Inheritance, variation & evolution.

Part 1Two ways to reproduce

Sexual reproduction involves the joining (fusion) of male and female gametes: the sperm and egg cell in animals, pollen and egg cells in flowering plants. Because there are two parents, the offspring get a mixture of genes from each — so they show variation and are not identical to either parent.

Asexual reproduction involves only one parent and no gametes. There is no mixing of genetic information, so the offspring are genetically identical to the parent — they are clones. Only one type of cell division, mitosis, is involved.

Part 2Meiosis — making gametes

Gametes are made by a special type of cell division called meiosis, which happens only in the reproductive organs (ovaries and testes). Your body cells each have 23 pairs of chromosomes — that's the diploid number, 46. Meiosis halves this number so each gamete has just one of each pair — the haploid number, 23.

Here's why halving matters: when a sperm fertilises an egg, their chromosomes combine. Half plus half restores the full number, so the fertilised egg has 46 again. Meiosis also shuffles the genes, so all the gametes are genetically different — this is a major source of variation.

Part 1From DNA to gene to chromosome

The genetic material in the nucleus of a cell is made of a chemical called DNA. DNA is a polymer made of two strands forming a double helix (a twisted ladder shape).

The DNA is contained in structures called chromosomes. A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which join together to make a specific protein. Your proteins decide how your cells — and so your whole body — are built and work.

Part 2The genome and why it's useful

The genome of an organism is its entire genetic material. The whole human genome has now been studied, and this has had a big impact on medicine and biology. You need three reasons why understanding it is important:

It lets scientists search for genes linked to different diseases; it helps in understanding and treating inherited disorders; and it is used to trace human migration patterns from the past.

Part 1The words you must know cold

Most characteristics are controlled by many genes working together, but some are controlled by a single gene — and those are the ones used in exam crosses. To handle them you need a precise vocabulary.

Different versions of the same gene are called alleles. You have two of each gene — one inherited from each parent — so you have two alleles for each characteristic. A dominant allele is always expressed even if only one copy is present; a recessive allele is only expressed if two copies are present (no dominant allele to mask it).

Part 1How to do a genetic cross

A Punnett square predicts the offspring of a cross. Each parent puts one allele into each gamete (they're haploid, remember). The grid pairs every possible egg with every possible sperm, showing the genotypes the offspring could have, and in what ratio.

Part 2Inherited disorders

Some disorders are caused by the inheritance of certain alleles. You need two:

Polydactyly (having extra fingers or toes) is caused by a dominant allele — so a child can inherit it from just one affected parent. Cystic fibrosis (a disorder of cell membranes affecting the lungs and digestion) is caused by a recessive allele — so a child must inherit two copies, one from each parent, to have it. A person with just one recessive allele is a carrier: unaffected, but able to pass it on.

Embryo screening: embryos made by IVF can be tested for the alleles that cause genetic disorders. This raises economic, social and ethical issues you may be asked to discuss — for example the cost, whether it is right to choose embryos, and worries about where "screening out" disorders might lead.

Part 3Sex determination

Of your 23 pairs of chromosomes, 22 pairs control your ordinary characteristics. The 23rd pair are the sex chromosomes. In females they are the same, XX; in males they are different, XY.

All eggs carry an X. Sperm carry either an X or a Y. So the sperm decides the sex of the child — and a Punnett square shows a 50 : 50 ratio of males to females.

Part 1Three causes of variation

The differences in the characteristics of individuals in a population are called variation. There are three causes you need:

Genetic variation comes from differences in the genes (alleles) you inherit — e.g. natural eye colour or blood group. Environmental variation comes from the conditions you grow up in — e.g. a scar, a sun tan, or a language you speak. Most variation is caused by a combination of genes and the environment — e.g. your height (set partly by genes, partly by your diet).

Part 2Mutation

There is usually extensive genetic variation within a population of a species, and it is constantly being added to by mutations. A mutation is a random change in the DNA (the base sequence). Mutations occur continuously.

Most mutations have no effect on the phenotype; some influence it a little; and very rarely a single mutation will significantly affect the phenotype. Very occasionally a mutation produces a new characteristic that is beneficial — and this is the raw material that natural selection acts on (Topic 6).

