Genotype, phenotype and inheritance (3.14, 3.15)
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- Genotype: the genetic makeup of an organism; the alleles it possesses.
- Phenotype: the observable characteristics of an organism.
Phenotype results from the interaction between an organism’s genotype and its environment. For example, height in humans is influenced by both genes and nutrition.
Epigenetics refers to changes in gene expression that do not alter the DNA sequence. Instead, epigenetic changes can activate or silence genes.
The following are mechanisms that regulate gene expression:
- DNA methylation: addition of methyl groups () to DNA, usually at cytosine bases. This reduces transcription, so the gene is more likely to be silenced.
- Histone Modification: addition or removal of chemical groups such as acetyl or methyl groups to histone proteins. Acetylation loosens DNA around histones and results in gene activation. Deacetylation and methylation cause DNA to get more tightly wrapped around histones and results in gene silencing.
Some epigenetic changes can be maintained through mitosis, allowing daughter cells to
retain the same patterns of gene expression as the parent cell. This helps maintain their specialised structure and function.
Some epigenetic changes are reversible, allowing gene expression to be dynamically regulated in response to developmental or environmental signals.
Phenotypic variation can be caused by environmental factors.
Environmental factors influence phenotype through external conditions, for example:
- Diet: affects growth, development and other characteristics in animals.
- Light: lack of light can cause etiolation in plants, causing elongated stems and reduced leaf development as they try to grow towards a source of light.
Environmental factors can influence the expression of traits and, together with genetic factors, contribute to continuous variation.
Continuous variation shows a range of phenotypes with no distinct categories (e.g., height, body mass).
Continuous variation is usually controlled by many genes (polygenic inheritance), each contributing a small additive effect. It is also influenced by environmental factors (e.g., nutrition, climate).
When data are collected from a large population and plotted, they typically form a normal distribution curve.