B5: Genes, Inheritance and Selection

What you'll learn

This module covers how genetic information is stored, passed from parents to offspring, and how this creates variation within populations. You'll explore DNA structure, how genes control characteristics, inheritance patterns including genetic diagrams, and the role of natural selection in evolution. This topic forms the foundation for understanding both classical genetics and modern applications like genetic engineering.

Key terms and definitions

Gene — a section of DNA that codes for a specific protein, which determines a particular characteristic

Allele — a different version of the same gene (e.g., brown eye allele vs blue eye allele)

Dominant allele — an allele that is expressed even when only one copy is present; represented by a capital letter

Recessive allele — an allele that is only expressed when two copies are present (no dominant allele present); represented by a lowercase letter

Genotype — the combination of alleles an organism has (e.g., Bb, bb)

Phenotype — the observable characteristics resulting from the genotype and environmental factors

Homozygous — having two identical alleles for a gene (e.g., BB or bb)

Heterozygous — having two different alleles for a gene (e.g., Bb)

Core concepts

DNA structure and function

DNA (deoxyribonucleic acid) is the molecule that carries genetic information in all living organisms. It is located in the nucleus of cells, within chromosomes.

Structure of DNA:

• Double helix shape (like a twisted ladder)
• Made of two strands wound around each other
• Each strand consists of repeating units called nucleotides
• Each nucleotide contains: a sugar molecule, a phosphate group, and one of four bases (A, T, C, G)
• The bases pair up in a complementary way: adenine (A) pairs with thymine (T), cytosine (C) pairs with guanine (G)
• The sugar and phosphate molecules form the "backbone" of the DNA strand
• The paired bases form the "rungs" of the ladder

How DNA codes for proteins:

• A gene is a sequence of bases that codes for a particular protein
• The sequence of bases determines the order of amino acids in a protein
• Three bases code for one amino acid (a triplet or codon)
• Proteins determine the structure and function of cells, and therefore the characteristics of organisms
• Different versions of genes (alleles) code for slightly different proteins, producing different characteristics

Chromosomes and cell division

Human body cells contain 46 chromosomes (23 pairs). One chromosome from each pair comes from each parent. The 23rd pair are the sex chromosomes: XX in females, XY in males.

Mitosis produces two genetically identical daughter cells for growth and repair:

• DNA replicates so each chromosome becomes two identical chromatids
• Chromosomes line up at the cell's equator
• Chromatids separate and move to opposite poles
• Two daughter cells form, each with 46 chromosomes (diploid number)
• Genetically identical to parent cell and each other

Meiosis produces four non-identical gametes (sex cells) for sexual reproduction:

• Occurs in reproductive organs (testes and ovaries)
• DNA replicates, then the cell divides twice
• Produces four daughter cells, each with 23 chromosomes (haploid number)
• Gametes are genetically different from each other due to random assortment and recombination
• At fertilisation, gametes fuse to restore the diploid number (46 chromosomes)

Monohybrid inheritance

Monohybrid inheritance examines how a single characteristic controlled by one gene is inherited.

Key principles:

• Each parent has two alleles for each gene
• Gametes receive only one allele from each pair (during meiosis)
• Offspring inherit one allele from each parent
• Dominant alleles mask recessive alleles in the phenotype

Using genetic diagrams:

1. Identify the characteristic and the alleles involved
2. Determine parents' genotypes
3. Work out the possible gametes each parent can produce
4. Use a Punnett square to show all possible offspring combinations
5. Determine the ratio of genotypes and phenotypes

Standard notation:

• Use the same letter for both alleles of a gene
• Capital letter for dominant allele
• Lowercase letter for recessive allele
• Always show both alleles (e.g., BB, Bb, bb)

Ratios in monohybrid crosses:

• Heterozygous × heterozygous (Bb × Bb) gives 3:1 phenotype ratio (dominant:recessive)
• Heterozygous × homozygous recessive (Bb × bb) gives 1:1 phenotype ratio
• Genotype ratios can differ from phenotype ratios when dominant alleles are present

Genetic variation and mutation

Variation exists within all species. This variation can be genetic, environmental, or a combination of both.

Sources of genetic variation:

• Sexual reproduction combines alleles from two parents
• Random fusion of gametes at fertilisation
• Meiosis produces genetically different gametes through independent assortment of chromosomes
• Mutations create new alleles

Mutations:

• A mutation is a random change in DNA sequence
• Can occur spontaneously or be caused by mutagens (radiation, chemicals)
• Most mutations have no effect on phenotype
• Some mutations are harmful (e.g., cause genetic disorders)
• Rarely, mutations are beneficial and increase survival chances
• Mutations are the only source of new alleles in a population
• If a mutation occurs in a gamete, it can be inherited

Environmental variation:

• Characteristics influenced by environmental factors include: language spoken, scars, muscle mass
• Most characteristics result from both genetic and environmental factors (e.g., height, skin colour, intelligence)

Natural selection and evolution

Evolution is the change in inherited characteristics of a population over time, through natural selection.

Natural selection process:

1. Variation — individuals in a population show variation due to genetic differences
2. Competition — organisms compete for limited resources (food, mates, territory)
3. Selection — individuals with characteristics best suited to their environment are more likely to survive
4. Reproduction — survivors reproduce and pass on advantageous alleles to offspring
5. Inheritance — over many generations, advantageous alleles become more common in the population

Key points:

• Natural selection acts on phenotypes, not genotypes directly
• Organisms do not adapt during their lifetime; populations evolve over generations
• The environment determines which characteristics are advantageous
• Takes many generations — evolution is not a quick process

Examples of natural selection:

• Antibiotic resistance in bacteria: bacteria with resistance alleles survive treatment and reproduce
• Peppered moths: dark moths were better camouflaged during industrial revolution when trees were soot-covered
• Darwin's finches: different beak shapes evolved to exploit different food sources on Galápagos Islands

Selective breeding and genetic engineering

Selective breeding (artificial selection) involves humans choosing organisms with desirable characteristics to breed together over many generations.

