B18: Genetic Variation
Variation, mutations and genetic engineering
Genetic Variation
📋 Key Concepts
Genetic Variation: Variation exists between individuals of the same species due to genetic and environmental factors. Mutations create new alleles. Genetic engineering and selective breeding allow humans to modify organisms for specific purposes.
Key Terms
- variation
- genetic variation
- environmental variation
- continuous variation
- discontinuous variation
- mutation
- genetic engineering
- restriction enzymes
- ligase enzymes
- vector
- plasmid
- GM crops
- selective breeding
📝 Variation
Variation: Differences in characteristics between individuals of the same species. Variation can be caused by genetic factors, environmental factors, or a combination of both.
| Type of Variation | Caused by | Examples |
|---|---|---|
| Genetic variation | Differences in alleles inherited from parents | Eye colour, blood group, natural hair colour, inherited diseases |
| Environmental variation | Conditions an organism lives in | Accent, scars, leaf size (due to light), hair dyed a different colour, weight from diet |
| Both genetic and environmental | Genetics set the potential range; environment determines where within that range | Height, weight, skin colour (genetic baseline + sun exposure), intelligence |
Continuous vs discontinuous variation:
- Continuous: A range of values with no distinct categories (e.g. height, weight). Usually influenced by many genes and the environment. Shown as a line graph or histogram.
- Discontinuous: Distinct categories with no in-between values (e.g. blood group A/B/AB/O, eye colour). Usually controlled by a single gene. Shown as a bar chart.
Example 1
Classify each characteristic as continuous or discontinuous variation: (a) blood group, (b) height, (c) tongue rolling ability, (d) foot size.
Solution:
(a) Blood group — discontinuous (distinct categories: A, B, AB, O)
(b) Height — continuous (a range of values with no gaps)
(c) Tongue rolling ability — discontinuous (can roll or cannot roll)
(d) Foot size — continuous (a range of values)
📝 Mutations
Mutation: A change in the DNA sequence of a gene. Mutations happen spontaneously (randomly) and continuously. Most mutations have no effect on the organism.
Effects of mutations:
- No effect: Most mutations have no observable effect on the phenotype (the genetic code is degenerate, so some changes don't alter the protein)
- Harmful: Some mutations produce a protein that does not work properly, which can cause disorders (e.g. cystic fibrosis)
- Beneficial: Rarely, a mutation produces a new protein that gives an advantage, which can lead to evolution by natural selection
Mutation rate can be increased by: ionising radiation (e.g. X-rays, UV light), and certain chemicals (e.g. chemicals in tobacco smoke). These are called mutagens.
Example 2
A mutation occurs in a gene but has no effect on the organism's phenotype. Explain how this is possible.
Solution:
The genetic code is degenerate, meaning that some amino acids are coded for by more than one codon (triplet of bases). A mutation may change a codon to one that codes for the same amino acid, so the protein produced is unchanged. Alternatively, the mutation may occur in a non-coding region of DNA, or the change in the protein may not affect its function.
📝 Genetic Engineering
Genetic engineering: Modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic. The organism that receives the gene is called a genetically modified (GM) organism.
Process of genetic engineering (e.g. producing human insulin in bacteria):
- Identify and isolate the desired gene (e.g. the human insulin gene)
- Cut out the gene from human DNA using a restriction enzyme
- Cut open a bacterial plasmid (the vector) using the same restriction enzyme
- Insert the human gene into the plasmid using ligase enzyme (which joins DNA strands together)
- The plasmid is now a recombinant plasmid (contains DNA from two different organisms)
- Insert the recombinant plasmid into a bacterium
- The bacterium divides and produces human insulin, which can be harvested and purified
Key enzymes:
Restriction enzymes — cut DNA at specific sequences
Ligase enzymes — join DNA pieces together
Restriction enzymes — cut DNA at specific sequences
Ligase enzymes — join DNA pieces together
Example 3
Describe how bacteria are genetically engineered to produce human insulin.
