Genetic variationOxford AQA IGCSE Biology: Revision notes
Section 1
What are genes, alleles and how do they control characteristics?
Genes are sections of DNA located on chromosomes in the nucleus that code for specific proteins. These proteins determine our characteristics (phenotype). Each gene may have different versions called alleles, which code for different forms of the same characteristic.
Chromosomes are made entirely of DNA and carry many genes arranged in sequence. Human cells contain 46 chromosomes (23 pairs), with each pair containing two copies of each gene—one from each parent.
How genes control characteristics:
- A gene codes for a specific protein
- The protein is made in the cell using the gene's instructions
- The protein determines the characteristic displayed
For example, a gene might code for the protein that produces eye pigment. Different alleles of this gene code for slightly different versions of the protein, resulting in different eye colours.
Think of a gene as a recipe for a protein, and different alleles as slightly different versions of that recipe. Just as different recipes produce different cakes, different alleles produce different versions of the protein, leading to different traits.
Section 2
What do the terms genotype, phenotype, dominant and recessive mean?
Genotype is the genetic makeup of an organism—the alleles it possesses. It is always written using letters (e.g. Aa, BB, tt).
Phenotype is the observable characteristics displayed by the organism. It is what you can see or measure (e.g. tall, blue eyes, smooth skin).
Dominant alleles are expressed in the phenotype even when only one copy is present. They are represented by capital letters (e.g. A, B, T).
Recessive alleles are only expressed in the phenotype when two copies are present (homozygous recessive). They are represented by lowercase letters (e.g. a, b, t).
| Term | Definition | Example |
|---|---|---|
| Homozygous | Two identical alleles for a gene | AA or aa |
| Heterozygous | Two different alleles for a gene | Aa |
| Homozygous dominant | Two dominant alleles | AA (expresses dominant trait) |
| Homozygous recessive | Two recessive alleles | aa (expresses recessive trait) |
Key point: A heterozygous individual (Aa) will show the dominant phenotype because only one dominant allele is needed to be expressed.
Students often assume that because an allele is recessive, it will be 'less common' in a population. This is wrong—recessive alleles can be very common. Recessive only describes how the allele is expressed, not its frequency.
When describing inheritance, examiners want you to use correct terminology: state whether alleles are dominant or recessive, whether genotypes are homozygous or heterozygous, and distinguish clearly between genotype and phenotype.
Section 3
How do we predict outcomes using Punnett squares and monohybrid crosses?
A monohybrid cross involves investigating the inheritance of a single characteristic controlled by one gene with two alleles.
Constructing a Punnett square:
- Identify the alleles for each parent (write as capital or lowercase letters)
- Draw a 2×2 grid (for monohybrid crosses)
- Write one parent's alleles across the top, the other parent's down the left side
- Fill each cell by combining the alleles from that row and column
- Determine the genotypic ratio (e.g. 1:2:1) and phenotypic ratio (e.g. 3:1)
Example: Cross between heterozygous tall plant (Tt) and homozygous recessive short plant (tt)
| t | t | |
|---|---|---|
| T | Tt | Tt |
| t | tt | tt |
Results: Genotypic ratio 1 Tt : 1 tt; Phenotypic ratio 1 tall : 1 short (50% tall, 50% short)
Test cross: Crossing an organism showing the dominant phenotype with a homozygous recessive individual reveals whether the dominant phenotype is homozygous (AA) or heterozygous (Aa).
Interpreting results: Always express ratios in their simplest form and convert to percentages when asked. State what the numbers mean (e.g. '1 in 4 of offspring' or '25%').
A heterozygous brown mouse (Bb) is crossed with a homozygous brown mouse (BB). Set up the Punnett square: gametes are B and b for the heterozygote, and B and B for the homozygote. The offspring are 50% BB (brown) and 50% Bb (brown), giving a 1:1 ratio, all brown.
Examiners want to see your working in Punnett squares. Always show parent genotypes, label rows and columns with alleles, and clearly state both genotypic and phenotypic ratios with correct terminology.
Section 4
What is codominance and how is it inherited?
Codominance occurs when both alleles are expressed equally in the phenotype of a heterozygous individual. Neither allele is dominant or recessive—both contribute to the observable characteristic.
Key difference: In complete dominance, the heterozygote shows the dominant phenotype. In codominance, the heterozygote shows a blend or intermediate phenotype.
Blood groups example (ABO system):
- I^A allele codes for A antigen
- I^B allele codes for B antigen
- i allele codes for neither antigen (recessive)
| Genotype | Phenotype (Blood group) |
|---|---|
| I^A I^A or I^A i | A |
| I^B I^B or I^B i | B |
| I^A I^B | AB (both A and B antigens expressed) |
| ii | O (neither antigen) |
In the AB blood group, both A and B antigens are present because both alleles are fully expressed—this is codominance.
Sickle cell anaemia example (HT): Heterozygous individuals (HbA HbS) produce both normal and sickle haemoglobin. This provides some protection against malaria while causing mild symptoms under stress. This is codominance where both proteins are expressed.
Students often confuse codominance with incomplete dominance or simple dominance. Remember: codominance means BOTH alleles are fully expressed (e.g. AB blood group shows both A and B antigens), not blended together.
