Inheritance, Variation and Evolution

What is sexual reproduction and what are gametes?

Reproduction involving the fusion of male and female gametes, producing offspring with genetic information from two parents. Sex cells. They contain one set of chromosomes, which is half the chromosome number of a body cell.

Which gametes fuse in animals and plants, and what is fertilisation?

In animals, sperm and egg cells fuse. In flowering plants, male gametes carried in pollen fuse with egg cells in ovules. The fusion of the nuclei of male and female gametes to form a zygote with the full chromosome number.

Why does sexual reproduction produce genetic variation?

Meiosis makes genetically different gametes, and fertilisation combines genetic information from two gametes in a new combination.

What is asexual reproduction, and why does it produce clones?

Reproduction involving one parent and no fusion of gametes, so there is no mixing of genetic information. They are produced by mitosis from one parent, so they are genetically identical to the parent unless a mutation occurs. A genetically identical copy of a cell or organism.

What is meiosis and where does it occur in humans?

Cell division that produces four genetically different gametes, each with half the chromosome number of the original cell. In the reproductive organs: testes produce sperm and ovaries produce egg cells.

What happens before and during meiosis?

The DNA is copied, so each chromosome has two chromatids. One cell divides twice. Chromosome pairs are separated, then chromatids are separated, producing four haploid cells.

What do haploid and diploid mean, and how many chromosomes do human cells have?

Haploid means one set of chromosomes, as in gametes. Diploid means two sets, as in body cells. A human body cell has 46 chromosomes in 23 pairs. A human gamete has 23 chromosomes.

Why is meiosis needed, and what happens after fertilisation?

It halves the chromosome number, so fertilisation restores the normal number instead of doubling it each generation. The zygote divides by mitosis to form an embryo, and cells later differentiate.

What are the advantages and disadvantages of sexual reproduction?

It produces genetic variation, which may help a population survive environmental change or disease. It usually needs two parents, takes more time and energy, and produces fewer offspring more slowly.

What are the advantages and disadvantages of asexual reproduction?

Only one parent is needed, so many offspring can be produced quickly when conditions are favourable. It produces little genetic variation, so all offspring may be vulnerable to the same environmental change or disease.

Why use both types of reproduction, and how does the malaria parasite do this?

Asexual reproduction allows rapid increase in good conditions; sexual reproduction produces variation when conditions change. They reproduce asexually in the human host and sexually in the mosquito vector.

How do fungi, daffodils and strawberries reproduce sexually and asexually?

They can reproduce asexually by spores and sexually when genetic material from different fungal cells combines. They reproduce sexually using flowers and seeds, and asexually using structures such as bulbs or runners.

Where is DNA found and how is it arranged?

In the nucleus, arranged in chromosomes. Long molecules of DNA, associated with proteins.

What are a gene and a genome?

A section of DNA that codes for a protein or functional RNA. The entire genetic material of an organism.

Why is understanding the human genome useful?

It helps scientists search for genes linked to disease, understand inherited disorders, and trace human migration patterns.

Describe DNA structure and the components of a nucleotide.

DNA is a polymer made of two strands coiled into a double helix. A DNA building block made of a sugar, a phosphate group and one base.

Which DNA bases are present and how do they pair?

A, T, C and G. A pairs with T, and C pairs with G.

How does a DNA base sequence code for a protein?

The order of amino acids in a protein. The base sequence is read in triplets, and each triplet codes for a particular amino acid.

How do proteins form, and why does their shape matter?

Amino acids join in a specific order to form a polypeptide, which folds into a specific protein shape. The shape controls how the protein works, for example the active site shape of an enzyme.

How are proteins synthesised at ribosomes? [HT]

A copy of the gene carries its code from DNA to a ribosome in the cytoplasm. Carrier molecules bring specific amino acids, which are joined in the coded order to form a protein. The chain folds into its specific shape, allowing it to work as an enzyme, hormone or structural protein.

What is a mutation and why do many have no effect?

A change in the DNA base sequence. They may not change the amino acid sequence, or they may change a part of the protein that does not affect its function.

How can a mutation affect phenotype?

It may change the protein enough to alter its shape or function, which can change a characteristic.

How can non-coding DNA affect gene expression and phenotype? [HT]

They can affect whether genes are switched on or off, changing how much protein is made. The process by which a gene is used to make a functional product, usually a protein.

What are alleles, homozygous and heterozygous genotypes?

A different version of the same gene. Having two identical alleles for a gene, such as BB or bb. Having two different alleles for a gene, such as Bb.

How do dominant and recessive alleles differ?

An allele that is expressed in the phenotype when only one copy is present. An allele that is expressed in the phenotype only when two copies are present.

How do genotype and phenotype differ?

The alleles an organism has for a particular gene. The observable characteristics of an organism, caused by its genotype and environment.

How are genetic crosses shown?

Using allele symbols, a Punnett square or genetic diagram, and the possible offspring genotypes and phenotypes.

