CIE iGCSE Biology-17.4 Monohybrid inheritance- Study Notes- New Syllabus

CIE iGCSE Biology-17.4 Monohybrid inheritance- Study Notes – New syllabus

CIE iGCSE Biology-17.4 Monohybrid inheritance- Study Notes -CIE iGCSE Biology – per latest Syllabus.

Key Concepts:

Core

Describe inheritance as the transmission of genetic information from generation to generation
Describe genotype as the genetic make-up of an organism and in terms of the alleles present
Describe phenotype as the observable features of an organism
Describe homozygous as having two identical alleles of a particular gene
State that two identical homozygous individuals that breed together will be pure-breeding
Describe heterozygous as having two different alleles of a particular gene
State that a heterozygous individual will not be pure-breeding
Describe a dominant allele as an allele that is expressed if it is present in the genotype
Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype
Interpret pedigree diagrams for the inheritance of a given characteristic
Use genetic diagrams to predict the results of monohybrid crosses and calculate phenotypic ratios, limited to 1:1 and 3:1 ratios
Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes

Supplement

Explain how to use a test cross to identify an unknown genotype
Describe codominance as a situation in which both alleles in heterozygous organisms contribute to the phenotype
Explain the inheritance of ABO blood groups: phenotypes are A, B, AB and O blood groups and alleles are $I^A, I^B$ and $I^o$
Describe a sex-linked characteristic as a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other
Describe red-green colour blindness as an example of sex linkage
Use genetic diagrams to predict the results of monohybrid crosses involving codominance or sex linkage and calculate phenotypic ratios

CIE iGCSE Biology-Concise Summary Notes- All Topics

What is Inheritance?

Key Definition:

Inheritance is the transmission of genetic information from one generation to the next.

📘 Explanation:

Inheritance is how traits (like eye colour, blood type, or height) are passed from parents to offspring.
This happens through genes, which are sections of DNA carried on chromosomes.
During reproduction, each parent passes on one set of genes, which combine to determine the offspring’s traits.

🔗 How It Works:

Component	Role in Inheritance
DNA	Carries genetic instructions
Genes	Units of inheritance that control traits
Chromosomes	Structures that hold genes (humans have 23 pairs)
Gametes	Carry half the genetic information (from each parent)

🧠 Summary:
Inheritance ensures that offspring receive genetic material from both parents, which contributes to their characteristics and variations.

Understanding Genotype

Definition:

A genotype is the genetic makeup of an organism.

It refers specifically to the combination of alleles (forms of a gene) that an organism carries for a particular trait.

📘 Explanation:

Every individual inherits two alleles for each gene – one from each parent.
The genotype shows which two alleles are present, even if only one of them affects how the organism looks or behaves.
The genotype is written using letters to represent alleles (e.g., B = dominant allele, b = recessive allele).

🔎 Types of Genotype:

Type	Allele Combination	Example	Description
Homozygous dominant	Two dominant alleles	BB	Trait shows the dominant form
Homozygous recessive	Two recessive alleles	bb	Trait shows the recessive form
Heterozygous	One dominant, one recessive	Bb	Dominant trait is expressed

🧪 Example: Eye Colour Gene

Let’s say B = brown eyes (dominant) and b = blue eyes (recessive):

Genotype	Phenotype (Visible Trait)
BB	Brown eyes
Bb	Brown eyes
bb	Blue eyes

🧠 Key Points to Remember:

Genotype is not always visible – it’s what’s in the genes, not just what you see.
Genotype influences the phenotype (physical expression of traits).
Genetic variation in populations arises from different genotypes.

Phenotype – The Features We Can See

✅ What is a Phenotype?

The phenotype of an organism means all the observable features that we can see or measure. It includes things like:

Eye colour
Hair texture
Height
Skin colour
Freckles
Shape of leaves in a plant
Flower colour
Blood type
Resistance to disease

In simple words, phenotype is how an organism looks or behaves on the outside.

🔍 How Does Phenotype Happen?

Phenotype is the result of two main things:

Genotype – the genetic makeup (the genes the organism carries)
Environment – the conditions the organism lives in

🧬 Genes give the instructions, but how those instructions are used can change depending on the environment.

Example:

A person may have genes for tall height (genotype), but if they don’t get enough food growing up, they may not grow very tall.
So, their phenotype (actual height) is affected by environment.