Solution:
1. The human insulin gene is identified and cut out of human DNA using a restriction enzyme.
2. A bacterial plasmid (the vector) is cut open using the same restriction enzyme, creating complementary sticky ends.
3. The human insulin gene is inserted into the plasmid and joined using ligase enzyme.
4. The recombinant plasmid is inserted into a bacterium.
5. The bacterium divides, and each new cell contains the insulin gene. The bacteria produce human insulin, which is harvested and used to treat people with diabetes.
📝 GM Crops
| Advantages of GM Crops | Disadvantages of GM Crops |
|---|---|
| Increased crop yield (more food from same land) | Unknown long-term effects on human health |
| Pest resistance (reduces need for pesticide chemicals) | Could reduce biodiversity (e.g. harm non-target insects) |
| Disease resistance | GM genes could spread to wild plants (cross-pollination) |
| Can grow in difficult conditions (drought, poor soil) | GM seeds are expensive — farmers in poorer countries may not afford them |
| Enhanced nutritional value (e.g. golden rice with beta-carotene/vitamin A) | Dependence on large biotechnology companies for seeds |
📝 Selective Breeding
Selective breeding: Choosing parents with the desired characteristics, breeding them together, and selecting the best offspring to breed from again. This is repeated over many generations to develop a population with the desired trait.
Process of selective breeding:
- Choose parents with the desired characteristics from a mixed population
- Breed them together
- Select the best offspring showing the desired characteristics
- Breed these offspring together
- Repeat over many generations until all offspring show the desired characteristic
Examples of selective breeding:
- Dairy cows — bred to produce more milk
- Meat cattle — bred for lean muscle/meat yield
- Disease-resistant crops — bred to survive fungal or bacterial infections
- Domestic dogs — bred for temperament, appearance, or working ability
- Crops for larger or tastier fruit/vegetables
Disadvantages of selective breeding:
- Reduction in the gene pool (genetic variation decreases over generations)
- Inbreeding can lead to increased risk of inherited disorders (e.g. hip problems in some dog breeds)
- Vulnerability to new diseases (less genetic variation means less chance of resistance)
Example 4
Compare genetic engineering and selective breeding, giving one advantage and one disadvantage of each.
Solution:
Genetic engineering — advantage: can introduce characteristics from a different species (e.g. human insulin gene into bacteria); disadvantage: potential unknown long-term effects and ethical concerns.
Selective breeding — advantage: uses natural reproduction to enhance desired traits (no artificial gene transfer); disadvantage: takes many generations and reduces the gene pool, increasing susceptibility to disease.
❓ Practice Questions
Q1: Explain the difference between continuous and discontinuous variation, giving one example of each.
Q2: Describe what a mutation is and explain why most mutations have no effect on the phenotype.
Q3: Describe the process of genetic engineering used to produce human insulin in bacteria, naming the enzymes involved at each step.
Q4: Give two advantages and two disadvantages of GM crops.
Q5: Describe the process of selective breeding and explain one disadvantage of this technique.
✅ Answers
- Continuous variation has a range of values with no distinct categories (e.g. height, weight) and is usually influenced by many genes and the environment. Discontinuous variation has distinct categories with no intermediate values (e.g. blood group, eye colour) and is usually controlled by a single gene.
- A mutation is a change in the DNA sequence of a gene. Most mutations have no effect because the genetic code is degenerate (some amino acids are coded for by more than one codon), so a change may not alter the amino acid produced. Also, mutations may occur in non-coding regions of DNA.
- 1. Identify and cut out the human insulin gene from human DNA using a restriction enzyme. 2. Cut open a bacterial plasmid using the same restriction enzyme (creates complementary sticky ends). 3. Insert the human gene into the plasmid using ligase enzyme. 4. Insert the recombinant plasmid into a bacterium. 5. The bacterium multiplies and produces human insulin, which is harvested and purified.
