Inheritance is Cambridge International Biology 9700 Topic 16. The chromosome examples use a conventional diploid sexual-life-cycle model. It connects meiosis and fertilisation to genetic crosses, statistical testing, gene-protein-phenotype relationships and gene control in prokaryotes and eukaryotes. Theory owns chromosome reasoning, cross construction, chi-squared inference and regulatory mechanisms; specimen preparation and data-collection execution remain in the dedicated practical hub.
1. Ploidy and homologous chromosomes
A haploid cell has one set of chromosomes, written n. A diploid cell has two sets, written 2n. In a diploid organism, one chromosome of each homologous pair came from each parent.
Homologous chromosomes carry the same genes at the same loci but may carry different alleles. They are similar in length, centromere position and gene order. They are not necessarily genetically identical.
In this diploid model, haploid gametes fuse to restore the diploid chromosome number rather than doubling it each generation. Meiosis provides the reduction division; its products are not necessarily direct gametes in every life cycle.
2. Meiosis I
Before meiosis, DNA replicates so each chromosome has two sister chromatids. In prophase I, homologous chromosomes pair as bivalents. Crossing over can occur between non-sister chromatids at chiasmata. Chromosomes condense, the nuclear envelope breaks down and a spindle forms.
At metaphase I, homologous pairs align at the equator with random orientation. At anaphase I, homologous chromosomes separate to opposite poles while sister chromatids remain joined. Telophase I and cytokinesis can produce two haploid cells whose chromosomes still contain two chromatids.
This overview follows prophase I as one stage, without establishing current assessment exclusions. The important distinction is that homologues separate in the first division.
3. Meiosis II and image recognition
No further DNA replication occurs before meiosis II. At prophase II, chromosomes condense and new spindles form. At metaphase II, chromosomes align individually at the equator.
At anaphase II, centromeres divide and sister chromatids separate. Telophase II and cytokinesis produce four haploid cells. Chromosome behaviour resembles mitosis in the second division, but the cells are already haploid and may contain recombinant chromatids.
In images, determine whether homologous pairs or individual chromosomes align, whether centromeres have divided and how many cells or nuclei are present. Do not identify a stage from condensation alone.
4. Sources of genetic variation
Crossing over exchanges corresponding DNA segments between non-sister chromatids of homologous chromosomes, producing new combinations of linked alleles. Random orientation of homologous pairs at metaphase I produces independent assortment of maternal and paternal homologues.
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Random orientation of chromosomes and sister chromatids during meiosis II adds further combinations where chromatids differ after crossing over. Finally, random fusion of genetically different gametes at fertilisation multiplies possible genotypes.
Meiosis generates variation by reshuffling existing alleles. Mutation is the ultimate source of new alleles; distinguish creation of new alleles from reshuffling existing ones.
5. Core genetic vocabulary
A gene is a DNA sequence contributing to a functional product, and its locus is its chromosome position. Alleles are alternative forms of a gene. In a complete-dominance model, the heterozygous phenotype matches the dominant homozygote and the recessive phenotype is masked. This describes phenotype, not whether an allele is transcribed or translated. Codominant alleles both contribute to the heterozygous phenotype.
Genotype is an organism's allele combination; phenotype is its observable characteristic. Homozygous individuals carry two identical alleles at a locus, while heterozygous individuals carry different alleles.
The F1 generation is the first filial offspring of a parental cross. Crossing F1 individuals can produce an F2 generation. A test cross mates an individual showing a dominant phenotype with a homozygous recessive individual to reveal the unknown genotype.
6. Constructing monohybrid and dihybrid crosses
A complete genetic diagram states parental phenotypes and genotypes, uses defined allele symbols, lists gametes, shows fertilisation outcomes and reports offspring genotype and phenotype ratios or probabilities.
Monohybrid crosses follow one locus. Dihybrid crosses follow two. For independently assorting loci, a double heterozygote can produce four gamete types. Do not apply a memorised ratio unless dominance, independence and viability conditions match the problem.
Multiple alleles means more than two alleles exist in the population, although a diploid individual still carries only two. In codominance, use notation that does not imply one allele dominates the other.
7. Sex linkage and autosomal linkage
A sex-linked gene lies on a sex chromosome. In a conventional XY model, a male is hemizygous at an X-linked locus without a corresponding Y copy. The absence of a second allele matters for inheritance, but does not guarantee identical phenotypic effects for every variant. Genetic diagrams must show chromosome-linked notation and different male and female gametes.
Physically linked autosomal genes lie on the same chromosome. Nearby loci often show non-independent inheritance, but widely separated loci can have a recombination frequency near one half and behave as if independently assorting. A heterozygote's allele arrangement matters: coupling places two specified alleles on one homologue, while repulsion places them on opposite homologues.
