Inheritance

• Meiosis I separates homologues, meiosis II separates chromatids; a gamete carries one allele of each gene; a gene is expressed only when nothing is holding it off.
• Meiosis I separates homologues, meiosis II separates chromatids. One allele of each gene per gamete. v = c − 1. Lactose binds the repressor; gibberellin breaks down DELLA.

• 16.1.1 I can explain — explain the meanings of the terms haploid (n) and diploid (2n)
• 16.1.2 I can explain — explain what is meant by homologous pairs of chromosomes
• 16.1.3 I can explain — explain the need for a reduction division during meiosis in the production of gametes
• 16.1.4 I can describe — describe the behaviour of chromosomes in plant and animal cells during meiosis and the associated behaviour of the nuclear envelope, the cell surface membrane and the spindle (names of the main stages of meiosis, but not the sub-divisions of prophase I, are expected: prophase I, metaphase I, anaphase I, telophase I, prophase II, metaphase II, anaphase II and telophase II)
• 16.1.5 I can interpret — interpret photomicrographs and diagrams of cells in different stages of meiosis and identify the main stages of meiosis
• 16.1.6 I can explain — explain that crossing over and random orientation (independent assortment) of pairs of homologous chromosomes and sister chromatids during meiosis produces genetically different gametes
• 16.1.7 I can explain — explain that the random fusion of gametes at fertilisation produces genetically different individuals
• 16.2.1 I can explain — explain the terms gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F₁, F₂, phenotype, genotype, homozygous and heterozygous
• 16.2.2 I can interpret — interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of monohybrid crosses and dihybrid crosses that involve dominance, codominance, multiple alleles and sex linkage
• 16.2.3 I can interpret — interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of dihybrid crosses that involve autosomal linkage and epistasis (knowledge of the expected ratios for different types of epistasis is not expected)
• 16.2.4 I can interpret — interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of test crosses
• 16.2.5 I can use — use the chi-squared test to test the significance of differences between observed and expected results (the formula for the chi-squared test will be provided, as shown in the Mathematical requirements)
• 16.2.6 I can explain — explain the relationship between genes, proteins and phenotype with respect to the: • TYR gene, tyrosinase and albinism • HBB gene, haemoglobin and sickle cell anaemia • F8 gene, factor VIII and haemophilia • HTT gene, huntingtin and Huntington's disease
• 16.2.7 I can explain — explain the role of gibberellin in stem elongation including the role of the dominant allele, Le, that codes for a functional enzyme in the gibberellin synthesis pathway, and the recessive allele, le, that codes for a non-functional enzyme
• 16.3.1 I can describe — describe the differences between structural genes and regulatory genes and the differences between repressible enzymes and inducible enzymes
• 16.3.2 I can explain — explain genetic control of protein production in a prokaryote using the lac operon (knowledge of the role of cAMP is not expected)
• 16.3.3 I can state — state that transcription factors are proteins that bind to DNA and are involved in the control of gene expression in eukaryotes by decreasing or increasing the rate of transcription
• 16.3.4 I can explain — explain how gibberellin activates genes by causing the breakdown of DELLA protein repressors, which normally inhibit factors that promote transcription

Precise vocabulary is part of the biology. Water moves down a water potential gradient; an active site is complementary to its substrate; enzymes are denatured, not killed; ATP releases energy when it is hydrolysed, and respiration never produces energy. Give a calculated answer to the same number of significant figures as the least precise data, or one more, with its unit. A fifth of the qualification is experimental: Papers 3 and 5 test AO3 only, and their questions may be set in contexts outside the syllabus content, so the practical work in this chapter is set out as the skills those papers test — method, recording, analysis and evaluation — rather than as theory.

