How one genome produces hundreds of different cell types — from gene expression to polygenic inheritance.
Every cell in your body has the same DNA. So why does a neuron look and behave completely differently from a muscle cell?
Learning Objectives
AO1 Know that a locus is the location of genes on a chromosome
AO1 Understand the linkage of genes on a chromosome
AO2 Understand how cells become specialised through differential gene expression, producing active mRNA leading to protein synthesis
AO2 Understand how phenotypes are affected by multiple alleles or polygenic inheritance
Prior Knowledge Check
Diagnostic Quiz
Test what you remember from Topics 2B and 2C before we begin.
Diagnostic Q1
What is an allele?
A A section of DNA that codes for a protein
B A different form of a gene
C A type of chromosome
D A strand of mRNA
Diagnostic Q2
What does ‘homozygous’ mean?
A Having two different alleles for a gene
B Having two identical alleles for a gene
C Having only one allele for a gene
D Having three or more alleles for a gene
Diagnostic Q3
During which process is the diploid chromosome number halved?
A Mitosis
B DNA replication
C Meiosis
D Fertilisation
Diagnostic Q4
If a gene has alleles represented by B (dominant) and b (recessive), what is the phenotype of genotype Bb?
A The recessive phenotype
B A blend of both phenotypes
C The dominant phenotype
D Neither phenotype
Section 1
Gene Expression in Action
In a multicellular organism, every cell contains the same genomeThe complete set of genetic material (DNA) in an organism. — the same complete set of DNA. Yet a neuron looks and functions completely differently from a red blood cell. How?
The answer is cell differentiationThe process by which a less specialised cell becomes more specialised for a particular function.. Different types of cell produce different proteins. All cells make housekeeping proteins — structural proteins of membranes and enzymes needed for cellular respiration. But each cell type also produces additional, specialised proteins.
Protein Diversity Across Cell Types
Cell Type
Housekeeping Proteins
Extra Specialised Proteins
Cerebral cortex neuron
Yes
~318 extra proteins (neurotransmitters, ion channels)
Liver cell (hepatocyte)
Yes
Enzymes for detoxification, bile production
Red blood cell
Minimal (loses nucleus)
Haemoglobin
Smooth muscle cell
Yes
Mainly housekeeping proteins only
The degree of differentiation between cells can be measured. The proteomeThe entire set of proteins expressed by a cell, tissue, or organism at a given time. is the complete set of proteins produced by a cell. By comparing the proteomes of different cells, scientists can work out which genes have been expressed and which have been suppressed.
Interactive diagram — One genome, many cell types. Click each cell.
See — Think — Wonder
Scientists analyse the proteins from brain cells and liver cells of the same organism. Both cells have identical DNA, yet their protein profiles are completely different.
What do you SEE in this information?
What do you THINK is happening?
What do you WONDER about?
AO1 Retrieval
Which type of proteins are found in ALL cells regardless of specialisation?
A Haemoglobin
B Housekeeping proteins
C Neurotransmitter receptors
D Antibodies
Section 2
Multiple Alleles
A locusThe specific position of a gene on a chromosome. is the specific location of a gene on a chromosome. Each gene has at least two different forms, known as alleles. But some genes have more than two possible alleles — these are called multiple allelesWhen a gene has more than two possible alleles at a particular locus in a population..
The ABO Blood Group System
The best-known example is the human ABO blood group system. There are three alleles: IA, IB, and i. The alleles IA and IB are codominantWhen both alleles are expressed equally in the phenotype of a heterozygote. — both are expressed when present together. The allele i is recessive to both.
ABO Blood Group Genotypes
Genotype
Blood Group
Antigens on RBC
IAIA or IAi
A
A antigens
IBIB or IBi
B
B antigens
IAIB
AB
Both A and B antigens
ii
O
Neither antigen
Learning Tip
Remember: codominant means both alleles are fully expressed. This is different from incomplete dominance, where you get a blended phenotype. In blood group AB, both A and B antigens are present — not a mixture.
Productive Struggle — Try before you learn
A couple both have blood group A. Can they have a child with blood group O? Explain your reasoning.
Attempt this question before looking at the scaffolding below. Struggle is where learning happens.
Tiered Support
Using your knowledge of multiple alleles, determine whether two parents with blood group A can produce a child with blood group O. Show your working with a genetic cross.