When drawing Punnett squares for codominant traits, use different notation if specified (e.g. I^A and I^B for blood groups). Clearly state that the heterozygote shows both phenotypes equally expressed.
Section 5
How are sex-linked traits inherited and what are examples?
Sex-linked traits are controlled by genes located on the X chromosome. Humans have 23 pairs of chromosomes; the 23rd pair determines sex:
- Females: XX (two X chromosomes)
- Males: XY (one X chromosome and one Y chromosome)
Because males have only one X chromosome, they have only one allele for genes on the X chromosome. This makes males more likely to express recessive sex-linked traits, even when heterozygous (which is impossible for them).
Key terminology for sex-linked traits:
- X^A = dominant allele on X chromosome
- X^a = recessive allele on X chromosome
- Y = Y chromosome (no corresponding allele)
Colour blindness example (HT):
| Genotype | Phenotype (Female) | Phenotype (Male) |
|---|---|---|
| X^B X^B | Normal vision | — |
| X^B X^b | Normal vision (carrier) | — |
| X^b X^b | Colour blind | — |
| — | — | X^B Y (normal) |
| — | — | X^b Y (colour blind) |
A colour-blind male (X^b Y) only needs one recessive allele. A heterozygous female (X^B X^b) is a carrier but shows normal vision.
Haemophilia example (HT): Haemophilia is also X-linked recessive. Males with X^h Y are affected; females need X^h X^h to be affected. Heterozygous females (X^H X^h) are carriers.
Constructing Punnett squares for sex-linked crosses: Always write the alleles on the X and Y chromosomes clearly. A cross between an affected male (X^b Y) and a carrier female (X^B X^b) can produce affected daughters—this is often tested.
Cross: colour-blind male (X^b Y) × carrier female (X^B X^b). Punnett square shows: X^B X^b (normal female), X^b X^b (colour-blind female), X^B Y (normal male), X^b Y (colour-blind male). Result: 50% of daughters are colour-blind; 50% of sons are colour-blind.
For sex-linked inheritance questions, examiners expect you to include sex chromosomes in your Punnett square (write X^A, X^a, and Y clearly). State why males are more frequently affected—they have only one X chromosome.
Section 6
How do pedigree charts and family trees show inheritance patterns?
Pedigree charts (also called family trees) show the inheritance of a trait through multiple generations of a family. They use standard symbols and allow you to identify inheritance patterns and predict genotypes.
Standard pedigree symbols:
- Circles = females
- Squares = males
- Filled symbols = affected individuals (show the phenotype)
- Unfilled symbols = unaffected individuals
- Half-filled symbols = carriers (for recessive conditions)
- Horizontal lines = marriages/partnerships
- Vertical lines = parent-child relationships
Interpreting pedigree charts:
- Dominant inheritance: Trait appears in every generation; affected individuals usually have an affected parent; both males and females equally affected
- Recessive inheritance: Trait skips generations; unaffected parents can have affected children; usually requires both parents to be at least carriers; both sexes equally affected
- Sex-linked (X-linked) recessive inheritance: Predominantly affects males; affected males have carrier mothers; affected females are rare (requires affected father and carrier/affected mother); females may be carriers
- X-linked dominant inheritance: Affects both sexes but more females than males; affected males pass trait to all daughters but no sons
Determining genotypes from pedigrees:
- An unaffected individual with an affected parent for a recessive trait must be a heterozygous carrier
- An affected individual with two unaffected parents for a recessive trait must be homozygous recessive
- For sex-linked traits, always note whether the individual is male or female—this determines possible genotypes
Test your understanding: Can you explain why more males than females are affected by colour blindness using a pedigree chart?
In a pedigree chart, an unaffected couple have an affected child for a recessive condition. Both parents must be Aa (carriers), and the affected child is aa. If their next child is unaffected, they could be AA (1/3 chance given unaffected) or Aa (2/3 chance).
Examiners often ask you to state the genotypes of individuals on pedigree charts. Always use the inheritance pattern to deduce this—state which alleles must be present based on whether the individual is affected and the pattern shown in the family.
Must Know
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Genes are sections of DNA on chromosomes that code for proteins; alleles are different versions of a gene. A gene's genotype (alleles) determines its phenotype (observable characteristic) through protein production.
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Dominant alleles are expressed with one copy (capital letters); recessive alleles need two copies (lowercase letters). Homozygous means two identical alleles; heterozygous means two different alleles.
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Monohybrid crosses use Punnett squares to predict offspring ratios. For heterozygous × homozygous recessive crosses, you get a 1:1 phenotypic ratio. Always show parent genotypes and state both genotypic and phenotypic ratios.
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Codominance means both alleles are fully expressed in heterozygotes (e.g. AB blood group, sickle cell trait). This differs from dominance, where only the dominant phenotype appears in heterozygotes.
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Sex-linked traits are on the X chromosome; males (XY) express recessive X-linked traits with just one recessive allele, while females (XX) need two copies. Females are carriers if heterozygous; males cannot be carriers.
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Pedigree charts show inheritance patterns across generations. Dominant traits appear every generation; recessive traits skip generations with unaffected carrier parents; X-linked recessive traits predominantly affect males. Use pedigrees to deduce genotypes of family members.
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