1.6Unit 1: Lifestyle, Transport, Genes and Health
SNAB Inheritance — IAL Edexcel Linear Revision Notes & Topical Questions
10 min read 12 parts Lesson 7 of 29 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
1.6 Inheritance — IAL EDEXCEL LINEAR notes: Explain DNA replication, mutations and cystic fibrosis, use inheritance terms, pedigrees, monohybrid crosses and the chi-squared test, and evaluate genetic screening.Purpose — DNA is copied before mitosis so each daughter cell receives …
- Semi-conservative
- Leading strand
- Lagging strand
- Conservative prediction
- Gene mutation
- Insertion and deletion
Part 1 of 9Semi-conservative DNA replication
Purpose
- DNA is copied before mitosis so each daughter cell receives a full copy, ensuring genetic continuity.
Semi-conservative
Semi-conservative each new DNA molecule keeps one original strand and gains one new strand.
Timing
- Timing replication occurs in the S phase of interphase.
1
Helicase unwinds the double helix by breaking the hydrogen bonds between base pairs.
2
Each single strand acts as a template for new strand synthesis.
3
Free activated nucleotides (nucleoside triphosphates) pair with exposed complementary bases.
4
DNA polymerase joins adjacent nucleotides by condensation, cleaving two phosphates and using the energy to form phosphodiester bonds.
5
Hydrogen bonds form between the template and new strands, giving two identical DNA molecules.
Leading strand
- DNA polymerase builds only 5′ to 3′, so it follows the unwinding fork and synthesises this strand continuously.
Lagging strand
- Lagging strand polymerase moves away from the fork, making short Okazaki fragments that DNA ligase joins by phosphodiester bonds.
Part 2 of 9Gene mutations
Gene mutation
Gene mutation a change in the base sequence of DNA that may alter the polypeptide.
- Mutations occur spontaneously and often during replication.
- Most do not alter the polypeptide, or alter it only slightly, because the code is degenerate.
Insertion and deletion
- Insertion and deletion adding or removing a nucleotide changes the triplet and every triplet downstream (a frameshift),
- so the amino acid sequence can change dramatically.
Substitution
- Substitution one base is swapped for another, affecting only that triplet.
- Silent mutations do not change the amino acid because of degeneracy.
- Missense mutations change one amino acid, as in sickle cell anaemia.
- Nonsense mutations create a premature stop codon, giving an incomplete polypeptide, as can occur in cystic fibrosis.
Duplication
- Duplication one or more copies of a gene or chromosome region are produced.
- The original gene stays intact, so it is not harmful, and the extra copy can evolve new functions.
- Duplication is more common in plants and is important in evolution.
Inversion
- Inversion a gene is cut in two places and the section is reversed and rejoined, usually during crossing over in meiosis.
- Many amino acids are affected, often giving a non-functional protein.
- A working gene on the other chromosome of the pair can lessen the effect.
Part 3 of 9Cystic fibrosis
Cystic fibrosis
Cause
- Cause a recessive allele of the CFTR gene on chromosome 7 codes for non-functional chloride ion channels.
- Reduced chloride movement reduces water entering secretions by osmosis, giving thick sticky mucus.
- Around 1000 different mutations can cause it, and all are recessive.
Inheritance
- Inheritance heterozygous people are carriers and only homozygous recessive people are affected.
- Two carrier parents have a 1 in 4 (25%) chance of an affected child.
- If one parent is homozygous dominant the child cannot be affected.
Respiratory system
- Respiratory system thick mucus stops cilia clearing microorganisms,
- causing frequent infections and blocked airways with a reduced gas exchange surface.
- Physiotherapy loosens mucus and improves gas exchange.
Digestive system
- Digestive system mucus blocks the pancreatic duct and causes pancreatic cysts,
- so enzymes cannot reach the intestine and absorption through the mucus-coated lining is reduced.
Reproductive system
- Reproductive system in men the tubes of the testes can block, and in women thick cervical mucus prevents sperm reaching the oviduct.
Part 4 of 9Inheritance key terms
| Term | Meaning |
|---|---|
| Chromosome | Long DNA molecule containing many genes |
| Gene | Length of DNA coding for a single polypeptide |
| Locus | Position of a gene on a chromosome |
| Allele | Alternative form of a gene, arising by mutation, at the same locus |
| Homologous pair | Chromosomes matching in size, shape and genes, one from each parent |
| Homozygous | Two identical alleles at a locus |
| Heterozygous | Two different alleles at a locus |
| Genotype | Alleles possessed by an individual |
| Phenotype | Observable characteristics |
| Dominant | Allele always expressed in the phenotype |
| Recessive | Allele expressed only when no dominant allele is present |
Carrier
- Carrier a heterozygous individual who does not show a recessive trait but can pass the allele on.