💡 Key Points to Remember:

Phenotype = What we see
It is a result of both genes + environment
Phenotypes can change over time (e.g. skin tanning in the sun)
Even identical twins can show slight differences in phenotype if their environment is different

🧪 In Experiments:

Scientists often study phenotype to:

Understand how genes work
Identify traits in plants and animals
Improve crops or breed better animals

✏️ Simple Examples of Phenotypes:

Organism	Phenotype Example
Human	Curly hair, brown eyes
Cat	Black fur, short tail
Rose Plant	Red petals, thorny stem
Bacteria	Resistance to antibiotics

🎯 Final Tip:
Don’t confuse genotype (the hidden genetic code) with phenotype (the visible features).
You can see phenotype.
You cannot see genotype just by looking.

Homozygous – When Both Alleles Match

✅ What Does “Homozygous” Mean?
An organism is said to be homozygous for a particular gene when it has two identical alleles of that gene.
In other words, both copies of the gene are the same.

We inherit one allele from our mother and one from our father.
If both alleles are the same, the organism is homozygous.

🧾 Example:

Let’s say the gene for eye colour has two forms (alleles):

B = Brown eye allele (dominant)
b = Blue eye allele (recessive)

Now look at the combinations:

Combination	Type	Explanation
BB	Homozygous dominant	Both alleles are dominant (brown eyes)
bb	Homozygous recessive	Both alleles are recessive (blue eyes)

Both BB and bb are examples of homozygous genotypes. 🧠 Tip:
“Homo” means same
So, homozygous = same alleles

🔍 Why Is This Important?

Understanding if an organism is homozygous helps scientists to:

Predict inherited traits
Study genetic diseases
Control breeding in plants and animals

🧪 In Genetics:

A homozygous plant for tallness might have the alleles TT
A homozygous animal for short fur might have the alleles ff

This helps in predicting the phenotype – the trait that actually shows up.

Pure-Breeding from Homozygous Parents

✅ Statement:
When two identical homozygous individuals breed together, the offspring will always be pure-breeding.

💡 What Does That Mean?

If both parents have the same homozygous genotype, they will pass on the same allele to all their offspring.

This means that the next generation will also be homozygous, just like the parents.

That’s what we call pure-breeding — because the trait stays the same across generations.

🧾 Example:

Let’s say both parents are homozygous for tallness in pea plants:

Parent 1: TT (tall)
Parent 2: TT (tall)

Every offspring will also get a T from each parent → TT → all will be tall and homozygous.

This is pure-breeding for the tall trait.

Same goes for bb × bb → all offspring will be bb (pure-breeding for the recessive trait).

🧠 Key Point:
Pure-breeding happens when:

Both parents are homozygous
And their alleles are identical
So, their children also show the same trait with no variation.

Heterozygous – When the Alleles Are Different

What Does “Heterozygous” Mean?

An organism is said to be heterozygous for a particular gene when it has two different alleles of that gene – one from each parent.
In other words, the gene pair has one dominant and one recessive allele.

🧾 Example:

Let’s look at a gene for flower colour:

R = Red flower allele (dominant)
r = White flower allele (recessive)

If a plant has the alleles Rr, it is heterozygous.

Even though it has one white allele (r), the dominant red allele (R) will show up in the flower’s appearance.

🔍 Genotype vs Phenotype:

Genotype	Type	Phenotype (what you see)
RR	Homozygous dominant	Red flowers
Rr	Heterozygous	Red flowers (dominant trait shows)
rr	Homozygous recessive	White flowers

🧪 Why Is Heterozygous Important in Genetics?

It helps explain why dominant traits show up even if only one allele is present.
It also explains why recessive traits can be hidden but still passed on to future generations.
Heterozygous individuals can carry a recessive gene without showing it themselves.

🧠 Tip:
“Hetero” = different
Different alleles → Heterozygous

Heterozygous Individuals Are Not Pure-Breeding

A heterozygous individual will not be pure-breeding.

🔍 What Does That Mean?

A heterozygous organism has two different alleles for a gene — for example, Rr (one dominant and one recessive).
When this organism breeds, it can pass on either the dominant (R) or the recessive (r) allele to its offspring.

This means the offspring could:

Be homozygous (RR or rr)
Or heterozygous (Rr)

So, the trait may not stay the same in all offspring.

That’s why it is not pure-breeding — it does not always produce offspring with the same trait.

🧾 Example:

If you cross two Rr (heterozygous red-flowered) plants:

Parent Genotype	Possible Offspring
Rr × Rr	RR, Rr, rr

Some plants will be red (RR or Rr)
Some will be white (rr)
This mix of traits means the parents were not pure-breeding.

🧠 Key Point to Remember:

Heterozygous = Mixed alleles = Mixed offspring

Pure-breeding only happens when both alleles are identical.

🧠 Tip:
Heterozygous parents = variety in offspring
No identical trait guaranteed!

Dominant Allele – Always Expressed If Present

Definition:

A dominant allele is an allele that is expressed in the phenotype whenever it is present in the genotype — even if there is only one copy of it.