- Advantages: increased yield, pest resistance (reduces pesticide use), enhanced nutrition, disease resistance. Disadvantages: unknown long-term health effects, potential harm to biodiversity, GM genes may spread to wild plants, expensive seeds, dependence on biotechnology companies.
- Choose parents with desired characteristics from a mixed population. Breed them together. Select offspring with the desired characteristics and breed them. Repeat over many generations. Disadvantage: reduces the gene pool (less genetic variation), which increases risk of inherited disorders and vulnerability to new diseases due to inbreeding.
🎯 Exam Tips
- Continuous variation = range (like a bell curve); discontinuous = distinct categories (like bar chart)
- Most mutations have NO effect — don't say "all mutations are harmful"
- Learn the genetic engineering steps in order and name the enzymes: restriction enzyme to CUT, ligase to JOIN
- The plasmid is the VECTOR — it carries the gene into the bacterium
- When discussing GM crops, always give balanced arguments (advantages AND disadvantages)
- Selective breeding is SLOW (many generations); genetic engineering is FASTER (direct gene transfer)
- Selective breeding reduces the gene pool — this is a key disadvantage to remember
- Don't confuse genetic engineering (transferring genes between species) with selective breeding (choosing parents within the same species)
🔢 Maths Skills
Mathematical Skills
Minimal maths in this topic: Key skills include interpreting bar charts and frequency diagrams for continuous vs discontinuous variation, and comparing proportions in GM crop yield data. When evaluating data on GM versus non-GM crops, calculate percentage differences: e.g. if GM yield is 8.2 tonnes/ha and non-GM is 6.5 tonnes/ha, the percentage increase = ((8.2 − 6.5) / 6.5) × 100 = 26.2%.
⚠️ Common Misconceptions
Watch Out!
1. Genetic engineering is the same as selective breeding. Wrong: genetic engineering and selective breeding are identical processes. Correct: genetic engineering transfers genes between different species (e.g. human insulin gene into bacteria); selective breeding chooses parents within the same species over many generations.
2. GM food is dangerous to eat. Wrong: eating GM food is harmful to human health. Correct: there is no scientific evidence that approved GM foods are harmful to eat; all GM crops are rigorously tested for safety before being approved for consumption.
✍️ 6-Mark Question
Extended Answer
6 marks: Evaluate the advantages and disadvantages of genetic engineering.
Genetic engineering has significant advantages: it can introduce desirable characteristics that would not be possible through selective breeding because genes can be transferred between species — for example, bacteria engineered to produce human insulin provide a reliable supply for diabetes treatment. GM crops can have increased yield, pest resistance (reducing pesticide use), and enhanced nutrition (e.g. golden rice with vitamin A). However, there are disadvantages and concerns: the long-term effects on human health and ecosystems are not fully known; GM genes could spread to wild plants through cross-pollination, potentially creating "superweeds"; GM seeds are expensive and create dependence on biotechnology companies; and some people have ethical objections to modifying organisms. Overall, the benefits of genetic engineering for medicine and food security are significant, but regulation and careful monitoring are essential to manage the risks.
Mark scheme: 1 mark — at least two advantages given and explained; 1 mark — advantage of cross-species gene transfer; 1 mark — at least two disadvantages given and explained; 1 mark — environmental or health risk explained; 1 mark — ethical or economic concern; 1 mark — balanced evaluation with a reasoned conclusion.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A study compared GM maize (pest-resistant) with non-GM maize over three years. Average yields: GM = 9.1 tonnes/ha, non-GM = 7.3 tonnes/ha. Pesticide used: GM = 1.2 kg/ha, non-GM = 3.8 kg/ha. However, the GM field had 30% fewer butterfly species than the non-GM field. Calculate the percentage difference in yield and pesticide use. Evaluate whether growing GM maize is justified, considering both the agricultural benefits and the ecological concerns. What further data would you need to make a more informed decision?
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