Sexual vs asexual reproduction

Sexual reproduction joins (fuses) a male and a female gamete — sperm + egg in animals, pollen + egg in plants. It mixes genetic information, so offspring are varied. Gametes are made by meiosis.

Asexual reproduction needs only one parent, with no fusion of gametes and no mixing of genes. The offspring are genetically identical clones. Only mitosis is involved.

Biology only: some organisms use both — the malaria parasite reproduces asexually in humans but sexually in the mosquito; many fungi use spores (asexual) but also reproduce sexually; strawberry plants spread by runners (asexual) yet also set seed (sexual). Sexual reproduction's big advantage is variation, which gives a survival advantage if the environment changes; asexual is faster, needs only one parent, and is energy-efficient.

Meiosis halves the chromosomes

Gametes are made by meiosis in the reproductive organs. Meiosis halves the chromosome number so each gamete has a single set; fertilisation restores the full number.

• Copies of the genetic information are made.
• The cell divides twice to form four gametes, each with a single set of chromosomes.
• All four gametes are genetically different from one another.

Watch out: a common error is thinking meiosis makes two identical cells — that's mitosis. Meiosis makes four cells and they are all genetically different. (You don't need the named stages of meiosis for AQA.)

Which type of reproduction?

Counting gametes

DNA, genes and the genome

The genetic material in a cell's nucleus is DNA. DNA is a polymer made of two strands forming a double helix. It is packaged into structures called chromosomes.

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids to make a specific protein.

The genome of an organism is its entire genetic material. The whole human genome has been studied — useful for finding genes linked to disease, understanding and treating inherited disorders, and tracing human migration patterns.

DNA structure & making proteins

(Biology only) DNA is a polymer of four nucleotides. Each nucleotide = a common sugar + a phosphate group + one of four bases (A, C, G, T). The strands are alternating sugar–phosphate sections with a base on each sugar.

A sequence of three bases codes for one amino acid. The order of bases controls the order amino acids are joined to build a particular protein.

(HT only) Proteins are made on ribosomes from a template; carrier molecules bring the correct amino acids in order. The finished chain folds into a unique shape so it can act as an enzyme, hormone or structural protein (e.g. collagen).

(HT only) Mutations happen continuously. Most don't change the protein, or only slightly. A few alter the protein's shape — an enzyme may no longer fit its substrate, or a structural protein may weaken. Non-coding DNA can switch genes on/off, so variants there may change how genes are expressed (and so the phenotype) even without changing the protein itself.

Naming the parts

The language of inheritance

• Allele — a different form of a gene.
• Dominant (written as a CAPITAL, e.g. B) — expressed even if only one copy is present.
• Recessive (lower case, e.g. b) — only expressed if two copies are present (no dominant allele).
• Homozygous — the two alleles are the same (BB or bb).
• Heterozygous — the two alleles are different (Bb).
• Genotype — the alleles present (e.g. Bb). Phenotype — the characteristic you see.

A few characteristics are controlled by a single gene (e.g. fur colour in mice, red–green colour blindness in humans), but most characteristics are controlled by many genes interacting.

Watch out: a dominant allele only needs one copy to show in the phenotype. A heterozygous individual (Bb) shows the dominant characteristic but still carries the recessive allele and can pass it on.

Genotype or phenotype?

Punnett squares & inherited disorders

A Punnett square shows every way the parents' alleles can combine. Some disorders are inherited from certain alleles:

• Polydactyly (extra fingers/toes) — caused by a dominant allele (so one copy causes it).
• Cystic fibrosis (a disorder of cell membranes) — caused by a recessive allele (needs two copies).

Two unaffected carrier parents (Ff × Ff) for cystic fibrosis. F = unaffected (dominant), f = cystic fibrosis allele (recessive):

Watch out: a 3 : 1 ratio is a probability, not a guarantee. Each child has an independent 1 in 4 chance of cystic fibrosis — a family could have four children all affected, or none. Embryo screening can check embryos for faulty alleles, raising economic, social and ethical issues.

Predict the offspring ratio

Inheriting sex: XX and XY

Human body cells have 23 pairs of chromosomes. 22 pairs control other characteristics; one pair carries the genes that determine sex:

• Females have two of the same sex chromosome: XX.
• Males have two different ones: XY.

The mother can only pass on an X; the father passes on either an X or a Y — so it is the father's gamete that determines the sex of the child.

What chance of a girl?

What causes variation?

The phenotype develops from the genome interacting with the environment. Differences between individuals in a population is called variation, and it can be due to:

• Genetic causes — the genes (alleles) inherited (e.g. blood group, eye colour).
• Environmental causes — the conditions during development (e.g. a scar, a language spoken).
• A combination of genes and environment (e.g. body mass, height).

There is usually extensive genetic variation within a species. All variants arise from mutations: most have no effect on the phenotype, some influence it, and very few determine it. Mutations occur continuously, and very rarely a mutation gives a new phenotype that — if it suits an environmental change — can spread relatively rapidly.

Link: this variation is the raw material that natural selection acts on (next).

Evolution by natural selection

Evolution is a change in the inherited characteristics of a population over time through natural selection, which may lead to a new species. All species evolved from simple life forms that first appeared more than three billion years ago.

Natural selection in four steps:

• Individuals show a wide range of variation (from mutation).
• Those with characteristics best suited to the environment are more likely to survive and breed.
• They pass on the alleles for those characteristics to the next generation.
• Over many generations the favourable characteristic becomes more common.

