1. Reproduction Recap: Where Variation Comes From
In sexual reproduction, a sperm cell (from the father) and an egg cell (from the mother) join together in fertilisation to form a new individual. Because that new individual receives a mixture of genetic information from both parents, it will never be genetically identical to either parent — this mixing is one of the main sources of variation between offspring.
This is different from asexual reproduction, where there's only one parent, so the offspring is genetically identical to it (a "clone"). Sexual reproduction is one of the reasons variation exists between individuals of the same species — but as you'll see below, it isn't the only one.
2. What Is Variation?
Variation means the differences that exist between individuals of the same species. No two humans (other than identical twins) are genetically identical — variation is what makes each of us unique.
Two Types of Variation
Continuous Variation in height of individuals
Discontinuous Variation observed blood groups
What Causes Variation?
Variation can be caused by:
1. Genetic causes: Differences inherited from parents (e.g. eye colour, blood group, natural hair colour).
2. Environmental causes: Differences caused by an organism's surroundings and lifestyle (e.g. a scar, muscle size from exercise, a suntan, a plant's height depending on how much light or water it received).
3. A combination of both: Many characteristics, like height and weight, are influenced by both genes AND environment (e.g. someone might be genetically tall, but poor nutrition could still limit how tall they actually grow).
4. Mutation: A random change in the DNA code itself.
Quick test: If you can change a characteristic through your environment or lifestyle (diet, exercise, sun exposure), it likely has an environmental cause. If nothing about your environment can change it (like your natural eye colour), it's genetic.
How Do Scientists Know Whether a Characteristic Is Genetic or Environmental? A Case of Twin Studies
One way scientists investigate this is by studying identical twins. Identical twins develop from a single fertilised egg that splits in two, so they share 100% of their genes. If identical twins are raised in different environments (different homes, diets, or countries) and still share a characteristic, scientists conclude that characteristic is mostly genetic. If the characteristic differs between them despite identical genes, environment must be playing a role.
Example: Studies of identical twins raised apart have shown that natural eye colour is almost always identical (strongly genetic), while things like body weight or exam results can differ noticeably (more strongly influenced by environment).
3. What Is Inheritance?
Inheritance is the passing on of characteristics from parents to offspring through genes. To understand inheritance, it helps to see how genes fit inside a cell.
Key Structures
Genotype vs Phenotype
Two terms that often confuse students, but are simple once explained clearly:
- Genotype: The actual combination of alleles an individual has (e.g. Rr). This is the genetic "code."
- Phenotype: The physical characteristic that is actually observed (e.g. "can roll tongue"). This is what the code produces.
Example: An individual with the genotype Rr has the phenotype "can roll tongue," because R (rolling) is dominant over r (non-rolling).
Dominant and Recessive Alleles
For many characteristics, an individual inherits two alleles for each gene — one from each parent. These alleles can be:
- Dominant: Only one copy is needed for the characteristic to appear; usually written as a capital letter (e.g. R)
- Recessive: Two copies are needed for the characteristic to appear; usually written as a lowercase letter (e.g. r)
If an individual has one dominant and one recessive allele (genotype Rr), the dominant characteristic will be the one that shows in the phenotype — but they can still pass the recessive allele on to their children. Someone with this genotype is called a carrier of the recessive allele.
Sex Determination: How X and Y Chromosomes Decide Biological Sex
One pair of human chromosomes is special — the sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Every egg cell carries an X chromosome, but sperm cells can carry either an X or a Y chromosome. This means it's the father's sperm that determines a baby's biological sex:
- If an X-carrying sperm fertilises the egg → XX → female
- If a Y-carrying sperm fertilises the egg → XY → male
Each outcome has an equal, roughly 50% chance, which is why sex ratios at birth are close to 1:1.
Mutation
A mutation is a random, spontaneous change in an organism's DNA code. Mutations can happen naturally during cell division, or be caused by exposure to things like radiation or certain chemicals. Most mutations have no effect or a harmful effect, but occasionally a mutation can be beneficial, giving an organism a survival advantage. Mutation is important because it's the original source of all new alleles — without mutation, there would be no new variation for evolution to act on.
Using a Punnett Square
A Punnett square is a simple grid used to predict the possible combinations of alleles that offspring could inherit from their parents.