Hint: Blood group A can be produced by two different genotypes. What are they? If both parents carry a hidden recessive allele, what could happen?
Parent 1 genotype: IA___ Parent 2 genotype: IA___
Complete this Punnett square:
Both parents are heterozygous: IAi × IAi
IA
i
IA
i
So the answer is: Yes — there is a 1 in 4 (25%) chance of a child with blood group O (genotype ii).
Exam Hint
Always show your Punnett square clearly. Write the parental phenotypes, genotypes, gametes, and then the square. Label the offspring ratios.
AO2 Application
A man with blood group AB and a woman with blood group O have children. Which blood groups are possible in their offspring?
A A and B only
B AB only
C A, B, AB, and O
D A, B, and O
Section 3
Gene Linkage
You have previously assumed that genes are inherited independently. This is true when genes are on different chromosomes. But what happens when two genes are on the same chromosome?
When genes are located on the same chromosome, they tend to be inherited together. This is called gene linkageWhen two or more genes are located on the same chromosome and tend to be inherited together.. Closely linked genes — those that are close together on the chromosome — rarely separate during crossing overThe exchange of genetic material between homologous chromosomes during meiosis I, producing recombinant chromosomes. in meiosis.
Linkage and Recombination
The tightness of linkage of a pair of genes is related to how close together the linked genes are located on the chromosome:
Closely linked genes → rarely separated by crossing over → behave as if they are one gene
Distantly linked genes → more likely to be separated by crossing over → more recombinant offspring
Genes on different chromosomes → always assort independently
Interactive — Click each gene locus to learn about linkage
Learning Tip
Think of linked genes like passengers on the same bus. Closely linked genes always get off at the same stop. Distantly linked genes might occasionally switch buses (crossing over) during the journey.
AO1 Fill the Blanks
Complete these sentences about gene linkage:
1. Genes on the same chromosome are said to be .
2. Closely linked genes are rarely separated during .
3. The closer two genes are on a chromosome, the likely they are to be separated.
4. Genes on different chromosomes assort .
Word Bank
linked | crossing over | less | independently
Section 4
Dihybrid Inheritance & Linkage in Drosophila
A dihybrid crossA genetic cross that tracks the inheritance of two different genes simultaneously. involves tracking the inheritance of two genes at the same time. When genes are on different chromosomes, they assort independently — producing the classic 9:3:3:1 ratio in F2.
Drosophila Example — Independent Assortment
In Drosophila (fruit flies): grey body (G) is dominant to ebony (g), and long wings (L) is dominant to vestigial wings (l).
Cross: GgLl × GgLl (both heterozygous for both traits)
If the genes are on different chromosomes, expected offspring ratio:
Phenotype
Ratio
Grey body, long wings
9
Grey body, vestigial wings
3
Ebony body, long wings
3
Ebony body, vestigial wings
1
But What If the Genes Are Linked?
When Drosophila with dominant broad abdomens and long wings were crossed with flies displaying recessive narrow abdomens and vestigial wings, scientists expected a 9:3:3:1 ratio. Instead, they observed approximately a 3:1 ratio.
This happens because the genes for abdomen width and wing length are on the same chromosome — they are linkedGenes on the same chromosome that tend to be inherited together.. The alleles travel together during meiosis as if they were a single unit.
Exam Hint
If you see offspring ratios that deviate significantly from 9:3:3:1 in a dihybrid cross, think about gene linkage. A ratio close to 3:1 strongly suggests the two genes are linked on the same chromosome.
AO2 Sequencing
Put these steps in order to explain how gene expression leads to cell differentiation:
?⋮⋮ Specific proteins determine cell structure and function
?⋮⋮ Certain genes in the cell are switched on (expressed)
?⋮⋮ Cell becomes specialised (differentiated)
?⋮⋮ Active genes are transcribed into mRNA
?⋮⋮ mRNA is translated into specific proteins at ribosomes
AO3 — Explain the Discrepancy
A student crosses two heterozygous Drosophila (BbLl × BbLl) and expects a 9:3:3:1 ratio. They observe 301 broad+long : 99 narrow+vestigial : 12 broad+vestigial : 8 narrow+long. Explain these results.
