4.6 Inheritance

"What is sexual reproduction?

The fusion (joining) of male and female GAMETES. In animals: sperm and egg. In flowering plants: pollen and egg. Produces offspring with MIXED genetic information from both parents."

"What is asexual reproduction?

Reproduction involving only ONE parent and no fusion of gametes. There's no mixing of genetic information → offspring are genetically IDENTICAL CLONES of the parent."

"Which cell division produces gametes?

MEIOSIS — produces non-identical cells (gametes)."

"Which cell division is involved in asexual reproduction?

MITOSIS only — produces identical cells (clones)."

"Why does sexual reproduction produce variety?

Because there's mixing of genetic information from two parents. Each offspring inherits a unique combination of alleles."

"Where does meiosis happen?

In the REPRODUCTIVE ORGANS — to form gametes (sperm/eggs in animals; pollen/eggs in plants)."

"What happens during meiosis?

1) Copies of the genetic information are made (chromosomes duplicate); 2) The cell divides TWICE; 3) Four genetically DIFFERENT gametes are formed, each with HALF the normal number of chromosomes (a single set)."

"Reading a meiosis DNA-mass graph — what each value represents

Starting plateau (e.g. 4 units) = body (somatic) cell DNA before replication. Rising section (S phase) = DNA being COPIED. Peak (e.g. 8 units) = doubled DNA, ready to divide. First drop = first meiotic division. Second drop = second meiotic division → gametes (lowest value, ¼ of peak). LOWEST value = sperm or egg. STARTING plateau = body cell."

"Meiosis graph: when does DNA replicate, when does cell divide?

DNA copying = during the RISE on the graph (S phase) — between resting value and peak (often shown 3–4 hours on AQA-style graphs). Cell division = at the DROPS (e.g. 5h and 6h: peak → half → quarter for gametes). Two drops = two divisions of meiosis. ALWAYS tick a box on multiple choice — never leave blank."

"How is meiosis different from mitosis?

Meiosis: 2 divisions, 4 daughter cells, half the chromosomes, all genetically different (gametes only). Mitosis: 1 division, 2 daughter cells, full chromosome number, genetically identical clones."

"Why must gametes have HALF the chromosomes?

So that when two gametes fuse at fertilisation, the full chromosome number is restored. (Otherwise the number would double each generation.)"

"What happens after fertilisation?

The new cell divides by MITOSIS — the number of cells increases. As the embryo develops, cells DIFFERENTIATE (specialise)."

"Sperm vs body cell vs zygote vs embryo cell — DNA mass quick reference

If a body cell = 4 units of DNA: Sperm = 2 (half, after meiosis). Egg = 2. Zygote = 4 (sperm 2 + egg 2 fuse at fertilisation). Embryo cells = 4 each (mitosis copies, doesn't halve, so they keep the body-cell amount). COMMON TRAP: confusing the PEAK value (8, post-replication, pre-division) with the gamete value (2)."

"Give 3 advantages of SEXUAL reproduction.

1) Produces VARIATION in offspring; 2) If environment changes, variation gives a survival advantage by natural selection; 3) Natural selection can be sped up by humans via selective breeding (e.g. to increase food production)."

"Give 4 advantages of ASEXUAL reproduction.

1) Only ONE parent needed; 2) More time and energy efficient (no need to find a mate); 3) FASTER than sexual reproduction; 4) Many identical offspring can be produced when conditions are favourable."

"Asexual vs sexual reproduction — quick comparison table (tick-box ready)

Cell division occurs: BOTH ✓. Fertilisation occurs: SEXUAL only ✓. Genes passed from parent to offspring: BOTH ✓ (in asexual, one parent's genes are copied). Offspring genetically identical: ASEXUAL only ✓. Sexual = variation; asexual = clones."

"Give 3 examples of organisms that reproduce BOTH sexually and asexually.

1) MALARIAL PARASITES — asexually in humans, sexually in mosquitoes; 2) MANY FUNGI — asexually by spores AND sexually for variation; 3) PLANTS — many produce seeds sexually but also reproduce asexually (e.g. strawberries by runners, daffodils by bulb division)."