What offspring ratios result from Bb x Bb and Bb x bb?

Genotypes: 1 BB : 2 Bb : 1 bb. Phenotypes: 3 dominant : 1 recessive. Bb x bb gives 1 Bb : 1 bb, so 50% dominant phenotype and 50% recessive phenotype.

Why can actual inheritance ratios differ, and what do family trees show?

Fertilisation is random and small sample sizes can give results that differ from expected probabilities. How a characteristic or inherited disorder passes through generations.

What are inherited disorders and how is polydactyly inherited?

A disorder caused by alleles passed from parents to offspring. It is caused by a dominant allele, so a person can have the disorder with one copy of the allele.

How is cystic fibrosis inherited and what is a carrier?

It is caused by a recessive allele, so a person must inherit two copies of the allele to have the disorder. A heterozygous person who has one recessive disorder allele but does not show the disorder.

If two cystic fibrosis carriers have a child, what are the chances for that child?

25% affected, 50% carrier and 25% unaffected non-carrier.

What is embryo screening, and what are its benefits and concerns?

Testing embryos for alleles linked to inherited disorders before deciding which embryo to implant. It can reduce the chance of a child being born with a serious inherited disorder. It can involve destroying embryos, may be expensive, and raises ethical concerns about selecting embryos.

How do sex chromosomes determine biological sex in humans?

Most females are XX and most males are XY. Egg cells carry an X chromosome. Sperm cells can carry either an X chromosome or a Y chromosome. Fertilisation by an X-carrying sperm gives XX, and fertilisation by a Y-carrying sperm gives XY.

What is variation and what causes it?

Differences in characteristics between individuals. Genetic differences, environmental differences, or a combination of both.

Give examples of genetic, environmental and combined variation.

Inherited eye colour, blood group and some inherited disorders. Language, scars and differences caused by diet or exercise. Body mass, height, or plant growth.

How do mutations affect phenotype and contribute to evolution?

A mutation is a DNA change. Most mutations do not affect phenotype, but some do. If a mutation gives an advantage, individuals with it may survive and reproduce more, so the allele becomes more common.

What is evolution and how does it occur?

A change in the inherited characteristics of a population over generations. Natural selection acting on genetic variation.

What is natural selection, and what is a population?

Individuals with advantageous characteristics are more likely to survive, reproduce and pass on the alleles for those characteristics. A group of organisms of the same species living in the same area.

How does competition change allele frequencies through natural selection?

Organisms compete for limited resources, so individuals with useful characteristics are more likely to survive. They become more common in the population.

How can evolution lead to a new species?

If populations become so different that they can no longer interbreed to produce fertile offspring, a new species has formed.

What is selective breeding and what are its steps?

The process where humans choose parents with desired characteristics and breed them together. Choose parents with desired characteristics, breed them, select the best offspring, and repeat for many generations.

Which characteristics are selected in animals and plants?

For features such as high milk yield, more meat, good temperament or disease resistance. For features such as high yield, large fruit, disease resistance or attractive flowers. It can increase yield and improve features such as taste, size or resistance to disease.

What is inbreeding and what are the risks of selective breeding?

Breeding closely related individuals. It can reduce the gene pool and increase the chance of inherited defects or disease vulnerability.

What is genetic engineering and why are bacteria useful for it?

Changing an organism's genome by introducing a gene for a desired characteristic. They reproduce quickly and can contain plasmids that are used to transfer genes.

How can plant cells be genetically engineered, and for which crop characteristics?

Genes can be inserted into plant cells, then the cells are grown into whole plants. Crops may be engineered to resist insect attack or herbicides.

What useful products and benefits can genetic engineering provide?

Bacteria can be engineered to produce human insulin or other useful substances. It can improve yield, reduce pesticide use, produce useful medicines and add useful characteristics quickly.

What are possible concerns about genetic engineering?

Inserted genes could affect food chains, spread to wild plants, reduce biodiversity or have unknown long-term effects.

Describe the steps of genetic engineering, including enzymes and vectors. [HT]

Enzymes cut out the desired gene, the gene is inserted into a vector, and the vector transfers the gene into the target organism. Something used to transfer a gene into a cell, such as a plasmid or virus. Enzymes cut out the useful gene and help insert it into a vector.

How do plant cuttings produce clones and why are they useful?

A cutting from a parent plant grows into a new plant genetically identical to the parent. They quickly produce many plants with the same desired characteristics.

What is tissue culture and why is it useful?

Growing many new plants from a small piece of plant tissue using sterile nutrient medium and hormones. It can quickly produce many identical plants, including rare plants or plants with useful characteristics.

How does embryo transplant cloning work?

Cells from an early embryo are split apart, and each cell grows into an identical embryo that is implanted into a host mother.

How does adult cell cloning work and whose nuclear genes does the clone have?

Remove the nucleus from an egg cell, insert a nucleus from an adult body cell, stimulate the egg to divide, then implant the embryo into a host mother. It has the same nuclear genetic information as the adult cell donor.