Crossing over can produce recombinant gametes. Parental combinations are generally more frequent when loci are linked, but the problem data should determine the inference.
8. Epistasis and test crosses
Epistasis occurs when an allele at one locus affects expression of a gene at another locus. Construct the cross from the stated biological rule, then translate each genotype into phenotype. This account derives expected ratios from the stated interaction; current assessment requirements need separate verification.
A test cross distinguishes an unknown dominant-phenotype genotype. All-dominant offspring can support homozygosity, but a finite sample does not exclude a heterozygous parent. Under the simple fully penetrant model, recessive offspring show that the tested parent carried the recessive allele. For linked loci, offspring classes can also reveal parental and recombinant gametes.
9. Chi-squared testing
The chi-squared test compares observed counts with counts expected under a genetic hypothesis. State a null hypothesis specifying the proposed class probabilities, with deviations attributed to sampling variation. The test assumes independent observations, appropriate counts and sufficiently large expected frequencies; a common introductory check is at least five expected per class. Calculate expected numbers from the proposed ratio and total sample size, then sum the contribution from every category using the supplied formula.
For k categories with probabilities fully specified in advance, degrees of freedom are k minus one. Fitting model parameters from the same counts can reduce the degrees of freedom further. Compare the calculated value with the critical value at the selected significance level, commonly 0.05. A value greater than the critical value leads to rejection of the null hypothesis; a smaller value means there is insufficient evidence to reject it.
Statistical non-significance does not prove the genetic model true. It means the observed deviation is compatible with sampling chance at the chosen threshold.
10. From genes to phenotype
The TYR gene codes for tyrosinase, an enzyme needed for melanin production. Pathogenic TYR variants can reduce or abolish tyrosinase activity and cause oculocutaneous albinism type 1; other forms of albinism involve other genes.
The HBB gene codes for beta-globin in haemoglobin. A sickle-cell allele changes the beta-globin amino-acid sequence, producing haemoglobin that can polymerise at low oxygen tension. In the HbSS disease model, red blood cells can distort and cause sickle-cell anaemia. An HbAS heterozygote usually has sickle-cell trait, rather than the same disease phenotype; the allele alone does not specify the whole clinical outcome.
The F8 gene codes for clotting factor VIII. Pathogenic F8 variants can reduce factor VIII amount or function and cause haemophilia A, with phenotype depending on the genotype and remaining clotting activity. Because F8 is X-linked, inheritance differs between males and females.
The HTT gene codes for huntingtin. A pathogenic CAG-repeat expansion produces altered huntingtin and underlies autosomal dominant Huntington disease. Penetrance and age of onset vary with repeat length and other factors, so not every expansion has the same outcome. Each example should follow gene sequence to protein structure or amount, cellular function and organism phenotype.
11. Gibberellin alleles and stem elongation
In the garden-pea example, Le codes for a gibberellin 3 beta-hydroxylase involved in producing active gibberellin. Under the comparison conditions, plants with at least one functional Le allele show normal stem elongation.
The studied recessive le allele produces an enzyme with greatly reduced activity, rather than necessarily no activity. Homozygous le plants have reduced active gibberellin production and a dwarf phenotype. This example shows how an allele can alter an enzyme, metabolic pathway, hormone concentration, cell elongation and whole-plant form.
Dominance describes phenotype in the heterozygote, not molecular strength or population frequency.
12. Structural and regulatory genes
The protein-coding structural genes discussed here produce proteins with cellular roles, such as enzymes or transport proteins; this does not imply that all functional gene products are proteins. Regulatory genes code for products that control expression of other genes.
Inducible enzymes are produced in response to the presence of a substrate or signal. Repressible enzymes are normally produced but their synthesis is reduced when a product or signal indicates they are not needed.
These terms describe regulation of production, not whether the enzyme itself can be chemically inhibited after it has formed.
13. The lac operon without lactose
The lac operon contains structural genes for proteins involved in lactose use, along with promoter and operator control regions. A separate regulatory gene produces a repressor protein.
When lactose is absent, the active repressor binds the operator. This blocks RNA polymerase from transcribing the structural genes, so the lactose-use enzymes are not produced in substantial amounts.
The operator is a DNA sequence, not a protein. The repressor is the regulatory protein that binds it.
14. The lac operon with lactose
When lactose is present, an inducer derived from lactose binds the repressor and changes its shape. The repressor can no longer bind the operator. RNA polymerase transcribes the structural genes, and translation produces proteins needed for lactose uptake and metabolism.