Part 1Darwin's theory

Evolution is a change in the inherited characteristics of a population over time, through a process of natural selection, which may result in the formation of a new species. The theory was proposed by Charles Darwin after the observations he made on his round-the-world voyage. Learn it as four steps:

1. Variation — individuals in a species show variation (from mutations). 2. Competition & survival — those with characteristics best suited (adapted) to the environment are more likely to survive and breed. 3. Inheritance — the survivors pass on their successful alleles to their offspring. 4. Change over time — over many generations the helpful alleles become more common, so the species gradually changes.

Part 2Selective breeding

Selective breeding (artificial selection) is when humans breed plants and animals for chosen useful characteristics. It's the same idea as natural selection, but we do the choosing. The steps: choose parents with the desired feature → breed them together → select the best offspring → repeat over many generations until all the offspring show the feature.

We selectively breed for things like cows that give more milk, disease-resistant crops, dogs with a gentle nature, and large or unusual flowers. The drawback is inbreeding: closely related organisms share alleles, so there is reduced variation. This can make a whole breed prone to the same disease or inherited defect.

Part 3Genetic engineering

Genetic engineering is a process that changes the genome of an organism by transferring a gene for a desired characteristic from one organism to another. Plant crops have been engineered to be resistant to disease, insects or herbicides, or to produce bigger, better fruits. Crops that have had their genes modified are called genetically modified (GM) crops. Bacteria have been engineered to produce useful substances such as human insulin to treat diabetes.

You should be able to weigh up the benefits and risks. Benefits: bigger yields, more nutritious food, and medicines like insulin. Concerns: some worry about effects on populations of wild flowers and insects, and about long-term effects on human health that may not yet be known. As with embryo screening, these involve ethical as well as scientific issues.

Part 1Evidence from fossils

Fossils are the remains of organisms from millions of years ago, found in rocks. They form in several ways: from parts of organisms that have not decayed (because one or more conditions for decay were absent), when parts of the organism are replaced by minerals as they decay, and as preserved traces of organisms — such as footprints, burrows and rootlet traces.

Fossils let scientists see how organisms have changed over time — strong evidence for evolution. But we can't be certain how life began, because many early organisms were soft-bodied (so left few fossils) and many fossils have been destroyed by geological activity. The fossil record is therefore incomplete.

Part 2Antibiotic-resistant bacteria

Bacteria evolve fast because they reproduce quickly, so we can watch natural selection happen in real time. Antibiotic-resistant bacteria are evolution in action. By chance, a random mutation makes a few bacteria resistant to an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed but the resistant ones survive and reproduce, passing the resistance allele on. Soon the whole population is resistant. A well-known example is MRSA.

To slow this down: doctors should not over-prescribe antibiotics (and not use them for viral infections), and patients should complete the full course so that all the bacteria are killed and none survive to breed.

Part 3Extinction

Extinction means there are no remaining individuals of a species still alive. A species becomes extinct when it can no longer survive the conditions it faces. The factors that can cause extinction include:

A new predator, a new disease, a new (more successful) competitor, a change in the environment (e.g. climate change), a single catastrophic event (such as a volcanic eruption or a collision with an asteroid), and the destruction of habitats.

Part 1The Linnaean system

Long before we knew about DNA, the scientist Carl Linnaeus classified living things into groups based on their structure and characteristics. In the Linnaean system, organisms are divided into a kingdom, then increasingly small groups: kingdom → phylum → class → order → family → genus → species.

Organisms are named by the binomial system, using two Latin words: the genus (with a capital letter) and the species (lower case) — for example Homo sapiens. This gives every species a single, internationally agreed name.

Part 2The three domains

As microscopes and the understanding of biochemical processes improved, classification became more detailed. Then evidence from studying the chemistry of cells (including DNA) led Carl Woese to propose the three-domain system. Organisms are now divided into three large groups called domains:

Archaea — primitive bacteria, often living in extreme places like hot springs. Bacteria — the "true bacteria". Eukaryota — everything whose cells have a nucleus: protists, fungi, plants and animals.