Epigenetics and continuous variation

Genes, environment and epigenetics

Phenotype is the result of genotype interacting with the environment: for example a plant's height depends on its alleles and on light, water and minerals.

Epigenetic changes alter whether genes are switched on or off without changing the DNA base sequence:

• DNA methylation: methyl groups are added to cytosine bases. This usually prevents transcription, switching the gene off.
• Histone modification: DNA is wound round proteins called histones. Adding acetyl groups (acetylation) loosens the packing, so genes can be transcribed; removing them packs DNA tightly, switching genes off.

These changes can be caused by environmental factors (such as diet, stress or toxins) and can be passed on to daughter cells when cells divide.

Discontinuous and continuous variation

• Discontinuous variation: distinct categories, usually controlled by one gene, for example ABO blood group.
• Continuous variation: a range of values with no clear categories (for example height, skin colour), usually forming a normal distribution. It arises because the trait is polygenic (controlled by many genes, each with a small additive effect) and is also influenced by the environment.

Multiple alleles

Some genes have more than two alleles in the population, although each person has only two. The ABO blood group gene has three: I^A and I^B are codominant and I^O is recessive. So there are four phenotypes (A, B, AB, O) from six genotypes.

Worked example

A parent with genotype I^A I^O and a parent with I^B I^O have a child. What is the probability the child is blood group O?

1. Gametes: I^A or I^O; I^B or I^O.
2. Offspring: I^A I^B, I^A I^O, I^B I^O, I^O I^O.
3. Only I^O I^O is group O: 1 in 4.

Answer: 0.25

Check yourself

1. Parents with genotypes I^A I^O and I^B I^O have a child. What is the percentage chance the child is blood group AB?
   Show answer

   25 %. I^A I^B is 1 of 4 outcomes: 25%.

2. How many different ABO blood group phenotypes are there?
   Show answer

   4. A, B, AB and O.

3. What is the usual effect of methylation of DNA in a gene?
   • The base sequence changes
   • The gene is copied more
   • The gene is switched off, as transcription is prevented
   • The gene is deleted
   Show answer

   The gene is switched off, as transcription is prevented. Methylation silences genes without changing the sequence.

4. Acetylation of histones usually
   • packs DNA tightly so genes are switched off
   • loosens DNA packing so genes can be transcribed
   • changes the base sequence
   • removes introns
   Show answer

   loosens DNA packing so genes can be transcribed. Looser chromatin lets transcription factors and RNA polymerase reach the gene.

5. Which statement about epigenetic changes is correct?
   • They are always mutations
   • They cannot be passed to daughter cells
   • They only happen in bacteria
   • They change gene expression without changing the DNA base sequence
   Show answer

   They change gene expression without changing the DNA base sequence. Epi = on top of the genes.

6. Why does human height show continuous variation?
   • It is controlled by one gene with two alleles
   • It is polygenic and also affected by the environment
   • It is sex-linked
   • It is only determined by diet
   Show answer

   It is polygenic and also affected by the environment. Many genes plus environment give a smooth range.

7. Which is an example of discontinuous variation?
   • Height
   • Body mass
   • Leaf length
   • ABO blood group
   Show answer

   ABO blood group. Blood groups fall into distinct categories.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.