The enzymes are inducible because their production rises when the substrate is available. This overview focuses on repressor control. Removing repression does not guarantee maximal transcription, because other controls, including glucose-dependent regulation, still act. Current official assessment scope needs separate verification.
Removing lactose permits active repressor to bind again and reduce transcription, conserving resources.
15. Eukaryotic transcription factors and DELLA proteins
Transcription factors are proteins that bind DNA and increase or decrease transcription rate. They allow gene expression to respond to cell type, developmental state and signals.
In the simplified growth-promoting branch described here, DELLA proteins act as repressors that inhibit factors promoting transcription. Gibberellin signalling causes DELLA breakdown. The transcription-promoting factors are released from inhibition, target genes are expressed and growth responses such as elongation or germination can proceed.
Gibberellin does not act as a transcription factor by directly binding DNA. It changes the stability of a repressor.
Worked application: linkage evidence and a statistical decision
Consider a hypothetical test cross producing 82 AB, 78 ab, 21 Ab and 19 aB offspring. Assume the classes report gametes from a double heterozygote, with accurate scoring, equal viability and independent sampled offspring. Under these conditions, the two large classes suggest parental combinations and the two small classes suggest recombinants. A fully specified 1:1:1:1 independent-assortment model predicts 50 in each class out of 200. Squaring each observed-minus-expected difference, dividing by 50 and adding gives a chi-squared statistic of 72.2. With four classes and no fitted parameters, there are three degrees of freedom. The upper-tail 0.05 critical value is 7.815, so reject this null model. This is consistent with linkage under the assumptions, but a departure from the ratio alone cannot rule out unequal viability, segregation distortion or scoring errors in real data.
Common misconceptions and corrections
Calling haploid cells chromosome-free. They contain one chromosome set.
Calling homologous chromosomes identical. They may carry different alleles.
Saying sister chromatids separate in anaphase I. Homologues separate.
Adding DNA replication between meiosis I and II. None occurs.
Using this overview as proof of prophase I assessment exclusions. Verify current official requirements separately.
Saying crossing over occurs between sister chromatids. It involves non-sister chromatids.
Calling independent assortment a directed arrangement. Orientation is random.
Saying meiosis creates new alleles. It reshuffles alleles.
Calling a locus an allele. It is the gene's chromosome position.
Calling dominant alleles more common. Dominance concerns heterozygous expression.
Saying a diploid person carries every multiple allele. An individual carries two at a locus.
Using a standard dihybrid ratio for linked genes. Linkage changes gamete frequencies.
Ignoring coupling or repulsion. Allele arrangement determines parental gametes.
Memorising epistasis ratios without reading the rule. Derive the ratio from the stated biological interaction.
Calling every dominant-phenotype cross a test cross. The partner must be homozygous recessive.
Using percentages instead of counts directly in chi-squared without justification. Expected and observed categories must share a count scale.
Saying non-significance proves the null hypothesis. It is not rejected.
Saying TYR codes directly for melanin. It codes for tyrosinase.
Saying HBB codes for a whole red blood cell. It codes for beta-globin.
Calling F8 autosomal. It is X-linked in this example.
Saying Le is the gibberellin molecule. It is an allele coding for an enzyme.
Calling an operator a regulatory protein. It is a DNA control sequence.
Treating lactose as the only determinant of lac expression. Removing repressor binding does not guarantee maximal transcription.
Calling DELLA a transcription-promoting factor. It represses such factors.
Saying gibberellin binds DNA directly. It promotes DELLA breakdown.
Assessment guidance
For meiosis, name the division and track homologues, sister chromatids, centromeres, spindle and ploidy. Genetic diagrams need defined symbols, parental genotypes, gametes and translated phenotypes. Treat linkage and epistasis from the information given rather than forcing standard ratios. Chi-squared answers require a null hypothesis, expected counts, degrees of freedom, critical comparison and a bounded conclusion. Gene examples should connect DNA to protein to cellular function to phenotype. Keep the lac sequence centred on repressor, operator and structural-gene transcription while recognising that this is a simplified regulatory account, and distinguish gibberellin signalling from DELLA and transcription-factor roles.
Retrieval practice
Track chromosomes and chromatids through all eight named meiosis stages and identify variation points. Construct crosses for dominance, codominance, multiple alleles, sex linkage, autosomal linkage, epistasis and a test cross. Complete a chi-squared decision from raw offspring counts. Build four gene-protein-phenotype chains and the Le-to-stem pathway. Finish by comparing structural and regulatory genes, switching the lac operon off and on, and tracing gibberellin through DELLA breakdown to transcription.
