3 Reproduction and inheritance

A clone is genetically identical, not necessarily physically identical: environmental conditions can still change its phenotype. Sexual reproduction creates variation through gamete formation and random fertilisation.

Fertilisation creates one cell; it does not create a complete embryo immediately. The embryo forms through repeated mitotic cell division and development after the zygote is produced.

Pollination is pollen transfer, not fertilisation. Wind-pollinated flowers invest in abundant light pollen and exposed structures rather than insect attractants.

The pollen grain does not travel whole to the ovary: its tube grows through the style while the male nucleus travels inside it. Ovules become seeds; the ovary becomes the fruit.

Seeds need oxygen for aerobic respiration, not photosynthesis. Light is required by some species but is not one of the three universal conditions tested here; temperature must be suitable because enzymes are involved.

The embryo cannot rely on photosynthesis before green leaves are exposed to light. Starch is a store; it is first digested into soluble sugar before that sugar can be transported and respired.

Asexual propagation preserves an existing genotype; it does not generate a new desired trait. Because clones share susceptibility, one disease or environmental change can affect many of them similarly.

Fertilisation usually occurs in an oviduct, not the uterus. The urethra carries both urine and semen in males at different times, but the female urethra is separate from the reproductive tract.

Oestrogen does not trigger ovulation directly: its high level helps stimulate LH release, and LH triggers ovulation. Progesterone maintains rather than initially rebuilds the lining.

FSH matures the follicle; LH releases the egg. Both come from the pituitary, not the ovary, and their peaks do not mean menstruation is occurring.

Maternal and fetal blood normally remain separate, so exchange occurs across a barrier rather than by direct blood mixing. The placenta transfers nutrients; it does not digest food for the fetus.

Amniotic fluid protects mechanically; it is not the embryo's main source of food or oxygen. Nutrient and gas exchange occurs through the placenta and umbilical cord.

Secondary sexual characteristics develop during sexual maturation but are not gametes or reproductive organs. Oestrogen and testosterone influence more than one tissue because target cells in those tissues have suitable receptors.

A genome is not one gene, and a gene does not directly code for a whole visible characteristic: it codes for a protein that can influence phenotype.

Genes are not separate objects floating beside chromosomes. They are sequences within chromosome DNA, and different chromosomes carry different sets of genes.

Base pairing is specific: A pairs with T and C pairs with G. The two strands are not identical; their sequences are complementary.

When writing an RNA sequence, pair A in the DNA template with U, not T. RNA is not simply one detached strand of the original DNA molecule.

A codon is a three-base sequence on mRNA; an anticodon is the complementary triplet on tRNA. Transcription makes RNA, while translation makes a polypeptide.

Gene and allele are not synonyms: the gene names the inherited instruction, while alleles are alternative forms of that instruction.

Dominant does not mean common, stronger or better. A heterozygote carries a recessive allele even when the dominant phenotype masks it.

Codominance is not blending into an intermediate allele and neither allele masks the other. Use distinct superscripts rather than dominant/recessive capitalisation when helpful.

Polygenic means many genes influence one characteristic, not one gene has many alleles and not many genes each control unrelated characteristics.

Gametes carry one allele, not two. A Punnett square shows probabilities for each fertilisation event; it does not guarantee that four actual offspring match its four boxes.

Infer genotypes from all relatives, not shading alone. An unaffected person can carry a recessive allele, whereas an affected recessive individual must be homozygous recessive.

Each birth is a new independent event: earlier children do not change the next child's genetic probability. Add mutually exclusive alternatives; multiply independent requirements.

X and Y are only one chromosome pair, not all of a person's chromosomes. An egg always contributes X; a sperm contributes X or Y.

The fertilising sperm determines whether the zygote is XX or XY. A 1:1 probability does not require every small family to contain equal numbers of girls and boys.

DNA replication doubles the amount of DNA temporarily, not the chromosome number of the resulting daughter cells. Mitosis does not halve chromosome number.

Mitosis can contribute to both growth and reproduction, but it does not itself fuse gametes or create the genetic variation associated with meiosis and sexual reproduction.

Meiosis does not make four identical diploid cells. Homologous chromosomes and alleles are assorted into different gametes, creating variation.

Fertilisation restores the diploid number; it does not deliberately choose advantageous gametes. The randomness of which gametes meet creates new genotype combinations.

Diploid means two sets and haploid means one set. A human gamete has 23 chromosomes, not 23 pairs; fertilisation combines two haploid sets.

Environmental variation is not inherited through DNA, although parents may shape offspring environments. A genetically influenced feature can still be modified by environment.

Organisms do not mutate because they need a trait. Mutations occur randomly with respect to usefulness; selection acts afterward on inherited variation.

A DNA change does not alter phenotype directly; its effect is mediated through gene expression and protein function.

Mutation does not always mean disease and it is not automatically beneficial. Its effect depends on where the change occurs and on the environment.

A mutagen raises risk; it does not guarantee a particular mutation or phenotype. Mutations remain individual DNA changes, not purposeful responses to exposure.

Individuals do not evolve during life and organisms do not acquire a useful allele because they need it. Populations evolve across generations as inherited variants differ in reproductive success.

The antibiotic does not teach an individual bacterium to become resistant. Overuse increases selection pressure and removes competitors, favouring resistant lineages.