IB DP Biology: Inheritance: alleles, monohybrid crosses, pedigrees and continuous variation (DP Biology) – Revision Notes

These are condensed notes for D3.2.1 to D3.2.15 (Inheritance, first part) in the IB Biology guide, first assessment 2025, for DP Biology. Everything here is SL and HL. Full explanations are in the study guide; test yourself with the practice questions. Course hub: /boards/ib/ib-dp/biology/. Checklist: /checklists/ib/ib-dp/biology/. Check your readiness with the free diagnostics.

Key terms

Term	Meaning
Gamete	Haploid sex cell made by meiosis
Zygote	Diploid cell formed when two gametes fuse
Gene	Heritable factor controlling a trait
Allele	One version of a gene
Genotype	The combination of alleles inherited
Phenotype	Observable traits: genotype plus environment
Homozygous	Two identical alleles of a gene
Heterozygous	Two different alleles of a gene
Dominant	Same phenotype in heterozygote and homozygote
Recessive	Shows only in the homozygote
Carrier	Heterozygote for a recessive allele, usually unaffected
Gene pool	All alleles of all genes in a population

D3.2.1 and D3.2.2 Gametes and plant crosses

• Meiosis makes haploid gametes. Fusion gives a diploid zygote. This holds for all eukaryotes with a sexual life cycle.
• Diploid cells carry two copies of every autosomal gene; a gamete carries one.
• Terms: P generation (parents), F1 (their offspring), F2 (F1 crossed together), Punnett grid.
• Pollen holds male gametes; female gametes are in the ovary, so pollination is required.
• Peas have male and female parts in one flower, so they can self-pollinate and self-fertilise.
• Crosses are used to breed new crop and ornamental varieties.

Method: a controlled cross in a flowering plant

1. Remove the anthers from the female parent before pollen is shed.
2. Cover the flower.
3. Transfer pollen from the chosen male parent to the stigma.
4. Cover again, collect the seeds, sow them and score the F1.

D3.2.3 to D3.2.6 Genotype, phenotype, dominance, plasticity

• A gene is not an allele. Pod shape is a gene; inflated and constricted are determined by alleles.
• Genotype only: ABO blood group. Environment only: a scar. Both: height.
• A dominant allele usually codes for a working polypeptide, so one copy gives enough product. That is why homozygous-dominant and heterozygous organisms look the same.
• Plasticity: traits suited to the environment arise through varying gene expression. The genotype does not change. The change may be reversible.

Plasticity examples to quote

• Water crowfoot: finely divided leaves when submerged, broad leaves in air.
• Humans at altitude: more red blood cells, reversible on return to low altitude.
• In each case say the genotype is unchanged and gene expression varies with the environment.

Phenotype sorting table

Trait	Main cause
ABO blood group	Genotype only
A scar	Environment only
Adult height	Genotype and environment
Outcome of PKU	Genotype and diet

D3.2.7 PKU

• Autosomal recessive. The gene codes for the enzyme that converts phenylalanine to tyrosine.
• Two carriers: 1/4 affected, 1/2 carrier, 1/4 unaffected non-carrier per child.
• Diet low in phenylalanine changes the environment, so the phenotype, not the genotype.

Worked reminder. Two PKU carriers expect three children. Each child is affected with probability 1/4, independently. The probability that none is affected is (3/4)³ = 27/64, which is about 0.42.

D3.2.8 to D3.2.10 SNPs, multiple alleles, ABO, dominance patterns

• SNP: a one-base difference at a position in the genome between individuals.
• Many alleles may exist in the gene pool; an individual has two.
• With n alleles there are n(n + 1)/2 genotypes.

Pattern	Heterozygote shows	Example
Complete dominance	Dominant phenotype	Pod shape
Codominance	Both phenotypes at once	Group AB (IᴬIᴮ)
Incomplete dominance	Intermediate phenotype	Pink Mirabilis jalapa flowers

ABO alleles: Iᴬ, Iᴮ, i. Group A = IᴬIᴬ or Iᴬi. Group B = IᴮIᴮ or Iᴮi. Group AB = IᴬIᴮ. Group O = ii.

Method: solving a cross

1. Write the parental genotypes.
2. Write the gametes of each parent.
3. Fill in a Punnett grid.
4. Read off genotype and phenotype ratios, then the probability asked.

Worked reminder. A group A father (Iᴬi) and a group A mother (Iᴬi) can have a group O child. Chance = 1/4, because the child must receive i from both.

