Genotype, environment and epigeneticsEdexcel International A Level Biology: Revision notes

The genotype is the set of alleles an organism has. The phenotype is its observable characteristics. Phenotype results from the interaction of genotype and environment.

Examples: genetically identical hydrangea plants have blue flowers in acidic soil and pink flowers in neutral soil; height depends on both genes and nutrition. The environment changes how genes are expressed; it does not change the alleles.

Epigenetic modifications change gene activity without changing the DNA base sequence.

In DNA methylation, methyl groups are added to cytosine bases, often in the promoter region of a gene. This stops transcription factors binding, so transcription is reduced and the gene is switched off (silenced). Removing methyl groups can switch a gene back on.

DNA is wound around histone proteins. Chemical changes to histones alter how tightly the DNA is held.

• Removing acetyl groups (deacetylation) makes histones bind DNA more tightly. The chromatin is condensed, transcription factors and RNA polymerase cannot reach the gene and it is switched off.
• Adding acetyl groups loosens the chromatin, so the gene can be transcribed and is activated.

Epigenetic marks are copied and passed on when a cell divides: the pattern of DNA methylation is copied onto the new strand after replication, and histone modifications are retained. Daughter cells therefore keep the same genes switched on and off, which helps cells stay specialised.

The environment (diet, smoking, stress) can alter epigenetic marks. This helps explain why identical twins become more different as they age even though their DNA sequences are identical.

Some genes have more than two alleles in a population, although an individual still carries only two. Example: ABO blood groups, controlled by one gene with three alleles, $I^A$, $I^B$ and $I^O$. $I^A$ and $I^B$ are codominant and both are dominant to $I^O$. The genotypes give four phenotypes: A, B, AB and O. This is discrete variation: separate categories with no intermediates.

Polygenic inheritance is when a characteristic is controlled by many genes at different loci, each with a small, additive effect. Examples are height and skin colour.

The many allele combinations, plus environmental effects, give a continuous range of phenotypes called continuous variation. A graph of the data is a bell-shaped (normal) distribution. Discrete variation, by contrast, usually comes from one gene and is little affected by the environment.