Genes, DNA and alleles, genetic crosses, variation, mutation and natural selection.
Chromosomes, genes and DNA
Inside the nucleus of almost every cell are chromosomes — long, coiled threads made of a molecule called DNA (deoxyribonucleic acid). Human body cells contain 23 pairs of chromosomes (46 in total), one of each pair inherited from each parent.
A gene is a short section of DNA on a chromosome that codes for a particular protein, and so controls a feature or characteristic. The order of bases in the DNA is the code that tells the cell which protein to make.
Key terms
DNA — the molecule that carries genetic information.
Gene — a length of DNA that codes for a protein and controls a characteristic.
Chromosome — a thread of DNA carrying many genes, found in the nucleus.
Alleles, genotype and phenotype
Most genes come in different versions called alleles. For example, the gene for tongue-rolling has a "can roll" allele and a "cannot roll" allele. Because chromosomes come in pairs, you carry two alleles for each gene — one on each chromosome of the pair.
T.t.The combination of alleles an organism carries is its genotype. The characteristic this produces — what you actually see — is its phenotype.
| Genotype | Name | Phenotype |
|---|---|---|
TT | homozygous dominant | can roll tongue |
Tt | heterozygous | can roll tongue |
tt | homozygous recessive | cannot roll tongue |
TT or tt).Tt).Watch out
Always use the same letter for both alleles of one gene. A capital and lower-case of the same letter (e.g.
Tandt) — never two different letters likeTandr.
Mitosis and meiosis
Cells divide in two different ways, and you must not confuse them.
Mitosis produces two genetically identical daughter cells, each with the full number of chromosomes (diploid). It is used for growth, repair of tissues, and asexual reproduction.
Meiosis produces gametes (sex cells — sperm and egg). It halves the chromosome number (the cells are haploid), so that when two gametes fuse at fertilisation the full number is restored. Meiosis also shuffles the alleles, creating genetic variation between the gametes.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | 2 | 4 |
| Chromosome number | stays the same | halved |
| Genetically | identical | varied |
| Used for | growth, repair, asexual reproduction | making gametes |
Monohybrid inheritance and genetic diagrams
A monohybrid cross follows the inheritance of a single gene. We can predict the offspring using a genetic diagram or Punnett square.
Worked example
In pea plants, tall (
T) is dominant to dwarf (t). Cross two heterozygous tall plants:Tt × Tt. What are the offspring ratios?Parents:
TtandTtGametes: each parent makes
Tandtgametes.Filling the Punnett square gives genotypes
TT,Tt,Tt,tt.Genotype ratio: 1
TT: 2Tt: 1tt.Phenotype ratio: 3 tall : 1 dwarf — the classic 3:1 ratio.
The test cross
A plant or animal showing the dominant phenotype could be homozygous (TT) or heterozygous (Tt) — you cannot tell by looking. A test cross finds out: cross the unknown with a homozygous recessive (tt).
Tt).TT).Codominance and blood groups
Sometimes neither allele is fully dominant — both are expressed together. This is codominance. A good example is the ABO blood group system, controlled by three alleles: , and .
| Genotype | Blood group |
|---|---|
| or | A |
Sex determination
Sex is decided by one pair of chromosomes, the sex chromosomes.
All egg cells carry an X. Sperm carry either an X or a Y. The sperm that fertilises the egg decides the sex of the child, giving a 1:1 (50:50) ratio of boys to girls.
Variation
Variation is the differences between individuals. It has two causes:
Many features result from both genes and environment together (e.g. body mass).
Variation is also described by its pattern:
Exam tip
Continuous = measured (numbers on a scale). Discontinuous = counted in groups. If you can sort everyone neatly into a few boxes, it's discontinuous.
Mutation
A mutation is a random change in the DNA (the base sequence of a gene). Mutations are the original source of all new alleles and therefore of genetic variation.
Natural selection and evolution
Evolution is the gradual change in the inherited features of a species over many generations. It happens by natural selection:
- Individuals in a population show variation (due to mutation and sexual reproduction).
- They compete for limited resources (food, mates, space) and face predators and disease.
- Individuals with the best-suited alleles are more likely to survive — "survival of the fittest".
- Survivors reproduce and pass on their advantageous alleles to offspring.
- Over many generations, these alleles become more common and the species changes.
Real world
Antibiotic-resistant bacteria. In any population of bacteria, a few carry a random mutation giving resistance to an antibiotic. When the antibiotic is used, the non-resistant bacteria die but the resistant ones survive and reproduce. Soon the whole population is resistant — for example MRSA. This is natural selection happening fast enough for us to watch.
Another classic example is the peppered moth: during the Industrial Revolution, soot darkened tree bark, so dark moths were better camouflaged from birds, survived more, and became common.
These ideas were set out by Charles Darwin in On the Origin of Species (1859). After his voyage on HMS Beagle, he proposed that species evolve by natural selection. His theory was controversial at the time but is now supported by huge amounts of evidence, including fossils and DNA.
Exam tip
When explaining natural selection, never say an organism "tries" or "wants" to adapt. Use the chain: variation → competition → survival of the fittest → reproduction → alleles passed on → species changes.
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