• “Homologous chromosomes are identical copies of each other.” Correct Homologous chromosomes are not identical: one came from each parent, and they carry the same genes at the same loci but may carry different alleles. Sister chromatids are the identical ones, copies made by replication.
• “In anaphase I the centromeres divide and the chromatids separate.” Correct In anaphase I homologous chromosomes separate, each still made of two chromatids; the centromeres divide only in anaphase II.
• “Crossing over happens between sister chromatids.” Correct Crossing over is between non-sister chromatids of a homologous pair, at a chiasma, in prophase I. Swapping stretches between identical copies would change nothing.
• “Tt gametes” or “the gametes are TT and tt”. Correct A gamete is haploid, so it carries one allele of each gene: T or t; for two genes, one of each, such as PT or pt.
• “χ² is larger than the critical value, so the results fit the ratio.” Correct A value greater than or equal to the critical value means the difference is significant: reject the null hypothesis. A small χ² is the one that fits.
• “Lactose binds to the operator and switches the genes on.” Correct Lactose binds to the repressor protein, which changes shape and can no longer bind the operator. The operator is where the repressor binds.
• “A dominant allele is the more common one.” Correct Dominance describes what is expressed in a heterozygote, not how common an allele is. The allele that causes Huntington’s disease is dominant and rare.
• “Meiosis produces two diploid cells.” Repair Meiosis produces four haploid cells, genetically different from each other and from the parent cell.
• “The chromosome number halves in meiosis II.” Repair It halves in meiosis I, when homologues separate. Meiosis II keeps the chromosome number and halves the chromatids per chromosome.
• “Independent assortment means genes are separated in anaphase II.” Repair Independent assortment is the random orientation of homologous pairs at metaphase I, each pair independently of the others.
• “The DNA replicates again before meiosis II.” Repair There is no S phase between the two divisions; one replication is followed by two divisions.
• “Pp × Pp gives 3 purple : 1 white, so four plants will be three purple and one white.” Repair A ratio is a probability for each offspring: each has a ¾ chance of being purple. Four offspring can easily be all purple.
• “The father passed haemophilia to his son.” Repair A father gives his son a Y, which carries no F8 allele; a son’s X, and so his sex-linked allele, comes from his mother.
• Writing XhYh for a man with haemophilia. Repair The Y carries no allele of a sex-linked gene: XhY.
• “The degrees of freedom are the number of offspring minus 1.” Repair v = the number of classes minus 1, and it must be recalled: the question gives the formula for χ² but not this.
• Using percentages for O, or rounding E to whole organisms. Repair O is the actual count; E is calculated from the total and not rounded to whole numbers.
• “The results prove the genes are unlinked.” Repair A χ² test gives a probability: the difference is not significant at p = 0.05, so the results are consistent with independent assortment.
• “Sickle cell anaemia is caused by a deletion that changes the reading frame.” Repair One base is substituted (GAG → GUG in the mRNA), changing one amino acid, glutamic acid to valine; the reading frame is unchanged.
• “Huntington’s disease is recessive, like the other three.” Repair The HTT allele is dominant: its altered huntingtin damages neurones even when a normal allele is present.
• “The le allele stops the plant responding to gibberellin.” Repair le codes for a non-functional enzyme in gibberellin synthesis; the plant responds normally to gibberellin sprayed on it.
• “Without lactose, the repressor is not made.” Repair The repressor is made all the time; lactose changes its shape, not its production.
• “Gibberellin is a transcription factor.” Repair Gibberellin is a hormone that causes the breakdown of DELLA proteins; that releases the transcription factor they were inhibiting.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe means “state the points of a topic / give characteristics and main features”. Explain means “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”. Compare means “identify/comment on similarities and/or differences”; contrast, “identify/comment on differences”. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations.
• Interleave with the chapters that use this one. When you reach topic 17, re-answer “how do meiosis and fertilisation produce variation?” and redo one χ² set; in topic 19, re-explain how the HTT and F8 alleles cause their conditions; with topic 15, re-explain DELLA and gibberellin in germination. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 16 is A Level content, so it is examined in Papers 4 and 5. A Level content: examined in Paper 4 (A Level structured, which also requires the AS content) and, as practical context, Paper 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• Topic 16 has no multiple-choice paper: Paper 1 covers AS topics only. In Paper 4 you describe chromosome behaviour stage by stage, explain how meiosis and fertilisation produce variation, explain or predict the results of a cross with a genetic diagram, calculate χ², and explain the lac operon and DELLA as sequences of cause and effect.
• Photomicrographs and diagrams of cells in meiosis to identify; genetic diagrams and Punnett squares to read or construct; tables of offspring counts to test with χ² (formula provided, degrees of freedom recalled, critical values supplied); base and codon sequences for HBB; diagrams of an operon or a signalling sequence to interpret.
• Topic 16 names no laboratory investigation. It supplies microscope work (identifying and counting meiosis stages on anther or testis slides, with drawings and a calibrated measurement) and data analysis (counting a cross such as a maize cob, then a χ² test with its null hypothesis, degrees of freedom and a conclusion in terms of probability).
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe means “state the points of a topic / give characteristics and main features”. Explain means “set out purposes or reasons / make the relationships between things clear / say why and/or how and support with relevant evidence”. Compare means “identify/comment on similarities and/or differences”; contrast, “identify/comment on differences”. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations.