation), not genes alone.

Try this

Q1. Define epigenetics and give two examples of epigenetic modifications. [2 marks]

• Cue. Heritable changes in gene expression not due to changes in DNA sequence; e.g. DNA methylation, histone acetylation.

Q2. Identical twins separated at birth show different rates of type 2 diabetes in middle age. Explain how epigenetics can account for this discordance despite identical genomes. [3 marks]

• Cue. Differences in diet, exercise and stress drive different methylation patterns at metabolic genes; phenotypes diverge with age.

Q3. Refer to honeybees. (a) State the trigger that switches a larva towards becoming a queen rather than a worker. (b) Identify the epigenetic mechanism. (c) Predict what would happen if you fed worker jelly to a larva but blocked DNA methylation pharmacologically. [2+2+2 marks]

• Cue. (a) Royal jelly diet. (b) DNA methylation differences at DNMT3 and developmental genes. (c) Methylation patterns would resemble those of queens; larva develops queen-like phenotype despite worker diet.