Inheritance

What you'll learn

Inheritance describes how characteristics pass from parents to offspring through genes. CIE IGCSE Biology exams test your ability to construct genetic diagrams, predict offspring ratios, and explain patterns of inheritance. Understanding this topic requires mastery of precise terminology and the ability to apply genetic principles to unfamiliar contexts.

Key terms and definitions

Gene — a section of DNA that codes for a specific protein, controlling a particular characteristic.

Allele — a variant form of a gene. Most genes exist in two or more different forms (e.g. the gene for eye colour has a brown allele and a blue allele).

Dominant allele — an allele that is expressed in the phenotype even when only one copy is present. Represented by a capital letter (e.g. B).

Recessive allele — an allele that is only expressed in the phenotype when two copies are present (homozygous). Represented by a lower-case letter (e.g. b).

Genotype — the genetic composition of an organism, describing which alleles are present (e.g. BB, Bb, or bb).

Phenotype — the observable characteristics of an organism, resulting from both genotype and environmental factors (e.g. brown eyes or blue eyes).

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

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

Core concepts

Chromosomes, genes and alleles

Human body cells contain 23 pairs of chromosomes (46 total). One chromosome in each pair comes from the mother, the other from the father. Genes are located at specific positions along chromosomes.

• Each gene may exist in different versions called alleles
• Diploid organisms have two copies of each gene (one on each chromosome in a pair)
• These two alleles may be the same (homozygous) or different (heterozygous)
• During gamete formation by meiosis, chromosome pairs separate so each gamete receives only one chromosome from each pair
• Gametes are haploid — they contain 23 chromosomes in humans (half the diploid number)
• At fertilisation, gametes fuse to restore the diploid number

Dominant and recessive inheritance

When an organism is heterozygous (e.g. Bb), only the dominant allele affects the phenotype. The recessive allele is present in the genotype but not expressed.

For a recessive characteristic to appear in the phenotype, the organism must be homozygous recessive (e.g. bb). This is why recessive characteristics can "skip generations" — two heterozygous parents (Bb) can produce homozygous recessive offspring (bb).

Key patterns:

• Dominant homozygous (BB) and heterozygous (Bb) individuals have identical phenotypes
• Only genetic diagrams or breeding experiments can distinguish between BB and Bb genotypes
• Two parents showing a dominant characteristic can have offspring showing the recessive characteristic if both parents are heterozygous
• Two parents showing a recessive characteristic (both bb) can only produce offspring showing the recessive characteristic

Constructing genetic diagrams

CIE IGCSE Biology requires you to construct clear, systematic genetic diagrams following these steps:

1. Define the alleles — State which letter represents each allele and what characteristic it controls. Example: "Let B = allele for brown eyes (dominant), b = allele for blue eyes (recessive)"

2. State parental phenotypes — Describe the observable characteristics of each parent

3. State parental genotypes — Write the genetic composition of each parent (e.g. Bb × Bb)

4. State gamete genotypes — Show which alleles each parent can pass to offspring. Use circles or underlining to show gametes clearly

5. Complete a Punnett square — Draw a 2×2 grid showing all possible offspring combinations

6. State offspring genotypes — List all possible genetic compositions and their ratios

7. State offspring phenotypes — Describe observable characteristics and their expected ratios

Punnett square method:

For a cross between two heterozygous parents (Bb × Bb):

        B    b
    -----------
  B | BB | Bb |
    -----------
  b | Bb | bb |
    -----------

Offspring genotypes: 1 BB : 2 Bb : 1 bb

Offspring phenotypes: 3 brown eyes : 1 blue eyes

Monohybrid crosses

A monohybrid cross examines the inheritance of a single gene. Exam questions typically involve:

Homozygous dominant × homozygous recessive (e.g. BB × bb):

• All offspring are heterozygous (Bb)
• All offspring show the dominant phenotype
• This is called the F1 generation (first filial generation)

Heterozygous × heterozygous (e.g. Bb × Bb):

