Contents
1. DNA and Chromosomes
DNA (deoxyribonucleic acid) carries genetic information as a sequence of bases (A, T, G, C). It is a double helix — two complementary strands twisted together. A pairs with T; C pairs with G.
- A gene is a section of DNA that codes for a specific protein.
- A chromosome is a long coiled molecule of DNA. Humans have 46 chromosomes (23 pairs) in body cells.
- Chromosomes occur in homologous pairs — one from each parent.
- Sex chromosomes: XX = female, XY = male.
An alternative form of a gene. A gene may have two or more alleles. For example, the gene for eye colour has alleles for brown and blue.
2. Cell Division
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes (sex cells) |
| Number of divisions | One | Two |
| Daughter cells | 2 identical diploid cells (2n) | 4 haploid cells (n) — genetically different |
| Chromosome number | Maintained (46 → 46) | Halved (46 → 23) |
| Genetic variation | None (clones) | Yes (crossing over, independent assortment) |
Gametes must be haploid (n=23) so that when egg and sperm fuse at fertilisation, the resulting zygote has the correct diploid number (2n=46). If gametes were diploid, the chromosome number would double with each generation.
3. Genetic Terminology
| Term | Definition | Example |
|---|---|---|
| Genotype | The allele combination of an organism | Tt, BB, aa |
| Phenotype | The observable characteristics resulting from genotype and environment | Tall, brown eyes, blood group A |
| Dominant allele | Expressed in the phenotype even when only one copy is present | T (tall) masks t (short) |
| Recessive allele | Only expressed when two copies are present (homozygous recessive) | t only expressed as tt |
| Homozygous | Two identical alleles for a gene | TT or tt |
| Heterozygous | Two different alleles for a gene | Tt |
| Codominance | Both alleles expressed simultaneously — neither dominates | Blood group AB (Iᴬ and Iᴮ both expressed) |
4. Monohybrid Crosses
Use a Punnett square to work out offspring genotype and phenotype ratios.
Cross: Tt × Tt (two heterozygous tall plants)
Gametes from each parent: T or t
Punnett square:
T t
T | TT Tt
t | Tt tt
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tall : 1 short
Sex determination
Mother (XX) × Father (XY)
Gametes: X or X (mother); X or Y (father)
Offspring: XX (female), XY (male), XX (female), XY (male)
Ratio: 1 female : 1 male (50% chance of each sex)
Sex-linked inheritance (e.g. colour blindness)
The gene for colour vision is on the X chromosome. The allele for normal vision (Xᴺ) is dominant; colour blindness (Xⁿ) is recessive. Males have only one X chromosome, so if they inherit Xⁿ they are colour blind. Females need two copies (XⁿXⁿ) to be colour blind.
Mother XᴺXⁿ (carrier) × Father XᴺY (normal)
Offspring: XᴺXᴺ (normal female), XᴺXⁿ (carrier female), XᴺY (normal male), XⁿY (colour blind male)
Result: 50% of sons are colour blind; all daughters have normal vision (50% carriers)
5. Natural Selection and Evolution
The process by which organisms with advantageous characteristics survive and reproduce more successfully, passing their alleles to offspring. Over generations, the frequency of advantageous alleles increases in the population.
Four steps of natural selection
- Variation: individuals in a population vary in their characteristics (due to mutations and sexual reproduction).
- Selection pressure: a factor in the environment (predation, disease, competition for food) reduces survival or reproduction.
- Differential survival and reproduction: individuals with the advantageous characteristic survive and reproduce more successfully.
- Inheritance: the advantageous alleles are passed to offspring; over generations their frequency increases.
1. Variation: some bacteria in a population have a mutation that confers resistance to an antibiotic.
2. Selection pressure: antibiotic is introduced — kills non-resistant bacteria.
3. Resistant bacteria survive and reproduce rapidly (bacteria reproduce quickly).
4. Resistance allele is inherited by all offspring — population becomes predominantly resistant.
- Dominant: expressed in Aa AND AA. Recessive: ONLY expressed in aa.
- Phenotype ratio from Tt x Tt = 3 dominant : 1 recessive (genotype ratio = 1 TT : 2 Tt : 1 tt)
- Sex-linked trait: gene on X chromosome. Males (XY) only need ONE recessive allele to show the trait. Females need TWO.
- Meiosis produces gametes (haploid, n=23). Mitosis produces identical body cells (diploid, 2n=46).
- Natural selection acts on PHENOTYPE. Beneficial phenotype -> survive and reproduce -> allele frequency increases.
6. Common Exam Traps
In a Tt × Tt cross, the genotype ratio is 1 TT : 2 Tt : 1 tt. The phenotype ratio is 3 tall : 1 short (because TT and Tt both look tall). Always state which ratio you are giving.
Natural selection acts on what the organism looks and functions like (phenotype). Alleles that produce a beneficial phenotype are selected — not the alleles directly. Write "organisms with the characteristic survive" not "organisms with the allele survive".
Because males are XY, they have only one X chromosome. One copy of the recessive allele (Xⁿ) is sufficient to make a male colour blind. Females need two copies (XⁿXⁿ). This is why colour blindness is much more common in males.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.