Process:

1. Select parents with desired characteristics
2. Breed them together
3. Select offspring showing desired characteristics most strongly
4. Repeat over many generations

Advantages:

• Produces organisms with specific, desirable features
• Used in agriculture to improve crop yield, disease resistance
• Used to breed domestic animals for temperament, appearance, productivity

Disadvantages:

• Reduces genetic variation (gene pool becomes smaller)
• Increases risk of genetic disorders
• Inbreeding can cause health problems
• Organisms may be less able to cope with environmental changes

Genetic engineering involves directly modifying an organism's genome by inserting genes from another organism.

Applications:

• Bacteria engineered to produce human insulin for diabetes treatment
• Golden rice modified to contain beta-carotene (vitamin A precursor)
• Crops engineered for herbicide resistance or pest resistance

Ethical considerations:

• Long-term effects may be unknown
• Concerns about "playing God" or unnatural interference
• Potential benefits (feeding populations, treating disease) vs potential risks
• Genetically modified organisms might affect ecosystems if released

Worked examples

Example 1: Monohybrid cross

Question: In pea plants, tall stem (T) is dominant to short stem (t). A heterozygous tall plant is crossed with a short plant. Show the cross using a genetic diagram and determine the ratio of tall to short plants in the offspring. (4 marks)

Answer:

Parents' genotypes: Tt × tt

Parents' phenotypes: tall × short

Gametes: T and t from first parent; t and t from second parent

Punnett square:

	t	t
T	Tt	Tt
t	tt	tt

Offspring genotypes: Tt, Tt, tt, tt (or 2 Tt : 2 tt)

Offspring phenotypes: 2 tall : 2 short, or ratio 1:1

Mark scheme: 1 mark for correct parent genotypes, 1 mark for correct gametes, 1 mark for correct offspring genotypes, 1 mark for correct ratio.

Example 2: Natural selection

Question: Explain how antibiotic resistance in bacteria provides evidence for natural selection. (4 marks)

Answer:

• Variation exists in bacterial populations due to random mutations (1)
• Some bacteria possess alleles that give resistance to antibiotics (1)
• When antibiotics are used, bacteria without resistance die, but resistant bacteria survive (1)
• Resistant bacteria reproduce, passing on resistance alleles to offspring, so resistance becomes more common in the population (1)

Mark scheme: Look for reference to variation/mutation, survival advantage, reproduction, and inheritance of advantageous alleles over generations.

Example 3: DNA structure

Question: (a) State which bases pair together in DNA. (2 marks) (b) Explain why DNA replication must occur before cell division. (2 marks)

Answer:

(a)

• Adenine pairs with thymine / A pairs with T (1)
• Cytosine pairs with guanine / C pairs with G (1)

(b)

• So each daughter cell receives a complete copy of DNA/genetic information (1)
• To ensure daughter cells have the same number of chromosomes as the parent cell / to maintain chromosome number (1)

Mark scheme: Accept any sensible wording showing understanding of complementary base pairing and the need for identical genetic information in daughter cells.

Common mistakes and how to avoid them

• Confusing genes and alleles — Remember: genes are sections of DNA coding for characteristics; alleles are different versions of the same gene. Use precise terminology in answers.

• Incorrect genetic diagram notation — Always use the same letter for both alleles (Bb not Bt), show both alleles even for homozygous organisms (BB not B), and ensure capital letters are clearly distinguishable from lowercase.

• Mixing up genotype and phenotype — Genotype is the combination of alleles (letters); phenotype is the observable characteristic (words describing appearance). Label these clearly in genetic diagrams.

• Stating organisms "need" to evolve — Evolution has no direction or purpose. Don't say "bacteria became resistant because they needed to survive." Instead, say "bacteria with resistance alleles survived when others died; they reproduced and passed on these alleles."

• Forgetting meiosis produces haploid gametes — Gametes have half the chromosome number (23 in humans), not 46. This is essential so fertilisation restores the diploid number.

• Assuming dominant alleles are "better" — Dominant and recessive refer only to how alleles are expressed, not to their quality or frequency in populations. Recessive alleles can be common and beneficial.

Exam technique for "B5: Genes, Inheritance and Selection"

• Command words matter — "State" requires a simple answer with no explanation; "Explain" requires reasoning; "Describe" requires a sequence or account without reasoning. "Suggest" means the answer may not be directly from specification content.

• Show working in genetic diagrams — Even if you know the answer, show parent genotypes, gametes, and Punnett square. Marks are awarded for method, not just the final ratio.

• Use specific examples — When discussing natural selection or selective breeding, give named examples (antibiotic resistance in bacteria, not just "bacteria evolve"). This demonstrates depth of understanding.

• Check ratio format — Express ratios in simplest form (3:1 not 6:2) and specify what the ratio represents (genotype or phenotype, and which is which).

Quick revision summary

DNA is a double helix made of nucleotides with complementary base pairs (A-T, C-G). Genes code for proteins that determine characteristics. Mitosis produces identical cells; meiosis produces haploid gametes. In inheritance, alleles can be dominant or recessive. Monohybrid crosses predict offspring ratios using genetic diagrams. Variation arises from sexual reproduction and mutations. Natural selection occurs when organisms with advantageous characteristics survive, reproduce, and pass on beneficial alleles, causing populations to evolve over generations. Selective breeding and genetic engineering allow humans to modify organisms deliberately.