Inheritance
Summary: Inheritance is the transmission of genetic information from parents to offspring. DNA is organised into chromosomes in the nucleus; each chromosome contains many genes. A gene is a section of DNA that codes for a particular protein. Alleles are different versions of the same gene. Mitosis produces genetically identical diploid cells (growth and repair). Meiosis produces genetically varied haploid gametes (sexual reproduction). Monohybrid crosses using Punnett squares predict offspring genotypes (e.g. 3:1 ratio for heterozygous crosses). Sex is determined by X and Y chromosomes (XX = female, XY = male). Tags: igcse biology inheritance DNA mitosis meiosis monohybrid punnett-square sex-determination Created: 2026-07-16 Last Updated: 2026-07-16
1. Chromosomes, Genes, and Alleles
| Term | Definition |
|---|---|
| Chromosome | A thread-like structure made of DNA found in the nucleus of cells. Humans have 46 chromosomes (23 pairs) in each body cell |
| Gene | A section of DNA on a chromosome that codes for a specific protein (or polypeptide). Genes determine our characteristics (traits) |
| Allele | A different version (variant) of the same gene. For example, the gene for eye colour may have alleles for brown, blue, or green. An individual inherits two alleles for each gene — one from each parent |
| Genome | The entire DNA sequence of an organism — all the genes |
DNA Recap
DNA (deoxyribonucleic acid) is a double helix molecule made of nucleotides. Each nucleotide contains a sugar (deoxyribose), a phosphate group, and a nitrogenous base (A, T, C, or G). The sequence of bases determines the sequence of amino acids in a protein. See Biological Molecules for more detail.
2. Key Genetic Terms
| Term | Definition |
|---|---|
| Genotype | The genetic makeup of an organism — the combination of alleles an individual possesses (e.g. BB, Bb, or bb) |
| Phenotype | The observable (physical) characteristics of an organism — the result of the genotype interacting with the environment (e.g. brown eyes, tall height) |
| Homozygous | Having two identical alleles for a gene (e.g. BB or bb). “True-breeding” |
| Heterozygous | Having two different alleles for a gene (e.g. Bb). “Hybrid” or “carrier” |
| Dominant allele | An allele whose phenotype is expressed even when only one copy is present (in a heterozygote). Represented by a capital letter (e.g. B for brown eyes) |
| Recessive allele | An allele whose phenotype is only expressed when both copies are present (homozygous recessive). Represented by a lowercase letter (e.g. b for blue eyes) |
| Diploid | Cells containing the full set of chromosomes — 46 in humans (23 pairs). All body cells are diploid |
| Haploid | Cells containing half the number of chromosomes — 23 in humans. Only gametes (sperm and egg) are haploid |
3. Mitosis
Mitosis is the type of cell division that produces two genetically identical diploid daughter cells from one parent cell.
Where it occurs: In all body cells (somatic cells) — for growth, repair, and replacement of cells.
Key features:
- Produces two daughter cells
- Daughter cells are genetically identical to each other and to the parent cell
- Daughter cells are diploid (46 chromosomes in humans — the same as the parent)
- One round of division
- No genetic variation is introduced (unless a mutation occurs)
- Chromosomes are copied once, then the cell divides once
Example uses in the body:
- Growth of an organism (increasing cell number)
- Repair of damaged tissues (e.g. skin healing after a cut)
- Replacement of worn-out cells (e.g. red blood cells, skin cells)
- Asexual reproduction in some organisms
4. Meiosis
Meiosis is the type of cell division that produces four genetically different haploid gametes from one parent cell.
Where it occurs: Only in the reproductive organs (ovaries and testes) — to produce gametes (eggs and sperm).
Key features:
- Produces four daughter cells
- Daughter cells are genetically different from each other and from the parent cell
- Daughter cells are haploid (23 chromosomes in humans — half the parent cell’s number)
- Two rounds of division (Meiosis I and Meiosis II)
- Genetic variation is introduced through:
- Crossing over — homologous chromosomes exchange sections of DNA during Prophase I
- Independent assortment — chromosomes are randomly distributed to daughter cells
- Chromosomes are copied once, then the cell divides twice
Why meiosis is important:
- Produces haploid gametes — when sperm and egg fuse at fertilisation, the diploid number (46) is restored
- Without meiosis, chromosome number would double each generation
- Introduces genetic variation — essential for evolution by natural selection
5. Monohybrid Crosses
A monohybrid cross involves one gene with two alleles.
Mendel’s Experiments
Gregor Mendel studied pea plants and discovered the basic principles of inheritance:
- Characteristics are determined by discrete units (now called genes)
- Each organism inherits two copies of each gene (one from each parent)
- One allele may be dominant over the other (recessive)
- Alleles segregate during gamete formation — each gamete receives only one copy
Punnett Squares
A Punnett square is a grid used to predict the possible genotypes (and phenotypes) of offspring from a given cross.