"What is DNA?

Deoxyribonucleic acid. A POLYMER made of two strands forming a DOUBLE HELIX. It carries the genetic instructions for an organism."

"Where is DNA found in a cell?

In the NUCLEUS, contained in structures called CHROMOSOMES."

"What is a chromosome?

A long structure made of DNA, found in the nucleus. Humans have 23 PAIRS (46 in total) in body cells."

"What is a gene?

A small section of DNA on a chromosome. Each gene codes for a particular SEQUENCE OF AMINO ACIDS, to make a specific PROTEIN."

"What is the genome?

The ENTIRE genetic material of an organism. (For humans: all 46 chromosomes' worth of DNA.) The human genome has now been studied."

"Why is understanding the human genome important?

1) Search for genes linked to different diseases; 2) Understanding and treatment of INHERITED DISORDERS; 3) Tracing human MIGRATION PATTERNS from the past."

"What are nucleotides?

The building blocks (monomers) of DNA. Each nucleotide consists of a SUGAR, a PHOSPHATE group, and one of four BASES."

"What are the four bases in DNA?

A (adenine), C (cytosine), G (guanine), T (thymine)."

"How does DNA code for an amino acid?

A sequence of THREE bases codes for one amino acid. The order of bases controls the order in which amino acids are assembled to make a protein."

"Which bases pair with each other in DNA? (HT)

A pairs with T; C pairs with G. (Complementary base pairing — held by hydrogen bonds.)"

"Describe protein synthesis. (HT)

Proteins are synthesised on RIBOSOMES, according to a template (a copy of the gene's code). CARRIER MOLECULES bring specific amino acids to add to the growing protein chain in the correct order."

"What happens to a protein chain once it's complete? (HT)

It FOLDS UP into a unique 3D shape. This shape enables the protein to do its job (as an enzyme, hormone, or structural protein like collagen)."

"What is a mutation? (HT)

A change in the DNA base sequence. Mutations happen continuously. Most don't alter the protein, or only alter it slightly so its function is unchanged."

"How can a mutation affect a protein's function? (HT)

A few mutations code for an altered protein with a DIFFERENT SHAPE. E.g. an enzyme may no longer fit its substrate; a structural protein may lose its strength."

"What is non-coding DNA, and why does it matter? (HT)

Not all DNA codes for proteins. Non-coding parts can switch GENES ON AND OFF, so variations in non-coding DNA may affect how genes are EXPRESSED (whether or not they make their protein)."

"What is a gamete?

A sex cell. MALE GAMETE: SPERM (animals) or POLLEN (flowering plants — NOT 'angiosperm', which means the flowering plant itself). FEMALE GAMETE: egg cell in both animals and plants. Contains HALF the normal number of chromosomes."

"Male gametes — precise names by organism (DON'T say 'angiosperm')

ANIMALS: sperm (or sperm cell). FLOWERING PLANTS: pollen (or pollen grain, or nucleus in pollen grain). NOT 'angiosperm' — that means flowering plant ITSELF, not its gamete. Female gametes: egg cell in both."

"What is an allele?

A different VERSION (form) of a gene. (E.g. a gene for eye colour might have a 'brown' allele and a 'blue' allele.)"

"What is a dominant allele?

An allele that is ALWAYS expressed (shows in the phenotype) even if only ONE copy is present. Usually shown as a capital letter (e.g. B)."

"What is a recessive allele?

An allele that is ONLY expressed if TWO copies are present (no dominant allele). Usually shown as a lower-case letter (e.g. b). IMPORTANT: 'recessive' describes the ALLELE, not the genotype. A heterozygote (Bb) carries a recessive allele but is NOT 'recessive' — its phenotype shows the dominant trait."

"What does homozygous mean?

Both alleles are the SAME for that gene. (E.g. BB or bb.)"

"What does heterozygous mean?

The two alleles are DIFFERENT for that gene. (E.g. Bb.) For a recessive disorder, a heterozygote is called a CARRIER — doesn't show the disorder but can pass the recessive allele on to offspring."

