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5.1.5Genes, Inheritance and Selection

Inheritance — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

10 min read 8 parts Lesson 6 of 9

📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision
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Part 1 of 8Key ideas

Key ideas
Nucleotide = phosphate + deoxyribose sugar + one base; four bases A, T, C, G; A pairs with T, C pairs with G.
Protein synthesis (Paper 2)
Protein synthesis (Paper 2) transcription — the DNA of a gene is copied into mRNA in the nucleus; translation — mRNA moves to a ribosome, tRNA brings amino acids, and each triplet of bases (codon) codes for one amino acid.
The order of bases in a gene determines the order of amino acids, and so the shape and function of the protein.
Sex determination
Sex determination females XX, males XY; the father's sperm decides sex; probability of a boy = 0.5 (50 %).
Humans
46 chromosomes in body cells (diploid), 23 in gametes (haploid); fertilisation restores diploid number.
Meiosis produces variation through random assortment of chromosomes (and crossing over); random fertilisation adds more.
Variation is genetic, environmental, or both; continuous (e.g. height, range of values) vs discontinuous (e.g. blood group, distinct categories).
Natural selection
Natural selection variation → overproduction → competition/struggle for survival → best-adapted survive and reproduce → pass on advantageous alleles → allele frequency changes over generations (evolution).
Antibiotic resistance
Antibiotic resistance a mutation gives a bacterium resistance; antibiotic kills non-resistant bacteria; resistant ones survive, reproduce and the resistant population increases.

Part 2 of 8Learning objectives

1
Describe DNA structure and the relationship between genes, alleles, chromosomes and genome.
2
Construct and interpret genetic diagrams and family pedigrees, including codominance and sex determination.
3
Compare mitosis and meiosis.
4
Explain variation, mutation, evolution by natural selection and antibiotic resistance.
5
Describe protein synthesis (transcription and translation).

Part 3 of 8Essential knowledge

Essential knowledge
The nucleus contains chromosomes; each chromosome is one DNA molecule carrying many genes; chromosomes occur in homologous pairs, one from each parent.
DNA is a double helix; the two strands are held together by complementary base pairing (A–T, C–G), which lets DNA be copied accurately.
Transcription
Transcription a gene's base sequence is copied into a single-stranded mRNA molecule (uracil replaces thymine). Translation: at the ribosome, each codon of three bases is matched by a tRNA anticodon carrying a specific amino acid; amino acids are joined into a polypeptide.
Most characteristics are controlled by several genes, but monohybrid crosses consider one gene with two alleles.
Pedigree diagrams
Pedigree diagrams if two unaffected parents have an affected child, the condition is caused by a recessive allele and both parents are heterozygous carriers.
Codominance
Codominance in ABO blood groups, IA and IB are codominant and both dominant to IO; IAIO gives group A, IOIO gives group O.
Mitosis produces two genetically identical diploid cells: chromosomes are copied, line up, and separate so each new nucleus receives the full set; used in growth, repair, cloning and asexual reproduction.
Meiosis in the gonads gives four haploid, genetically different cells; halving the chromosome number keeps it constant across generations after fertilisation.
Mutations are random; ionising radiation (e.g. X-rays, gamma) and chemical mutagens (e.g. in tobacco smoke) increase the rate. A mutation may produce a new allele and a new variation.
Darwin's theory
Darwin's theory individuals vary; more offspring are produced than can survive; those with advantageous variations are more likely to survive and reproduce, passing on their alleles; over many generations the species changes.
Resistant bacteria (e.g. MRSA) arise by mutation and are selected by antibiotic use; reduce spread by finishing courses and not overusing antibiotics.
RNA structure
RNA is a single strand of nucleotides containing the sugar ribose and the base uracil instead of thymine. mRNA carries the copy of a gene to the ribosome, tRNA brings specific amino acids, and rRNA forms part of the ribosome.
Transcription detail
In the nucleus the DNA of a gene unwinds as hydrogen bonds between bases break. Free RNA nucleotides pair with the exposed template strand and are joined into mRNA, which leaves through a nuclear pore.
End of translation
At the ribosome, neighbouring amino acids are joined by peptide bonds. Translation stops when a stop codon is reached; the finished chain then folds into the protein's specific shape (e.g. an enzyme or antibody).
Polygenic inheritance
Characteristics controlled by several genes, such as human height, skin colour and body mass, are polygenic. They show a continuous range of phenotypes and cannot be predicted with a simple Punnett square.
Genes and environment
Genes set the potential for a feature, but the environment (diet, climate, lifestyle, accidents) affects how it develops: a child of tall parents may stay short if poorly fed. Discontinuous features such as blood group are mainly genetic; continuous ones usually involve both.
Types of gene mutation
A substitution swaps one base and alters at most one codon. An insertion or deletion of a base shifts every following triplet, so many amino acids after that point may change and the protein is usually non-functional.
Effects of mutations
Most mutations have no or little effect on the protein or phenotype. A change in shape can stop a protein working, e.g. an enzyme whose active site no longer fits its substrate, or cause disorders such as sickle-cell anaemia; a few give an advantage. Mutagens raise cancer risk.
F1 and F2 crosses
Crossing two pure-breeding (homozygous) parents, TT × tt, gives an F1 generation that is all Tt. Interbreeding the F1 gives the F2 generation in a 3 dominant : 1 recessive phenotype ratio.
Reading pedigrees
In a family pedigree, squares are males and circles are females; shaded symbols are affected individuals, a horizontal line joins parents and a vertical line leads to their children. Use the pattern to deduce genotypes and probabilities.