Incomplete dominance
- Incomplete dominance both alleles influence the phenotype, giving a third phenotype, such as pink snapdragons (Rr) between red (RR) and white (rr).
Part 5 of 9Pedigree diagrams
Pedigree
Pedigree a family tree tracing the inheritance of a trait through generations.
- Squares are males, circles are females, and shading marks affected individuals.
- Horizontal lines join parents, vertical lines link parents to children and Roman numerals show generations.
Information gained
- Information gained it can show whether an allele is dominant or recessive, whether a sex is more affected, genotypes and the probability of inheritance.
Deduction
- Deduction two unaffected parents with an affected child show a recessive allele and that both parents are carriers.
- Affected individuals are homozygous recessive, which gives a starting point for other genotypes.
Part 6 of 9Monohybrid crosses
Monohybrid inheritance
Monohybrid inheritance inheritance of one gene whose alleles pass on in gametes by meiosis with equal chance.
Genetic diagram
- Genetic diagram it lists parental phenotypes, genotypes and gametes, then uses a Punnett square to show offspring genotypes.
- Bb × Bb gives 1 BB : 2 Bb : 1 bb.
- The phenotype ratio is 3 dominant : 1 recessive.
Chance
- Chance predictions are probabilities, so real results can differ from them.
Part 7 of 9Chi-squared test
Purpose
- Purpose it tests whether the difference between observed and expected results is significant, for example due to gene linkage, using categorical data.
Null hypothesis
- Null hypothesis it states that there is no significant difference between observed and expected results.
Obtain observed (O) and expected (E) results.
Calculate O − E for each class and square it.
Divide each squared value by E and add them to give χ² = Σ (O − E)² ÷ E.
Degrees of freedom
Degrees of freedom number of classes − 1.
Compare χ² with the critical value at the 0.05 (5%) probability level.
Conclusion
- Conclusion if χ² is greater than or equal to the critical value there is a significant difference, a factor other than chance acts and the null hypothesis is rejected.
- If χ² is smaller than the critical value, differences are due to chance and the null hypothesis is accepted.
Probability range
- Probability range comparing χ² with other probability levels shows how likely the difference is to be due to chance.
Part 8 of 9Genetic screening
Genetic screening
Carrier testing
- Carrier testing it identifies individuals who carry an allele for a disorder such as cystic fibrosis.
- Couples can find the chance of an affected child and make informed decisions before having children.
Preimplantation genetic diagnosis (PGD)
- Preimplantation genetic diagnosis (PGD) cells from embryos made by IVF are tested before implantation into the uterus.
- It reduces the chance of a child with a genetic disorder and avoids abortion.
Prenatal testing
- Prenatal testing the fetus is tested by taking fetal DNA.
- Chorionic villus sampling removes placental cells with a fine needle at about 11–14 weeks, with a 1–2% miscarriage risk.
- Amniocentesis removes fetal cells from amniotic fluid at about 15–20 weeks, with a 1% miscarriage risk.
- Results help parents make informed decisions and prepare for care of the child.
Part 9 of 9Ethical and social issues of screening
Genetic counsellors
- Genetic counsellors they discuss the probability of a disorder, termination, treatments, finances and effects on siblings, then explain results.
Carrier identification
- Carrier identification it can cause emotional stress, affect finding a partner, reveal other disorders and be used by employers and insurers.
- Tests are not always accurate, so false positives or negatives can occur.
PGD
- PGD it raises concerns about designer babies selected for traits such as eye colour or sex,
- and false results could lead to wrong decisions about embryos.
Prenatal testing
- Prenatal testing each procedure carries miscarriage risk, false results can mislead and abortion raises ethical questions.
Key terms
| Term | Meaning |
|---|---|
| Semi-conservative replication | Copying of DNA so each new molecule has one original and one new strand. |
| Frameshift | Change in the reading of all triplets after an inserted or deleted nucleotide. |
| Allele | An alternative form of a gene at the same locus. |
| Genotype | The alleles an individual has for a gene. |
| Phenotype | The observable characteristics of an organism. |
| Carrier | A heterozygous individual with a recessive allele that is not expressed. |
| Null hypothesis | Statement that there is no significant difference between observed and expected results. |
| Amniocentesis | Prenatal test using fetal cells in amniotic fluid. |
Avoid these mistakes
1
Saying heterozygous people have cystic fibrosis — they are carriers and only homozygous recessive people are affected.
2
Confusing helicase and DNA polymerase — helicase unwinds the helix and DNA polymerase builds the new strand.
3
Saying the lagging strand is made continuously — it is made in Okazaki fragments joined by DNA ligase.
4
Concluding a significant chi-squared result proves linkage — it shows only that chance is unlikely to explain the difference.
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