"What is a genotype?

The combination of alleles an organism has for a particular gene. (E.g. BB, Bb, or bb.)"

"What is a phenotype?

The PHYSICAL characteristic that results from the genotype. (E.g. brown eyes, blue eyes.)"

"Are most characteristics controlled by a single gene?

NO — most are controlled by MULTIPLE genes interacting. Only a few traits are controlled by a single gene (e.g. fur colour in mice, red-green colour blindness in humans)."

"What is a Punnett square used for?

To predict the outcomes of a genetic cross — shows the possible combinations of alleles in the offspring of two parents."

"Phenotype labelling on a Punnett square — always do this

Under each genotype, WRITE THE PHENOTYPE: e.g. for a recessive disorder cross — NN = no disease (homozygous dominant); Nn = no disease (heterozygous CARRIER); nn = HAS the disease (homozygous recessive). Three labels for three distinct phenotypes. COMMON ERROR: calling Nn 'recessive' — Nn is HETEROZYGOUS, its phenotype is 'no disease' because the dominant N is expressed."

"How do you do a single-gene Punnett square?

Put one parent's alleles along the top, the other parent's down the side. Fill in the boxes by combining one allele from each parent. Each box represents a possible offspring genotype."

"Punnett square probability — accepted ways to express it

AQA accepts these for a 1-in-4 outcome: 0.25 / ¼ / 25% / 1 in 4. Also accepts 1:3 (ratio of has-disease to doesn't-have). Do NOT write 1 in 3 or 1:4 — these are wrong. Always sanity-check: the four probabilities should sum to 1 (or 100%)."

"What is polydactyly?

A genetic disorder where a person has EXTRA fingers or toes. Caused by a DOMINANT allele (so only one copy needed to inherit the condition)."

"What is cystic fibrosis?

A genetic disorder of CELL MEMBRANES. Caused by a RECESSIVE allele (so two copies needed — one from each parent — to inherit the condition). Causes thick mucus in the lungs and digestive system."

"Evidence from a pedigree that a condition is RECESSIVE — what to write

Cite SPECIFIC NUMBERED INDIVIDUALS: e.g. 'Persons 1 and 2 do not have the condition, but their child 5 does — so the parents must both be carriers, which means the allele is recessive.' Don't just say 'carriers pass it on' — that uses the answer as its own evidence. Always reference numbered individuals from the figure."

"What is embryo screening?

Testing embryos (e.g. during IVF) for genetic disorders before implantation. Raises ethical issues — should embryos with genetic disorders be discarded?"

"How many pairs of chromosomes do human body cells have?

23 PAIRS (46 chromosomes total). 22 pairs control characteristics; 1 pair is the SEX chromosomes."

"What are the sex chromosomes in females and males?

Females: XX (two X chromosomes). Males: XY (one X and one Y chromosome)."

"Who determines the sex of a child?

The FATHER — because his sperm carries either an X or a Y. (Mother's eggs all carry X.) X sperm + X egg = girl (XX); Y sperm + X egg = boy (XY)."

"What is the ratio of boys to girls expected from a sex Punnett square?

1:1 (50% boys, 50% girls). XX, XX, XY, XY in the Punnett square = 2 girls : 2 boys."

"What is variation?

Differences in the characteristics of individuals in a population."

"What 3 causes can variation have?

1) GENETIC — differences in inherited alleles; 2) ENVIRONMENTAL — differences in conditions of development; 3) Combination of both."

"Give an example of environmental variation.

Plant heights affected by amount of sunlight or nutrients in soil. Skin tan from sun exposure. Spoken language. Scars."

"Give an example of genetic variation.

Eye colour, blood group, natural hair colour, having attached/free earlobes."

"Where do all variants ultimately come from?

MUTATIONS. Most have no effect on phenotype; some influence phenotype; very few determine phenotype."

"How can a rare beneficial mutation lead to evolution?

If the new phenotype is SUITED to an environmental change, the organism is more likely to survive and reproduce. The new allele then spreads in the population — relatively rapid change in the species."