Part 4 of 8Process & sequence

    • State the phenotypes of the parents and choose symbols (capital = dominant).
    • Write the parental genotypes.
    • Write all possible gametes from each parent (one allele each, circled).
    • Draw a Punnett square combining every male gamete with every female gamete.
    • List offspring genotypes and their phenotypes.
    • State the ratio, probability or percentage asked for.

    Development of antibiotic resistancewrite these steps in this order; each step is a marking point.

    1. A population of bacteria shows variation; a random mutation gives one bacterium resistance.
    2. An antibiotic is used and kills the non-resistant bacteria.
    3. The resistant bacterium survives because it is better adapted.
    4. It reproduces rapidly (binary fission), passing on the resistance allele.
    5. Over time most of the population is resistant — the antibiotic is no longer effective.

Part 5 of 8Worked examples

Worked examples
Two heterozygous tall pea plants (Tt) are crossed. They produce 400 offspring. How many are expected to be short?
  • Gametes T, t from each parent.
    • Offspring: 1 TT : 2 Tt : 1 tt.
    • Short (tt) = 1/4.
    • Expected number = 400 × 1/4 = 100 short plants (300 tall).
A man with blood group AB (IAIB) and a woman with blood group O (IOIO) have a child. Give the probability the child is group A.
  • Father gametes IA, IB;
    • mother gametes IO.
    • Offspring: IAIO (group A) and IBIO (group B) in 1 : 1 ratio.
    • Probability of group A = 0.5 (50 %).

A mRNA strand is 900 bases long, all coding. How many amino acids are in the polypeptide?

  • Each codon = 3 bases codes for one amino acid. 900 ÷ 3 = 300 amino acids.

Part 6 of 8Summary

    • A gene is a section of DNA coding for a protein; alleles are different forms of a gene.
    • Use genetic diagrams to predict ratios such as 3 : 1 and 1 : 1.
    • Mitosis gives identical diploid cells; meiosis gives varied haploid gametes.
    • Mutations create new alleles; natural selection leads to evolution.

Part 7 of 8Keep going

  1. Inheritance Edexcel O Level Biology: what to do next

    These Inheritance Edexcel O Level Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Inheritance Edexcel O Level Biology understanding with the practice questions below, then reread any Inheritance Edexcel O Level Biology point you were unsure about before moving on.

    Inheritance Edexcel O Level Biology: revision checklist

    • Revise the main Inheritance Edexcel O Level Biology definitions until you can write them from memory.
    • Practise Inheritance Edexcel O Level Biology exam questions under timed conditions.
    • Link each Inheritance Edexcel O Level Biology idea to a diagram or a worked example.
    • List the common Inheritance Edexcel O Level Biology mistakes and how to avoid them.
    • Explain one Inheritance Edexcel O Level Biology idea out loud as if teaching a friend.
    • Summarise Inheritance Edexcel O Level Biology in five key points on a revision card.

Part 8 of 8Compare: Compare

Mitosis vs meiosis
FeatureMitosisMeiosis
Purposegrowth, repair, asexual reproductionproduces gametes
Divisionsonetwo
Daughter cells2, diploid4, haploid
Genetic make-upidentical to parent and each othergenetically different from each other
Where (humans)body cells everywheretestes and ovaries
Standard monohybrid ratios
CrossOffspring genotypesPhenotype ratio
AA × aaall Aaall dominant
Aa × Aa1 AA : 2 Aa : 1 aa3 dominant : 1 recessive
Aa × aa (test cross)1 Aa : 1 aa1 dominant : 1 recessive
XX × XY1 XX : 1 XY1 female : 1 male

Ratios are probabilitieswith small numbers of offspring actual results often differ.

Key terms

Command terms

Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
DNA · Chromosome · Gene · Allele · Genome · Genotype · Phenotype · Dominant allele · Recessive allele · Homozygous · Heterozygous · Codominance · Mutation · Diploid / haploid
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).
2Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
3Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
4
Convert units carefully (mg → g, cm3 → dm3) and show working for calculations.
5Avoid vague words
“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.
6
Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

1
the bacteria become immune / get used to the antibiotica random mutation made some bacteria resistant; these are selected and reproduce
2
a gene is a characteristica gene is a section of DNA that codes for a specific protein
3
dominant alleles are more commona dominant allele is expressed when only one copy is present
4
the organism mutates to adaptmutations are random; natural selection favours individuals that already have useful alleles
5
meiosis makes two cellsmeiosis produces four genetically different haploid cells

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