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16 · Inherited Change

Cambridge A Level Biology — Full F-drive source: 16 · Inherited Change — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 10 parts Lesson 18 of 18 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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16. Inherited Change: overview — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Outline meiosis, genetic variation, inheritance patterns, genes and proteins, and gene control.Diploid — two sets of chromosomes; haploid has one set.Fertilisation — n + n = 2n, so gamete…

  • Homologous chromosomes
  • Meiosis II
  • No interphase
  • Crossing over
  • Independent assortment
  • Random fertilisation

Part 1 of 8Haploid, diploid and homologous chromosomes

Diploid
  • Diploid two sets of chromosomes; haploid has one set.
Fertilisation
  • Fertilisation n + n = 2n, so gametes must be haploid.
Homologous chromosomes
  • Homologous chromosomes same genes at the same loci, one from each parent.

Part 2 of 8Stages of meiosis

1Meiosis I
  • Meiosis I homologous pairs separate;
    • prophase I includes crossing over;
    • anaphase I separates whole chromosomes.
2Meiosis II
  • Meiosis II chromatids separate; four haploid cells form.
3No interphase
  • No interphase there is no DNA replication between the divisions.

Part 3 of 8Sources of genetic variation

Crossing over
  • Crossing over non-sister chromatids exchange alleles at chiasmata.
Independent assortment
  • Independent assortment random orientation of pairs at metaphase I.
Random fertilisation
  • Random fertilisation any male gamete can fuse with any female gamete.
Mutation
  • Mutation produces new alleles.

Part 4 of 8Key terms in genetics

Key terms in genetics
Gene and allele
  • Gene and allele a gene codes for a protein; alleles are its alternative forms.
Genotype and phenotype
  • Genotype and phenotype alleles present and observable characteristics.
Dominance
  • Dominance dominant alleles show in heterozygotes; recessive only when homozygous.
Codominance
  • Codominance both alleles show in a heterozygote.

Part 5 of 8Predicting inheritance

Monohybrid
  • Monohybrid heterozygote × heterozygote gives 3 : 1 phenotypes.
Dihybrid
  • Dihybrid two double heterozygotes give 9 : 3 : 3 : 1 if genes are unlinked.
Linkage and epistasis
  • Linkage and epistasis change the expected ratios.
Test cross
  • Test cross with a homozygous recessive to find a genotype.
Chi-squared
  • Chi-squared compares observed with expected, with degrees of freedom classes − 1.

Part 6 of 8Genes, proteins and phenotype

Genes, proteins and phenotype
TYR
  • TYR a recessive allele gives no tyrosinase, causing albinism.
HBB
  • HBB a base substitution gives valine for glutamic acid, causing sickle cell haemoglobin.
F8
  • F8 an X-linked gene whose abnormal alleles cause haemophilia.
HTT
  • HTT more than 40 CAG repeats give Huntington's disease.
Le
  • Le a mutation gives a non-functional enzyme in the pathway forming gibberellin, giving dwarf plants.

Part 7 of 8Gene control

1Structural and regulatory genes
  • Structural and regulatory genes structural genes code for functional proteins, regulatory genes for control proteins.
2lac operon
  • lac operon lactose binds the repressor, so RNA polymerase can transcribe lacZ, lacY and lacA.
3Transcription factors
  • Transcription factors bind DNA to increase or decrease transcription.
4Gibberellin
  • Gibberellin breaks down DELLA, freeing PIF to activate the amylase gene.

Part 8 of 8More from the specification

1
Stages of Mitosis and Chromosome Movement
  • Mitosis proceeds through prophase, metaphase, anaphase,
  • and telophase to ensure accurate segregation of duplicated genetic material into two identical daughter nuclei.
2
Spindle Fibre Action and Centromeres
  • During metaphase, chromosomes align along the spindle equator;
    • in anaphase, spindle fibres contract and shorten, pulling sister chromatids apart by their centromeres toward opposite cell poles.
3
Telophase and Nuclear Envelope Reformation
  • In telophase, separated chromatids reach the poles and decondense into chromatin while a new nuclear envelope reforms around each group to establish two daughter nuclei.
4
Haploidy vs Diploidy in Animal Reproduction
  • Somatic animal cells contain a diploid (2n) number of chromosomes in homologous pairs,
  • whereas meiotic reduction division produces haploid (n) gametes containing half the chromosome complement.
5
Stages of Mitosis and Chromosome Movement
  • Mitosis proceeds through prophase, metaphase, anaphase,
  • and telophase to ensure accurate segregation of duplicated genetic material into two identical daughter nuclei.
6
Spindle Fibre Action and Centromeres
  • During metaphase, chromosomes align along the spindle equator;
    • in anaphase, spindle fibres contract and shorten, pulling sister chromatids apart by their centromeres toward opposite cell poles.
7
Telophase and Nuclear Envelope Reformation
  • In telophase, separated chromatids reach the poles and decondense into chromatin while a new nuclear envelope reforms around each group to establish two daughter nuclei.
8
Haploidy vs Diploidy in Animal Reproduction
  • Somatic animal cells contain a diploid (2n) number of chromosomes in homologous pairs,
  • whereas meiotic reduction division produces haploid (n) gametes containing half the chromosome complement.
9
Photomicrograph
  • A photograph taken through a light microscope.
10
Cytokinesis
  • Division of the cytoplasm.
11
Sexual reproduction
  • Reproduction involving fusion of two gamete nuclei, giving varied offspring.
12
Reduction division
  • Division that halves the chromosome number.
13
Daughter cell
  • Cell produced by cell division.
14
Spindle
  • Structure of microtubules that attaches to and moves chromosomes.
15
Telophase
  • Final stage in which chromosomes decondense and nuclear envelopes reform.
16
Centromere
  • Constricted region that joins sister chromatids and binds spindle fibres.
17
Nucleus
  • The part of the cell that controls its activities.
18
Equator
  • Middle of the spindle where chromosomes line up.
19
Cytoplasm
  • The jelly in which the cell's chemical reactions happen.

Key terms

TermMeaning
MeiosisDivision producing haploid cells.
AlleleAlternative form of a gene.
GenotypeAlleles an individual carries.
Test crossCross with a homozygous recessive.
OperonCluster of genes controlled by one promoter.
Transcription factorProtein controlling transcription.

Avoid these mistakes

1
Saying meiosis produces identical cells — it produces genetically different haploid cells.
2
Saying recessive alleles never show — they show when homozygous.
3
Saying the lac operon is switched on without lactose — it is repressed.
4
Saying a significant chi-squared result means the data fit the expected ratio — it means they do not.

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