Example 1: Tongue rolling (R = can roll tongue, dominant; r = cannot roll tongue, recessive)
If both parents have the genotype Rr (they can roll their tongues but each carry a recessive allele too), their children could inherit:
- RR — can roll tongue (1 in 4 chance)
- Rr — can roll tongue (2 in 4 chance)
- rr — cannot roll tongue (1 in 4 chance)
This gives an overall 3 in 4 (75%) chance their child can roll their tongue, and a 1 in 4 (25%) chance they cannot.
Example 2: An inherited condition (B = unaffected, dominant; b = affected, recessive — this is how conditions like cystic fibrosis are inherited, where two carrier parents can have an affected child)
If one parent is a carrier (Bb) and the other parent is affected (bb), their children could inherit:
- Bb — unaffected carrier (1 in 2 chance)
- bb — affected (1 in 2 chance)
This example shows why genetic counsellors use Punnett squares in real life — to help families understand the probability of passing on inherited conditions.
4. Selective Breeding: Humans Using Inheritance on Purpose
Once scientists understood inheritance, humans began deliberately using it to shape other species — this is called selective breeding (also called artificial selection).
How it works:
1. Choose parent organisms with a desired characteristic (e.g. cows that produce more milk, or wheat that grows faster).
2. Breed those organisms together.
3. From the offspring, select those that show the desired characteristic most strongly.
4. Repeat this process over many generations.
Over time, this concentrates the desired allele in the population. Selective breeding is how farmers have produced crops with higher yields, dogs with specific traits (like guide dogs bred for temperament), and livestock that grow faster or produce more meat or milk. It's essentially a human-controlled, faster version of the natural process of inheritance.
5. Key Terms Glossary (Quick Revision)
1. Variation: Differences between individuals of the same speciesContinuous variation – variation with a full range of values (e.g. height)
2. Discontinuous variation: Variation with distinct categories (e.g. blood group).
3. Inheritance: The passing on of characteristics from parents to offspring.
4. Gene: A section of DNA that codes for one characteristic.
5. Allele: A different version of a gene.
6. Genotype: The combination of alleles in an individual.
7. Phenotype: The physical characteristic that is actually observed.
8. Dominant allele: Only one copy is needed for the characteristic to show.
9. Recessive allele: Two copies are needed for the characteristic to show.
10. Carrier: An individual with one copy of a recessive allele, who doesn't show the characteristic but can pass it on.
11. Chromosome: A thread of DNA found in the nucleus.
12. Sex chromosomes: The chromosome pair (XX or XY) that determines biological sex.
13. DNA: The molecule that carries genetic instructions.
14. Mutation: A random change in the DNA code.
15. Punnett square: A grid used to predict possible offspring characteristics.
16. Selective breeding: Humans deliberately breeding organisms with desired characteristics.
Frequently Asked Questions
Q: What is the difference between genotype and phenotype?
A: Genotype is the actual combination of alleles an individual carries (the genetic code), while phenotype is the physical characteristic that results from that genotype (what you can observe).
Q: Is eye colour dominant or recessive?
A: Brown eye colour alleles are generally dominant over blue, though real eye colour inheritance involves multiple genes working together, making it more complex than a single dominant/recessive pair.
Q: What determines whether a baby is male or female?
A: The father's sperm determines biological sex — an X-carrying sperm produces a female (XX), while a Y-carrying sperm produces a male (XY), each with roughly a 50% chance.
Q: Is height a continuous or discontinuous variable?
A: Height is continuous — it can take any value within a range, rather than falling into fixed categories, and is influenced by both genes and environment (like nutrition).
Q: Can environment change your genes?
A: No. The environment can affect how characteristics develop (e.g. muscle size, tan, scars), but it does not change the genes themselves, which are passed down unchanged through inheritance.
Q: Why don't siblings look exactly alike (unless they're identical twins)?
A: Because each sibling inherits a different random mixture of alleles from their parents during sexual reproduction, resulting in variation between them.
Q: What is the difference between variation caused by mutation and variation caused by sexual reproduction?
A: Sexual reproduction mixes existing alleles from two parents in new combinations, while mutation actually creates brand new alleles that didn't exist before.