Inheritance, Variation & Evolution

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.

A dominant allele is expressed (shown) even if only one copy is present. Written as a capital letter, e.g. T.
A recessive allele is only expressed when two copies are present. Written as a lower-case letter, e.g. t.

The combination of alleles an organism carries is its genotype. The characteristic this produces — what you actually see — is its phenotype.

Homozygous = two identical alleles (TT or tt).
Heterozygous = two different alleles (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. T and t) — never two different letters like T and r.

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.

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: Tt and Tt

Gametes: each parent makes T and t gametes.

Filling the Punnett square gives genotypes TT, Tt, Tt, tt.

Genotype ratio: 1 TT : 2 Tt : 1 tt.

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).

If any recessive offspring appear (e.g. a dwarf plant), the unknown parent must have been heterozygous (Tt).
If all offspring show the dominant phenotype, the unknown parent was most likely homozygous (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: $I^A$, $I^B$ and $I^O$.

$I^A$ and $I^B$ are codominant with each other.
$I^O$ is recessive to both.

Sex determination

Sex is decided by one pair of chromosomes, the sex chromosomes.

Females are XX.
Males are XY.

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:

Genetic variation — caused by the alleles inherited (e.g. blood group, eye colour).
Environmental variation — caused by surroundings (e.g. a scar, a language spoken, plant height from sunlight).

Many features result from both genes and environment together (e.g. body mass).

Variation is also described by its pattern:

Continuous variation — a smooth range of values with any value possible, e.g. height or mass. Shown on a histogram as a bell shape.
Discontinuous variation — distinct categories with no in-betweens, e.g. blood group, tongue-rolling, sex.

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.

Most mutations have no effect or are harmful, but a few are beneficial.
The rate of mutation is increased by ionising radiation (such as X-rays, gamma rays, UV light) and by some chemicals (mutagens, e.g. those in tobacco smoke).

Natural selection and evolution

Evolution is the gradual change in the inherited features of a species over many generations. It happens by natural selection:

1. Individuals in a population show variation (due to mutation and sexual reproduction).
2. They compete for limited resources (food, mates, space) and face predators and disease.
3. Individuals with the best-suited alleles are more likely to survive — "survival of the fittest".
4. Survivors reproduce and pass on their advantageous alleles to offspring.
5. 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.