"What is evolution?

A CHANGE in the inherited characteristics of a population OVER TIME, through the process of NATURAL SELECTION — which may result in the formation of new species."

"How long ago did life on Earth begin?

More than THREE BILLION years ago. All species have evolved from simple life forms that first developed then."

"Describe the process of natural selection.

1) Individuals show VARIATION (mutations); 2) Those best suited to the environment SURVIVE and breed (survival of the fittest); 3) The beneficial alleles are PASSED ON to offspring; 4) Over generations, the population becomes better adapted."

"When does a new species form?

When two populations of one species become so DIFFERENT in phenotype that they can no longer INTERBREED to produce FERTILE OFFSPRING."

"What is selective breeding?

Also called ARTIFICIAL SELECTION — the process by which humans breed plants and animals for particular genetic characteristics. (Humans have been doing this for thousands of years.)"

"Describe the steps of selective breeding.

1) Choose parents with the DESIRED characteristic from a mixed population; 2) Breed them together; 3) From offspring, choose those with the desired characteristic and breed them together; 4) Repeat over many generations until all offspring show the desired characteristic."

"Give 4 examples of characteristics chosen for in selective breeding.

1) Disease resistance in food crops; 2) Animals producing more meat or milk; 3) Domestic dogs with a gentle nature; 4) Large or unusual flowers."

"What is inbreeding, and why is it a risk of selective breeding?

Breeding closely related individuals to fix desired characteristics. Risk: leads to less genetic variation in the population, making the breed more prone to disease and inherited defects."

"What is genetic engineering?

A process which involves MODIFYING the genome of an organism by introducing a GENE FROM ANOTHER ORGANISM to give a desired characteristic."

"Give an example of genetic engineering in plants.

Crops genetically engineered to be resistant to disease, herbicides, or insects — or to produce bigger/better fruits."

"Give an example of genetic engineering in bacteria.

Bacteria engineered to produce HUMAN INSULIN — used to treat diabetes."

"What are GM crops?

Genetically Modified crops — plants with genes from other organisms inserted. Often resistant to insect attack or herbicides. Usually show higher yields."

"Give 2 concerns about GM crops.

1) Effects on wild populations of flowers and insects; 2) Long-term effects on human health of eating GM crops haven't been fully explored."

"Describe the steps of genetic engineering. (HT)

1) ENZYMES are used to isolate the required gene; 2) The gene is inserted into a VECTOR (usually a bacterial plasmid or virus); 3) The vector is used to insert the gene into the required CELLS; 4) Genes are transferred at an EARLY STAGE so cells develop with the desired characteristic."

"What is a vector in genetic engineering? (HT)

Something that carries the gene into the new cell. Usually a BACTERIAL PLASMID (small circular DNA) or a VIRUS."

"What is tissue culture (for plants)?

Using small groups of cells from part of a plant to grow IDENTICAL new plants. Used to preserve rare plant species or commercially in nurseries."

"What are cuttings?

An older, simpler cloning method. Gardeners cut a piece off a parent plant (with leaves) and plant it; it grows roots and becomes an identical new plant."

"What is embryo transplanting?

Splitting cells from a developing animal embryo BEFORE they specialise, then transplanting the identical embryos into host mothers. All offspring are genetically identical."

"Describe the steps of adult cell cloning.

1) Nucleus removed from an unfertilised EGG cell; 2) Nucleus from an ADULT body cell (e.g. skin) inserted into the egg cell; 3) Electric SHOCK stimulates the egg cell to divide into an embryo; 4) When the embryo is a ball of cells, inserted into the womb of an adult female to develop. Result: a clone of the adult that donated the nucleus."

"Who proposed the theory of evolution by natural selection?

CHARLES DARWIN — based on observations from a round-the-world expedition (the Beagle), backed by years of experiments and discussion. Published On the Origin of Species in 1859."

"Summarise Darwin's three main observations.

1) Individual organisms within a species show wide VARIATION; 2) Individuals with characteristics best suited to the environment are more likely to SURVIVE and breed; 3) The characteristics enabling survival are PASSED ON to the next generation."