The ratio is approximately 3:1 (301+12 : 99+8 ≈ 313:107 ≈ 3:1), not 9:3:3:1. This indicates the genes for body width (B/b) and wing length (L/l) are linked — located on the same chromosome. They are inherited together. The small number of recombinant phenotypes (12 broad+vestigial and 8 narrow+long) are produced by crossing over during meiosis, which occasionally separates the linked alleles. The low recombination frequency (~5%) suggests the genes are closely linked.
Section 5
Polygenic Inheritance
Most traits in living organisms are not determined by a single gene at a single locus. They are polygenicA characteristic determined by the interaction of multiple genes at different loci. — controlled by the combined effect of several different genes at different loci, often on different chromosomes.
Characteristics such as eye colour, height, weight, skin colour, and intelligence are all polygenic. Each gene contributes a small effect, and the combined result produces a continuous range of phenotypes in a population.
Monohybrid vs Polygenic
Feature
Monohybrid
Polygenic
Number of genes
One
Many (at different loci)
Type of variation
Discontinuous (distinct categories)
Continuous (range of values)
Environmental effect
Usually minimal
Often significant
Example
Blood group (A, B, AB, O)
Height, skin colour
Distribution
Discrete categories
Normal distribution (bell curve)
Learning Tip
A helpful way to remember: monohybrid = one gene, distinct categories. Polygenic = many genes, continuous range. The more genes involved, the smoother the distribution curve becomes.
Retrieval Practice — Key Term Flashcards
Click each card to reveal the definition.
Locus
click to flip
The specific position of a gene on a chromosome.
Multiple Alleles
click to flip
When a gene has more than two possible alleles at a particular locus in a population.
Codominant
click to flip
When both alleles are expressed equally in the phenotype of a heterozygote.
Gene Linkage
click to flip
When genes on the same chromosome tend to be inherited together.
Polygenic
click to flip
A characteristic controlled by many genes at different loci, producing continuous variation.
Cell Differentiation
click to flip
The process by which a less specialised cell becomes specialised for a particular function.
Dihybrid Cross
click to flip
A genetic cross tracking the inheritance of two different genes simultaneously.
Crossing Over
click to flip
Exchange of genetic material between homologous chromosomes during meiosis I.
Exam Practice
Exam-Style Questions
2 Marks
Question 1
Explain what is meant by the term ‘gene linkage’.
Reveal model answer
Mark scheme: 1. Gene linkage is when two or more genes are located on the same chromosome (1 mark) 2. They tend to be inherited together / are not independently assorted (1 mark)
4 Marks
Question 2
A man has blood group A (genotype IAi) and a woman has blood group B (genotype IBi). Using a Punnett square, determine the possible blood groups of their children.
Reveal model answer
Mark scheme: 1. Correct gametes identified: Father = IA and i; Mother = IB and i (1 mark) 2. Correct Punnett square drawn (1 mark) 3. Offspring genotypes: IAIB, IAi, IBi, ii (1 mark) 4. Phenotypes: AB, A, B, O — all four blood groups possible, each with 25% probability (1 mark)
6 Marks — Extended Response
Question 3
A scientist crosses two Drosophila that are heterozygous for body colour (Gg) and wing length (Ll). The observed ratio of phenotypes is approximately 3:1 rather than the expected 9:3:3:1. Explain this observation.
Planning Checklist
State the expected ratio for independent assortment (9:3:3:1)
Explain that the observed 3:1 ratio suggests genes are NOT on different chromosomes
Define gene linkage — genes on the same chromosome
Explain that linked genes are inherited together as a unit
Mention that recombinant offspring (rare phenotypes) can occur via crossing over
Conclude that the low frequency of recombinants indicates close linkage
Reveal model answer
If the two genes were on different chromosomes, they would assort independently during meiosis, producing a 9:3:3:1 ratio in the F2 generation. However, the observed ratio of approximately 3:1 indicates that the genes for body colour (G/g) and wing length (L/l) are located on the same chromosome — they are linked. Because they are on the same chromosome, the alleles G and L travel together during meiosis, as do g and l. This means the offspring predominantly inherit either the dominant combination (grey body + long wings) or the recessive combination (ebony body + vestigial wings), producing a ratio close to 3:1. A small number of recombinant phenotypes (e.g. grey body + vestigial wings) may be observed due to crossing over during prophase I of meiosis, which can separate linked alleles. The low frequency of these recombinants suggests the two genes are closely linked — positioned close together on the chromosome, making crossing over between them a rare event.
