Phenotype, genotype and the environmentEdexcel A-Level Biology A: Revision notes
Section 1
Genotype, phenotype and the environment
The genotype is the alleles an organism has. The phenotype is its observable characteristics. The phenotype is the result of an interaction between the genotype and the environment.
For example, genetically identical hydrangea cuttings give pink flowers in alkaline soil and blue flowers in acidic soil. Identical twins have the same genotype but can differ in height, weight or health because of differences in diet, lifestyle and environment.
The environment can affect how genes are expressed, but it does not change the base sequence of the DNA.
Environmental effects on phenotype are not mutations. The DNA sequence is the same; gene expression differs.
Section 2
Epigenetics: DNA methylation
Epigenetic changes alter how genes are expressed without changing the base sequence of the DNA.
In DNA methylation, methyl groups are added to cytosine bases, often in the promoter region of a gene. Methylation stops transcription factors and RNA polymerase binding, so the gene is less transcribed or switched off. Less mRNA and protein are made, which can change the phenotype.
Example: in agouti mice, high methylation near the agouti gene leads to a brown, lean coat instead of a yellow, obese one.
Section 3
Epigenetics: histone modification
DNA is wound around histone proteins. The state of the histones affects how tightly the DNA is packed.
- Adding acetyl groups to histones loosens the DNA, so transcription factors and RNA polymerase can reach the gene and transcription increases.
- Removing acetyl groups (deacetylation) makes the DNA wind more tightly and transcription decreases.
So a gene can be switched on or off by modifying the histones, again without changing the base sequence.
Methylation of DNA usually switches genes off; acetylation of histones usually switches genes on.
Section 4
Passing on epigenetic changes
Environmental factors such as diet, smoking and stress can alter methylation and histone modification. These changes can be passed on to daughter cells when cells divide by mitosis, so the pattern is maintained in a tissue.
Identical twins show this. When young, their methylation patterns are similar, but as they get older they diverge because of different lifestyles and environments, and the differences can affect health and risk of diseases such as cancer. If methylation switches off a tumour suppressor gene, less of the protein that controls cell division is made, so cells may divide uncontrollably.
Section 5
Polygenic inheritance and continuous variation
Some characteristics are controlled by alleles at many loci (polygenic inheritance), often with each allele having a small effect. Examples are height, skin colour and mass.
Because there are so many combinations of alleles, and the environment also affects the phenotype, there is a continuous range of phenotypes with no distinct categories. This is continuous variation. The distribution is usually bell-shaped, with most individuals near the mean and few at the extremes.
In contrast, a characteristic controlled by one gene, such as a blood group, shows discontinuous variation, with distinct categories.
Continuous variation is caused by genes and the environment together, not by the environment alone.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Phenotype, genotype and the environment
- A gardener takes cuttings from a single hydrangea plant with pink flowers and grows them in two soils. Cuttings grown in alkaline soil have pink flowers, but cuttings grown in acidic soil have blue flowers. All the cuttings are genetically identical to the parent plant.Explain why the cuttings have the same genotype but different phenotypes.2 marks
- Mice with a particular allele of the agouti gene would normally have yellow coats and become obese. When pregnant mothers are fed a diet rich in methyl groups, many of their genetically identical offspring have brown coats and are lean. Cells from the brown mice have many more methyl groups attached to the DNA near the agouti gene than cells from the yellow mice.Explain how a difference in DNA methylation can lead to a difference in coat colour without a change in the base sequence of the DNA.2 marks
- Identical twins have the same genotype. A study compared patterns of DNA methylation in twin pairs. At the age of three, the patterns in the two twins were very similar, but by the age of fifty they differed greatly. In one pair, the twin with a tumour had 72% of the CpG sites in the promoter of a tumour suppressor gene methylated, compared with 18% in the twin without a tumour.Explain why the methylation patterns of the twins became different as they got older, and why the differences are found in many of their cells.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).