Quick questions on Genes, environment and epigenetics: VCE Biology Unit 2

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

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

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.

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.

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).

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

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.

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.

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

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.

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

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]

Refer to honeybees. (a) State the trigger that switches a larva towards becoming a queen rather than a worker. (b) Identify the epigenetic mechanism.