"Why was Darwin's theory only gradually accepted? (3 reasons)

1) It challenged the idea that GOD made all the animals and plants; 2) There was INSUFFICIENT EVIDENCE at the time to convince many scientists; 3) The mechanism of INHERITANCE wasn't known until 50 years later."

"What was Lamarck's theory of evolution?

Changes that occur in an organism DURING its lifetime can be INHERITED by its offspring. (E.g. a giraffe stretching its neck to reach high leaves would pass on its longer neck.) We now know this is WRONG in the vast majority of cases."

"Who proposed natural selection independently of Darwin?

ALFRED RUSSEL WALLACE. He worked worldwide gathering evidence. Published joint writings with Darwin in 1858, which prompted Darwin to publish On the Origin of Species in 1859."

"What is Wallace best known for?

His work on WARNING COLOURATION in animals and his theory of SPECIATION."

"Describe the steps that give rise to a new species (speciation).

1) Two populations of one species become geographically ISOLATED (e.g. by a mountain or sea); 2) Each population experiences different environmental conditions; 3) NATURAL SELECTION favours different characteristics in each; 4) Over many generations, the two populations become so different that they can no longer INTERBREED to produce fertile offspring — they are now separate species."

"Who was Gregor Mendel and what did he discover?

A monk who, in the mid-19th century, carried out breeding experiments on PLANTS (mostly peas). He concluded that inheritance of each characteristic is determined by 'units' (now called GENES) that are passed on unchanged."

"Why wasn't Mendel's work recognised in his lifetime?

His ideas were ahead of their time. The behaviour of chromosomes wasn't observed until LATE 19th century — AFTER Mendel died. Only then did scientists realise his 'units' behaved similarly."

"When was DNA structure determined?

In the MID-20th century — leading to the mechanism of gene function being worked out and the gene theory being developed."

"What are the main types of evidence for evolution?

1) FOSSILS — show how organisms have changed over time; 2) GENES — characteristics are passed on through genes; 3) ANTIBIOTIC RESISTANCE — bacteria evolving resistance is observable evolution."

"What is a fossil?

The remains of organisms from MILLIONS of years ago, found in rocks."

"How are fossils formed? (3 ways)

1) Parts of organisms that haven't DECAYED because conditions for decay were absent (e.g. low oxygen, very acidic); 2) Parts REPLACED by minerals as they decay; 3) PRESERVED TRACES — footprints, burrows, rootlet traces."

"Why can't we be certain about how life began on Earth?

Many early life forms were SOFT-BODIED → left few traces. Any traces have largely been DESTROYED by geological activity (erosion, plate tectonics, etc) — so the fossil record is INCOMPLETE."

"What is extinction?

When there are no remaining individuals of a species still alive."

"What factors can cause extinction? (5)

1) New PREDATORS; 2) New DISEASES/pathogens; 3) New COMPETITORS; 4) Environmental CHANGES (e.g. climate, sea level); 5) Catastrophic events (e.g. asteroid impact, super-volcano)."

"Why can bacteria evolve rapidly?

They REPRODUCE at a fast rate (some divide every 20 mins). So natural selection acts quickly across many generations."

"How does antibiotic resistance evolve in bacteria?

1) MUTATIONS produce new strains of bacterial pathogens; 2) Some are RESISTANT to antibiotics so aren't killed; 3) Resistant ones SURVIVE and REPRODUCE; 4) Resistant population RISES; 5) Spreads because people aren't immune and there's no effective treatment."

"Give an example of an antibiotic-resistant bacterium.

MRSA (methicillin-resistant Staphylococcus aureus) — resistant to many antibiotics."

"3 ways to reduce the development of antibiotic resistance?

1) Doctors should not prescribe antibiotics INAPPROPRIATELY (e.g. for viral infections); 2) Patients should COMPLETE their full course of antibiotics; 3) Restrict AGRICULTURAL use of antibiotics."

"Why is developing new antibiotics not a complete solution?

Development is COSTLY and SLOW — unlikely to keep pace with the emergence of new resistant strains."