🔍 Explanation:
Every organism has two copies of each gene (one from each parent). If at least one of those copies is a dominant allele, it will control the trait that appears.
It masks the effect of the recessive allele, if present.

🧾 Example:

Let’s say:

B = Brown eyes (dominant)
b = Blue eyes (recessive)

Then:

BB → Brown eyes
Bb → Brown eyes (dominant allele B is expressed)
bb → Blue eyes (only when dominant B is absent)

🧠 Key Point:
You only need one dominant allele for its trait to show up in the organism.

Recessive Allele – The Quiet One That Only Shows When Alone

✅ What Is a Recessive Allele?

A recessive allele is an allele that is only expressed (shows up in the organism’s features) when there is no dominant allele present in the genotype.

This means the organism must have two copies of the recessive allele for the trait to appear.

🧾 Example:

Let’s take hair colour:

A = Brown hair (dominant)
a = White hair (recessive)

Now look at the genotypes:

Genotype	Phenotype (What You See)
AA	Brown hair
Aa	Brown hair
aa	White hair

🧠 Easy Way to Remember:
Recessive = Hides behind the dominant one
It needs a pair to be seen (like bb, rr, tt)

Only people with bb have blue eyes.

So, the b allele is recessive – it only shows when the dominant B is not there.

Interpreting Pedigree Diagrams – Tracing Traits Through Generations

What Is a Pedigree Diagram?

A pedigree diagram is like a family tree used in genetics to track how a particular trait or disorder is inherited through generations.

Who is related to whom
Who has the trait (affected)
Who carries the gene (carriers, in some cases)
Whether the trait is dominant, recessive, or sex-linked

🔍 Basic Symbols Used:

Symbol	Meaning
○	Female (unaffected)
●	Female (affected)
☐	Male (unaffected)
■	Male (affected)
─	Horizontal line between male and female = marriage
│	Vertical line = children
◐ / ◑	Half-filled = carrier (recessive or sex-linked traits)

🧬 How to Interpret a Pedigree Diagram

Start with the first generation (usually at the top).
See how many people are affected and how traits are passed down.

Check how the trait appears:

In every generation → likely dominant
Skips generations → likely recessive

Compare parents and children:

If two unaffected parents have an affected child → trait is recessive
If at least one parent is affected and some children are affected → trait is likely dominant

Sex-linked traits (usually X-linked):

Often seen more in males (like haemophilia)
Affected males usually inherit the trait from their mother
Females may be carriers (one affected X and one normal X)

🧾 Example:

Let’s say in a pedigree:

Two unaffected parents have a child with a genetic condition.
This suggests the trait is recessive – both parents are likely carriers.

Or:
Every generation has affected individuals – this supports a dominant inheritance pattern.

🎯 Quick Tips:

Pattern	What It Suggests
Dominant Trait	Appears in every generation. Affected person usually has an affected parent.
Recessive Trait	May skip generations. Can appear in children even if parents don’t show the trait.
Sex-linked Recessive Trait	Mostly affects males. Females are usually carriers.

Monohybrid Crosses & Phenotypic Ratios

What is a Monohybrid Cross?

A monohybrid cross is a genetic cross that involves just one gene with two alleles – used to predict how a single trait is inherited.

🔍 Key Terms to Know:

Allele: A version of a gene (e.g., T or t)
Dominant allele: Expressed if present (e.g., T = tall)
Recessive allele: Only shows if no dominant allele is present (e.g., t = short)
Genotype: The allele combination (TT, Tt, tt)
Phenotype: The visible trait (tall or short)

✏️ Steps for a Genetic Diagram (Punnett Square):

Write genotypes of parents
Determine gametes (sperm & egg)
Draw Punnett square
Fill in offspring genotypes
Identify phenotypes
Write phenotypic ratio

🧪 Case 1: 3 : 1 Ratio

Cross: Heterozygous × Heterozygous (Tt × Tt)

Trait: T = tall (dominant), t = short (recessive)

	T	t
T	TT	Tt
t	Tt	tt

Offspring Genotypes: 1 TT, 2 Tt, 1 tt

Phenotypic Ratio: 3 Tall : 1 Short

🧪 Case 2: 1 : 1 Ratio

Cross: Heterozygous × Homozygous recessive (Tt × tt)

	t	t
T	Tt	Tt
t	tt	tt

Offspring Genotypes: 2 Tt, 2 tt

Phenotypic Ratio: 1 Tall : 1 Short

🧠 Summary Table:

Parent Genotypes	Offspring Ratio	Phenotypic Ratio
Tt × Tt	1 TT : 2 Tt : 1 tt	3 Tall : 1 Short
Tt × tt	2 Tt : 2 tt	1 Tall : 1 Short

🎯 Final Tips:
– Always label which allele is dominant and which is recessive
– Use capital letters for dominant, lowercase for recessive
– Ensure Punnett square matches parent genotypes
– Double-check your phenotypic ratio using actual counts

What is a Punnett Square?