Independent Practice — Dihybrid Genetics
Level 1: A plant has two traits — tall (T) dominant to short (t), and red flowers (R) dominant to white (r). Complete this Punnett square for TtRr × TtRr. Use the word bank.
The expected phenotypic ratio is .
The most common phenotype will be .
The rarest phenotype will be .
Word Bank
9:3:3:1 | tall red | short white
Level 2: In Drosophila, long wings (L) are dominant to vestigial wings (l), and grey body (G) is dominant to ebony body (g). Cross a heterozygous fly (GgLl) with a double recessive fly (ggll). Draw the Punnett square and state the expected ratio if genes are on different chromosomes.
Check your answer
GgLl × ggll → Gametes from GgLl: GL, Gl, gL, gl. Gametes from ggll: gl only. Offspring: GgLl (grey long), Ggll (grey vestigial), ggLl (ebony long), ggll (ebony vestigial). Ratio: 1:1:1:1 (this is a test cross).
Level 3 (Challenge): A genetics experiment with Drosophila produces these offspring: 412 grey+long, 398 ebony+vestigial, 46 grey+vestigial, 52 ebony+long. What can you conclude about the relationship between these genes? Calculate the recombination frequency.
Check your answer
The parental phenotypes (grey+long: 412, ebony+vestigial: 398) vastly outnumber the recombinant phenotypes (grey+vestigial: 46, ebony+long: 52). This indicates the genes are linked on the same chromosome. Recombination frequency = (46 + 52) / (412 + 398 + 46 + 52) × 100 = 98/908 × 100 = 10.8%. This means the genes are linked but not very closely — approximately 10.8 map units apart.
Final Assessment
Test your understanding of the whole lesson.
Q1
What is a locus?
A A type of allele
B The specific position of a gene on a chromosome
C A section of mRNA
D A pair of homologous chromosomes
Q2
Why do linked genes not follow the expected 9:3:3:1 ratio in a dihybrid cross?
A Because they are on different chromosomes
B Because they are on the same chromosome and tend to be inherited together
C Because one gene is always dominant over the other
D Because crossing over always separates them
Q3
In the ABO system, a person with genotype IAIB has blood group AB because:
A IA is dominant over IB
B Both alleles are recessive
C Both alleles are codominant — both are expressed
D The alleles show incomplete dominance
Q4
Which of the following is an example of a polygenic trait?
A ABO blood group
B Human height
C Sickle cell anaemia
D Cystic fibrosis
Q5
All cells in a multicellular organism contain the same DNA. Different cell types arise because:
A Different cells have different chromosomes
B Different genes are expressed in different cells
C Mutations occur during mitosis
D Cells lose unwanted genes during development
How confident do you feel?
Exit Ticket
Answer these before you finish:
1. Explain why every cell has the same DNA but different cells produce different proteins.
2. A couple both have blood group A. What is the maximum number of different blood groups their children could have? Explain.
3. How can you tell from offspring ratios whether two genes are linked?
Vocabulary Grid
Cell differentiation
The process by which a less specialised cell becomes specialised for a particular function.
Locus
The specific position of a gene on a chromosome.
Multiple alleles
When a gene has more than two possible alleles at a locus.
Codominant
Both alleles are expressed equally in the heterozygote phenotype.
Gene linkage
Genes on the same chromosome that tend to be inherited together.
Crossing over
Exchange of genetic material between homologous chromosomes in meiosis I.
Dihybrid cross
A genetic cross tracking the inheritance of two genes simultaneously.
Polygenic
A trait controlled by many genes at different loci.
Proteome
The complete set of proteins expressed by a cell at a given time.
Housekeeping proteins
Proteins produced by all cells, e.g. membrane proteins, respiratory enzymes.
Self-Assessment Checklist
I can define locus, allele, and gene linkage
I can explain how differential gene expression leads to cell differentiation
I can use Punnett squares with multiple alleles (ABO blood groups)
I can explain why linked genes produce a 3:1 rather than 9:3:3:1 ratio
I can distinguish between monohybrid and polygenic inheritance
I can calculate recombination frequency from experimental data