Example — Monohybrid Cross with Heterozygous Parents (Bb x Bb):
Let B = brown eyes (dominant), b = blue eyes (recessive). Parent genotypes: both Bb (heterozygous).
| Gametes | B | b |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Offspring genotypes: 1 BB : 2 Bb : 1 bb (1:2:1 ratio) Offspring phenotypes: 3 brown eyes : 1 blue eye (3:1 ratio)
The 3:1 phenotypic ratio is characteristic of a cross between two heterozygous parents — this ratio only appears when dominance is complete.
Common Monohybrid Ratios
| Cross | Genotypic Ratio | Phenotypic Ratio |
|---|---|---|
| Homozygous dominant x Homozygous recessive (BB x bb) | All Bb | All dominant phenotype |
| Heterozygous x Heterozygous (Bb x Bb) | 1 BB : 2 Bb : 1 bb | 3 dominant : 1 recessive |
| Heterozygous x Homozygous recessive (Bb x bb) — a test cross | 1 Bb : 1 bb | 1 dominant : 1 recessive |
A test cross is used to determine whether an organism showing the dominant phenotype is homozygous dominant (BB) or heterozygous (Bb) — cross it with a homozygous recessive (bb) individual and observe the offspring.
6. Codominance
In codominance, both alleles are expressed in the heterozygote — neither is dominant or recessive.
Example — ABO blood groups in humans:
- Three alleles: IA, IB, and IO
- IA and IB are codominant (both expressed when present together)
- IO is recessive to both IA and IB
| Genotype | Blood Group (Phenotype) |
|---|---|
| IA IA or IA IO | A |
| IB IB or IB IO | B |
| IA IB | AB (both A and B antigens expressed — codominance) |
| IO IO | O |
7. Sex Determination
In humans, sex is determined by the X and Y chromosomes:
- Females: XX — all eggs contain one X chromosome
- Males: XY — half of sperm contain an X chromosome, half contain a Y chromosome
Punnett square for sex determination:
| Gametes | X (egg) | X (egg) |
|---|---|---|
| X (sperm) | XX (female) | XX (female) |
| Y (sperm) | XY (male) | XY (male) |
The sex of offspring is determined by the sperm — a 50% chance of male (Y sperm) or female (X sperm) at each fertilisation.
8. Sex-Linked Inheritance
Sex-linked genes are carried on the X chromosome (the Y chromosome is much smaller and carries few genes).
Example — Red-green colour blindness:
- Caused by a recessive allele on the X chromosome
- Males are more commonly affected because they have only one X chromosome — if it carries the recessive allele, they express the trait (there is no second X to mask it)
- Females can be carriers (heterozygous) — they have one affected and one normal X chromosome, so they do not show the trait but can pass the affected X to offspring
Notation: XN = normal allele, Xn = colour-blind allele
| Genotype | Phenotype |
|---|---|
| XN XN | Female, normal vision |
| XN Xn | Female, normal vision (carrier) |
| Xn Xn | Female, colour-blind (rare) |
| XN Y | Male, normal vision |
| Xn Y | Male, colour-blind |
Related Notes
- Biological Molecules — DNA structure (double helix, nucleotides, base pairing)
- Cell Structure and Organisation — Nucleus contains chromosomes; ribosomes synthesise proteins from genetic code
- Variation and Selection — Genetic variation from meiosis enables natural selection
- Reproduction — Meiosis produces gametes for sexual reproduction
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Inheritance / DNA and genetics, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 12: Mendel’s Experiments and Heredity / Chapter 14: DNA Structure and Function, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 17: Inheritance, Cambridge Assessment International Education
- CK-12 Biology for High School — Genetics chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Dominant means ‘most common’ or ‘better‘“ | Dominant simply means the allele is expressed in the heterozygote. A dominant allele can be rare (e.g. polydactyly — extra fingers is caused by a rare dominant allele) |
| “All mutations are harmful” | Some mutations are neutral (no effect on phenotype) or even beneficial (providing an advantage for natural selection). Mutations are the ultimate source of genetic variation |
| ”Mitosis produces gametes” | Meiosis produces gametes (haploid). Mitosis produces identical diploid cells for growth and repair |
| ”The 3:1 ratio means exactly 3 offspring will show the dominant trait” | The ratio is a probability — in a large number of offspring, approximately 3/4 will show the dominant phenotype. With only a few offspring, the actual ratio can vary by chance |
| ”Males determine the sex of the baby, so the father is responsible for any sex-linked disorder” | True that the father determines sex (X or Y sperm), but for X-linked disorders, affected males inherit the condition from their mother (a carrier) — the father passes his Y chromosome to sons, not his X |
| ”Meiosis reduces chromosome number by dividing the parent cell in half” | Meiosis reduces chromosome number by separating homologous pairs (not by physically cutting chromosomes). The diploid number (46) is halved to the haploid number (23) |