• Offspring ratio is 1:2:1 genotypic (BB:Bb:bb)
• Offspring ratio is 3:1 phenotypic (dominant:recessive)
• This is the F2 generation when F1 individuals are crossed

Test cross (Bb × bb):

• Used to determine whether an individual showing the dominant phenotype is BB or Bb
• If parent is Bb, offspring ratio is 1:1 (50% dominant phenotype, 50% recessive phenotype)
• If parent is BB, all offspring show dominant phenotype

Sex determination in humans

Humans have 23 pairs of chromosomes. The 23rd pair comprises the sex chromosomes, which determine biological sex:

• Females have two X chromosomes (XX)
• Males have one X and one Y chromosome (XY)
• The Y chromosome carries the gene for male development

During gamete formation:

• All egg cells contain an X chromosome
• Sperm cells contain either an X or a Y chromosome (50% of each)
• At fertilisation, there is a 50% chance of XX (female) and 50% chance of XY (male)

Genetic diagram for sex determination:

Parental phenotypes: Female × Male
Parental genotypes:  XX × XY
Gametes:             X    X or Y

        X    X
    -----------
  X | XX | XX |
    -----------
  Y | XY | XY |
    -----------

Offspring ratio: 1 XX : 1 XY (1 female : 1 male or 50%:50%)

Codominance

In codominance, both alleles in a heterozygous individual are expressed in the phenotype. Neither allele is dominant or recessive to the other.

Example: ABO blood groups involve codominance (though full ABO genetics includes three alleles, IGCSE focuses on simpler examples).

A common IGCSE example uses coat colour:

• Allele R codes for red colour
• Allele W codes for white colour
• Heterozygous individuals (RW) show both colours — roan (red and white patches)

Codominant alleles are represented with capital letters and superscripts or different capital letters, not upper and lower case.

Environmental effects on phenotype

Phenotype results from both genotype and environmental factors. Exam questions test understanding that identical genotypes can produce different phenotypes in different environments.

Examples:

• Plant height depends on genes AND availability of water, minerals, and light
• Human body mass depends on genes AND diet and exercise
• Fur colour in Himalayan rabbits depends on genes AND temperature (darker fur grows in colder areas)
• Identical twins (same genotype) may develop different characteristics due to environmental differences

Genes set the potential range, but environment determines where within that range the actual phenotype falls.

Worked examples

Example 1: Monohybrid cross with seed shape

Question: In pea plants, the allele for round seeds (R) is dominant to the allele for wrinkled seeds (r). A heterozygous plant is crossed with a wrinkled seed plant. Draw a genetic diagram to show this cross and state the expected ratio of phenotypes in the offspring. [4 marks]

Answer:

Alleles: R = round seeds (dominant), r = wrinkled seeds (recessive)

Parental phenotypes: Round seeds × Wrinkled seeds

Parental genotypes: Rr × rr

Gametes: (R or r) × (r)

Punnett square:

        R    r
    -----------
  r | Rr | rr |
    -----------
  r | Rr | rr |
    -----------

Offspring genotypes: 2 Rr : 2 rr (or 1 Rr : 1 rr)

Offspring phenotypes: 2 round : 2 wrinkled (or 1:1 ratio, or 50% round and 50% wrinkled)

Mark scheme notes: Award 1 mark for correct parental genotypes, 1 mark for correct gametes, 1 mark for correct Punnett square, 1 mark for correct phenotype ratio.

Example 2: Predicting outcomes from data

Question: A breeder crosses two black guinea pigs. The offspring include 15 black guinea pigs and 5 brown guinea pigs. Explain these results using a genetic diagram. [5 marks]

Answer:

The 3:1 ratio (approximately 15:5) suggests both parents are heterozygous and black is dominant to brown.

Alleles: B = black fur (dominant), b = brown fur (recessive)

Parental phenotypes: Black × Black

Parental genotypes: Bb × Bb

Gametes: (B or b) × (B or b)

Punnett square:

        B    b
    -----------
  B | BB | Bb |
    -----------
  b | Bb | bb |
    -----------

Offspring genotypes: 1 BB : 2 Bb : 1 bb

Offspring phenotypes: 3 black : 1 brown

The brown offspring must be bb (homozygous recessive), which is only possible if both parents carry the b allele. Both parents must be Bb (heterozygous).