Watch out: natural selection is not an organism "trying" or "choosing" to change. The variation is already there from random mutation; the environment simply selects which variants survive to breed.

Antibiotic resistance

Selective breeding (artificial selection)

Selective breeding is when humans breed plants and animals for particular genetic characteristics. Humans have done it for thousands of years. The method:

• Choose parents with the desired characteristic from a mixed population.
• Breed them together.
• From the offspring, breed together those with the desired characteristic.
• Repeat over many generations until all offspring show it.

Used for: disease-resistant crops, animals giving more meat or milk, gentle-natured dogs, large or unusual flowers.

Risk: selective breeding reduces variation and can cause inbreeding, where a breed becomes prone to disease or inherited defects.

Genetic engineering & GM crops

Genetic engineering modifies an organism's genome by introducing a gene from another organism to give a desired characteristic. Examples: bacteria engineered to produce human insulin; GM crops resistant to insects or herbicides, usually with increased yields.

(HT only) The main steps in the process:

• Enzymes are used to cut out / isolate the required gene.
• The gene is inserted into a vector — usually a bacterial plasmid or a virus.
• The vector inserts the gene into the required cells, at an early stage of development so they grow with the new characteristic.

Benefits & risks: GM can raise yields and make medicines, and research is exploring it to treat inherited disorders. Concerns include effects on wild flowers and insects, and that the long-term health effects of eating GM crops are not fully known. Some people have ethical objections.

Cloning methods

(Biology only) Cloning makes genetically identical copies. Four methods to know:

• Tissue culture — small groups of cells from a plant grown into many identical new plants (preserves rare species; used commercially).
• Cuttings — an older, simple gardening method: many identical plants from one parent plant.
• Embryo transplants — split the cells of a developing animal embryo before they specialise, then transplant the identical embryos into host mothers.
• Adult cell cloning — see the steps below.

Adult cell cloning: remove the nucleus from an unfertilised egg cell → insert the nucleus from an adult body cell (e.g. a skin cell) → give an electric shock so the egg divides to form an embryo → when it is a ball of cells, insert it into the womb of an adult female to develop. The clone has the same genes as the adult body cell.

Match the technique to its description

Tap a term on the left, then its correct match on the right.

Darwin, Lamarck & Wallace

(Biology only) Charles Darwin, after observations on a round-the-world voyage plus years of experiments and discussion (and new ideas in geology and fossils), proposed evolution by natural selection, published in On the Origin of Species (1859).

His theory was only accepted slowly because:

• It challenged the religious idea that God made all living things.
• There was insufficient evidence at the time to convince many scientists.
• The mechanism of inheritance was not known until ~50 years later.

Jean-Baptiste Lamarck had an earlier, different theory: that changes acquired during an organism's lifetime can be inherited. We now know this is wrong in the vast majority of cases.

Alfred Russel Wallace independently proposed natural selection, published joint writings with Darwin in 1858, and did pioneering work on warning colouration and speciation. Gregor Mendel's 19th-century breeding experiments showed inheritance is determined by 'units' (now called genes) — but his work was not recognised until after his death.

Watch out — Lamarck's error: a giraffe stretching its neck does not pass a longer neck to its offspring. Acquired characteristics are not inherited. Evolution works on inherited alleles selected over generations, not on changes made during one life.

Darwin or Lamarck?

Tap the scientist whose idea each statement matches.

Evidence for evolution

Evolution by natural selection is now widely accepted. Evidence comes from genetics (characteristics passed on in genes), the fossil record, and the evolution of antibiotic resistance in bacteria.

Fossils are the remains of organisms from millions of years ago, found in rocks. They form:

• From parts that did not decay because a condition for decay was absent.
• When parts are replaced by minerals as they decay.
• As preserved traces — footprints, burrows, rootlet traces.

The fossil record is incomplete: many early life forms were soft-bodied and left few traces, and many traces were destroyed by geological activity. That's why scientists can't be certain how life began. Evolutionary trees use classification data (living organisms) and fossil data (extinct ones) to show how organisms are related.

Extinction happens when there are no remaining individuals of a species alive. Causes include new predators, new diseases, new competitors, environmental change, catastrophic events, and destruction of habitats.

Speciation

(Biology only) If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species.

The steps that give rise to a new species: an original population is isolated (e.g. separated geographically) → each group experiences different conditions and different natural selection on its variation / mutations → over many generations the two diverge so much that they can no longer interbreed to produce fertile offspring → two separate species. Wallace did much of the pioneering work on speciation; more evidence since has refined our understanding.

Classifying living things

Traditionally, Carl Linnaeus classified living things by structure and characteristics into:

Kingdom → Phylum → Class → Order → Family → Genus → Species

Organisms are named by the binomial system: Genus + species, e.g. Homo sapiens (Genus capitalised, species lower case, both in italics/underlined).

As microscopes and biochemistry improved, new models appeared. From chemical analysis, Carl Woese proposed the three-domain system:

• Archaea — primitive bacteria, often in extreme environments.
• Bacteria — true bacteria.
• Eukaryota — protists, fungi, plants and animals.

Evolutionary trees show how scientists believe organisms are related, using current classification data for living organisms and fossil data for extinct ones.

Naming & domains

The big ideas of 4.6

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