D3.2.11 and D3.2.12 Sex linkage and haemophilia

• Females XX; males XY. The sperm’s sex chromosome decides the sex of the zygote. The X carries far more genes than the Y.
• Write alleles as superscripts on X: Xᴴ normal, Xʰ haemophilia.
• Males have one X, so a single recessive allele gives the disorder.
• Carrier mother × unaffected father: 1/2 of sons affected, 1/2 of daughters carriers, no affected daughters.
• Affected father × non-carrier mother: all daughters carriers, all sons unaffected.
• A son always gets his X from his mother.
• Worked reminder: carrier mother (XᴴXʰ) × unaffected father (XᴴY). Gametes: Xᴴ or Xʰ; Xᴴ or Y. Offspring: XᴴXᴴ, XᴴY, XᴴXʰ, XʰY, each 1/4. Chance a child is affected = 1/4.

Must-know distinctions

• Gene or allele: the gene is the region controlling the trait; alleles are its versions.
• Codominance or incomplete dominance: codominance shows both phenotypes side by side; incomplete dominance gives a blend between the two.
• Carrier or affected: a carrier of a recessive autosomal condition has one copy of the allele and no symptoms.
• Plasticity or mutation: plasticity changes expression only; a mutation changes the base sequence.
• Discrete or continuous: count the categories, or measure on a scale.
• Inductive or deductive: from cases to pattern, or from pattern to a particular case.

D3.2.13 Pedigree rules

Observation	Conclusion
Two unaffected parents, affected child	Recessive; both parents carriers
Two affected parents, unaffected child	Dominant; both parents heterozygous
Unaffected father, affected daughter	Not X-linked recessive
Affected mother, unaffected son	Not X-linked recessive

• Inductive reasoning: generalising a pattern from some cases. Deductive reasoning: applying that pattern to find a specific genotype.
• Marriage between close relatives is prohibited in many societies. Relatives share recent ancestors, so both are more likely to carry the same rare recessive allele.

D3.2.14 and D3.2.15 Continuous variation and box plots

• Continuous: any value in a range, such as height or skin colour. Discrete: categories, such as ABO group.
• Causes of continuous variation: polygenic inheritance, environment, or both. Skin colour is the example.
• Mean = sum ÷ count. Median = middle value. Mode = most frequent value.

Method: a box-and-whisker plot

1. Order the data. Find the median.
2. Find the first quartile (median of the lower half) and third quartile (median of the upper half).
3. IQR = third quartile − first quartile.
4. Limits: first quartile − 1.5 × IQR and third quartile + 1.5 × IQR. Values beyond them are outliers, plotted separately.
5. Whiskers run to the smallest and largest values that are not outliers.

Worked reminder. Masses (g): 62, 64, 64, 66, 68, 69, 71, 90.

• Median = (66 + 68) ÷ 2 = 67. First quartile = (64 + 64) ÷ 2 = 64. Third quartile = (69 + 71) ÷ 2 = 70.
• IQR = 6, so 1.5 × IQR = 9. Limits: 55 and 79. The value 90 is an outlier.
• Whiskers: 62 and 71. Mean = 554 ÷ 8 = 69.25. Mode = 64.

Quick self-test

1. A plant has 2n = 20. How many chromosomes are in one of its gametes?
2. Why is a PKU carrier usually unaffected?
3. How many genotypes are possible for a gene with 3 alleles?
4. A woman of group O and a man of group AB have children. Which groups are possible?
5. A pink Mirabilis jalapa is crossed with a white one. State the offspring phenotype ratio.
6. A haemophiliac man and a non-carrier woman have a son. What is the chance he is affected?
7. Parents are unaffected and have an affected daughter. Give the inheritance pattern.
8. Why is skin colour described as continuous variation?
9. For the masses above, state the range and the IQR.
10. Name one feature that separates plasticity from a mutation.

Answers

2. The carrier has one working allele, which makes enough functional enzyme.
3. 3 × 4 ÷ 2 = 6.
4. Group A (Iᴬi) and group B (Iᴮi), each with probability 1/2.
5. 1 pink : 1 white.
6. Zero. The son gets his father’s Y and his mother’s Xᴴ.
7. Autosomal recessive.
8. It shows a smooth range of values, produced by many genes and by sun exposure.
9. Range = 90 − 62 = 28 g. IQR = 70 − 64 = 6 g.
10. Plasticity changes gene expression, not the DNA sequence, and can be reversible.

Where marks are usually lost

• Writing “the gene is dominant” when it is the allele.
• Leaving out the X on sex-linked genotypes.
• Giving a Punnett grid with no statement of the parental genotypes.
• Saying a carrier “has the disease”.
• Quoting a 1 : 2 : 1 ratio for codominance without saying the genotypes and phenotypes match.
• Naming an outlier without showing the 1.5 × IQR working.
• Describing dominance as “the stronger allele”.
• Giving “inheritance” instead of the specific pattern, such as autosomal recessive.
• Stating a pedigree genotype with no reasoning from the family’s pattern.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section D3.2 Inheritance (statements D3.2.1 to D3.2.15).