What are the benefits and concerns of cloning?

It can copy animals or plants with useful characteristics and help preserve rare species. It has a low success rate, can cause health problems, reduces genetic variation and raises ethical concerns.

What is Darwin's theory of evolution by natural selection?

Species have evolved from simpler life forms over long periods through natural selection.

Why was Darwin's theory controversial, and what evidence supported it later?

It challenged religious beliefs, there was limited evidence at the time, and the mechanism of inheritance was not yet known. Fossils, genetics, observed evolution and the discovery of how inheritance works.

How did Lamarck explain evolution, and why is his explanation mostly rejected?

He suggested that characteristics acquired during an organism's life could be passed to offspring. Acquired characteristics are usually not passed on genetically.

What did Wallace propose and study, and how did he influence Darwin?

A theory of evolution by natural selection. Wallace's ideas encouraged Darwin to publish his theory. Warning colouration in animals and the formation of new species.

What is speciation and how can isolation produce a new species?

The formation of a new species. Separated populations experience different conditions and selection pressures, so they evolve differently until they can no longer interbreed successfully. A group of organisms that can interbreed to produce fertile offspring.

Who was Mendel, and why was his work not accepted immediately?

A monk who studied inheritance in pea plants and discovered patterns of inheritance. Genes and chromosomes had not yet been discovered, and his work was not widely known.

What did Mendel's pea plant experiments show?

Characteristics are passed on by separate inherited factors, now called genes.

How did discoveries of chromosomes, genes and DNA improve understanding of inheritance?

In the late 19th and early 20th centuries, as microscopes improved and chromosomes were observed. It explained how genetic information is stored and passed on.

How do fossils and antibiotic-resistant bacteria provide evidence for evolution?

Fossils and antibiotic-resistant bacteria. They show that organisms have changed over long periods and that some organisms are now extinct. It shows natural selection happening in bacteria over short timescales.

What are fossils, and how can undecayed parts be preserved?

The preserved remains, traces or impressions of organisms from the past. Hard parts such as bones, teeth or shells may be preserved.

How do mineral replacement, casts and impressions form fossils?

Minerals gradually replace the remains, producing a rock-like fossil. An organism leaves a shape in soft material that later hardens into rock.

Why are early fossils scarce and the fossil record incomplete?

Early organisms were soft-bodied and many traces were destroyed by geological activity. Many organisms did not fossilise, fossils may have been destroyed, and many fossils have not been found.

What can scientists learn from fossils?

They can study how organisms changed over time and how different groups are related.

What is extinction, and how can environmental change cause it?

The permanent loss of all members of a species. If conditions change faster than a species can adapt, it may die out.

How can predators, competitors, disease and catastrophic events cause extinction?

They may reduce survival or access to resources. A disease can kill many individuals, especially if the population has little resistance. Events such as asteroid impacts or major volcanic eruptions can rapidly change habitats and climates.

How do resistant bacteria arise, and what happens when antibiotics are used?

Random mutations can produce bacteria that are resistant to an antibiotic. Non-resistant bacteria are killed, while resistant bacteria survive.

How does antibiotic resistance spread and become more common?

Resistant bacteria reproduce and pass on the resistance allele, so the resistant strain increases. Bacteria reproduce rapidly and resistance genes can be passed on during reproduction.

What is MRSA and why are resistant infections difficult to treat?

A strain of Staphylococcus aureus that is resistant to several antibiotics. Some antibiotics no longer kill the bacteria, so fewer effective treatments are available.

How can doctors and patients help reduce antibiotic resistance?

Avoid unnecessary antibiotic use and choose antibiotics carefully. It helps kill all bacteria and reduces the chance that resistant bacteria survive and reproduce.

Why do antibiotics not treat viruses, and why is developing new antibiotics difficult?

Antibiotics kill bacteria, not viruses. It is costly, slow, and bacteria may evolve resistance to new antibiotics too.

What is classification and what is the Linnaean hierarchy?

Organising living organisms into groups based on similarities and differences. A hierarchy of kingdom, phylum, class, order, family, genus and species.

What is binomial naming and why is it useful?

A two-part scientific name made from the genus and species. They give each species a standard name used by scientists worldwide.

How did new evidence lead to Woese's three-domain system?

Microscopes, biochemical tests and DNA analysis have shown new relationships between organisms. Archaea, Bacteria and Eukaryota. RNA and other molecular evidence showed that archaea are distinct from bacteria and eukaryotes.

Which organisms belong to Eukaryota, Bacteria and Archaea?

Organisms with cells containing a nucleus, including animals, plants, fungi and protists. Prokaryotic organisms with bacterial cell structure. Prokaryotic organisms that are genetically and biochemically different from bacteria, often found in extreme environments.

What are evolutionary trees, how are they constructed, and how do you identify close relatives?

A diagram showing how scientists think organisms are related through common ancestors. Current classification data, fossils and genetic evidence. They share the most recent common ancestor.