Mark scheme notes: Award 1 mark for identifying 3:1 ratio, 1 mark for correct allele notation, 1 mark for correct parental genotypes (Bb × Bb), 1 mark for correct genetic diagram, 1 mark for explanation linking offspring to parental genotypes.

Example 3: Sex determination application

Question: In a family of four children, all four are girls. Explain why the next child still has a 50% chance of being a boy. [3 marks]

Answer:

Sex is determined independently at each fertilisation event. Each sperm has an equal (50%) chance of carrying an X or a Y chromosome. Previous children do not affect which sex chromosome is present in the sperm that fertilises the next egg. The probability remains 50% male and 50% female for each pregnancy regardless of previous outcomes.

Mark scheme notes: Award 1 mark for stating sex is determined at fertilisation, 1 mark for explaining equal probability of X or Y sperm, 1 mark for stating each fertilisation is an independent event.

Common mistakes and how to avoid them

• Using incorrect letter notation: Students write "B" for both dominant and recessive alleles, or use completely different letters (A and B instead of A and a). Always use the same letter with capital for dominant and lower-case for recessive (e.g. B and b, not B and r).

• Confusing genotype and phenotype: Writing "the phenotype is Bb" is incorrect. Bb is always a genotype. The phenotype is the observable characteristic (e.g. "brown eyes" or "tall plant"). Keep these terms distinct.

• Forgetting to show gametes: Jumping straight from parental genotypes to offspring without showing gametes loses marks. Always include a clear gametes row showing which alleles can be passed on (e.g. gametes: B or b).

• Incorrect ratio format: Writing offspring ratios as fractions (1/4 BB) instead of ratios (1 BB : 2 Bb : 1 bb). Use colon notation for genotype ratios and phenotype ratios, or state as percentages (25% BB, 50% Bb, 25% bb).

• Misunderstanding predicted ratios: Expected ratios are probabilities, not guaranteed outcomes. A 3:1 predicted ratio doesn't mean exactly 3 and 1 in small sample sizes. Actual results approximate predicted ratios; larger samples give closer matches.

• Assuming sex ratio must be 1:1 in families: While each child has a 50% probability of being male or female, small families can show deviations (e.g. all boys or all girls). The 1:1 ratio is a probability for each individual birth, not a guarantee for small groups.

Exam technique for Inheritance

• Command word "construct" or "draw a genetic diagram": Requires all seven steps listed earlier (alleles, parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes). Missing any component loses marks. Allocate 4-6 marks for full genetic diagrams.

• Command word "explain" with inheritance data: Requires you to construct a genetic diagram AND link it to the data. State why the observed ratio supports your genetic explanation (e.g. "The 3:1 ratio indicates both parents are heterozygous"). Worth 4-5 marks typically.

• Command word "state the ratio": Give a simple ratio using colons (3:1) or percentages (75%:25%). Both genotypic and phenotypic ratios may be asked for — read carefully. Usually 1 mark.

• Show clear working: Even if your final answer is wrong, correct method earns partial marks. Draw Punnett squares neatly, label all components, and show the logical sequence from parents to gametes to offspring. Examiners follow your reasoning and award marks for correct steps even if you make an error in one part.

Quick revision summary

Genes exist as different alleles; dominant alleles are expressed in heterozygous individuals while recessive alleles require homozygous genotypes. Construct genetic diagrams systematically: define alleles, state parental phenotypes and genotypes, show gametes, complete Punnett squares, state offspring genotypes and phenotypes with ratios. Monohybrid crosses between heterozygous parents produce 3:1 phenotypic ratios. Sex determination follows 1:1 ratio (XX female, XY male). Environmental factors interact with genotype to produce final phenotype. Master the seven-step genetic diagram method for consistent exam success.