A Punnett square is a simple diagram used in genetics to predict the genotypes and phenotypes of offspring from a cross between two individuals.

🎯 When Do We Use It?
When parents have different alleles
When a cross can result in multiple genotype combinations
To find probability of traits in the offspring

🧪 Example: Cross Tt × Tt
(T = Tall, t = Short)

T	t
TT	Tt
Tt	tt

✏️ Genotypes:
TT – Homozygous dominant (Tall)
Tt – Heterozygous (Tall)
tt – Homozygous recessive (Short)

🔢 Genotype Ratio: 1 TT : 2 Tt : 1 tt

🌱 Phenotype Ratio: 3 Tall : 1 Short

🧬 Key Concepts:
– If at least one parent is heterozygous, multiple genotypes are possible.
– You can determine phenotypes by knowing which alleles are dominant or recessive.

👨‍👩‍👧‍👦 Interpreting Pedigree Diagram – Widow’s Peak Trait

🧾 Trait Details:

W = Widow’s peak (dominant)
w = No widow’s peak (recessive)

Genotypes:

WW or Ww → Widow’s peak
ww → No widow’s peak

🧭 Pedigree Key:

Symbol	Meaning
☐	Male (unaffected)
○	Female (unaffected)
■	Male with trait
●	Female with trait
◐	Carrier (if trait is recessive)
─ / │	Marriage / Offspring Line

👪 Generations in Pedigree (from diagram):

1st Generation – Grandparents
2nd Generation – Parents, Aunts, Uncles
3rd Generation – Two sisters (offspring shown in birth order, first-born on left)

✅ Example from Pedigree (Widow’s Peak Trait)

Parents: Ww × Ww

Possible Offspring Genotypes:

WW → Widow’s peak
Ww → Widow’s peak
ww → No widow’s peak

Genotype Ratio: 1 WW : 2 Ww : 1 ww
Phenotype Ratio: 3 with widow’s peak : 1 with no widow’s peak

Test Cross – Finding the Unknown Genotype

What is a Test Cross?

A test cross is used to find out the genotype of an organism that shows a dominant trait – but we don’t know if it’s homozygous (TT) or heterozygous (Tt).

🧪 How It Works:

Cross the unknown dominant individual with a homozygous recessive (tt) individual.
Then observe the offspring.

🎯 Two Possible Outcomes:

All offspring show dominant trait → unknown genotype is TT (homozygous)
Mix of dominant and recessive offspring → unknown genotype is Tt (heterozygous)

📌 Why It Works:

The recessive parent (tt) can only pass on t.

If the unknown parent has T and t, then both parents can contribute t → some offspring will be tt (recessive).

🧾 Example:

Unknown plant: tall (T?)
Test cross with: short plant (tt)

	t	t
T	Tt	Tt
t	tt	tt

Another example of flower is in the figure above.

🧠 Conclusion:
– If all offspring are tall → unknown genotype = TT
– If tall and short appear in 1:1 ratio → unknown genotype = Tt

Codominance – When Both Alleles Show Together

What is Codominance?

Codominance is a type of inheritance where both alleles in a heterozygous organism are fully and equally expressed in the phenotype.
This means neither allele is dominant or hidden instead, both traits appear together.

🧾 Key Feature:

The heterozygous individual shows a combination of both alleles, not a blend.

🧪 Example: Human Blood Group AB

A allele and B allele are codominant
Genotype: IAIB
Phenotype: Blood group AB (both A and B antigens are present)

🔍 Another Example: Coat Colour in Cattle

R = red hair
W = white hair
RW = roan coat (both red and white hairs visible)

🧠 Key Point:
Codominance means both alleles contribute equally – the phenotype clearly shows both traits side by side.

Inheritance of ABO Blood Groups

Blood Group Phenotypes:

Humans can have four blood groups (phenotypes): A, B, AB, and O.

🧬 The Alleles Involved:

IA = gives A-type antigen
IB = gives B-type antigen
Io = gives no antigen (O type)

🔍 Key Rules:

IA and IB are codominant → both are expressed when together
Io is recessive → only shows if both alleles are Io

🧾 Possible Genotypes and Phenotypes:

Genotype	Blood Group (Phenotype)
IAIA or IAIo	A
IBIB or IBIo	B
IAIB	AB (codominant – both A and B are expressed)
IoIo	O

🧠 Summary:
AB is the result of codominance (both A and B antigens show).
O appears only when both alleles are recessive (IoIo).
This is an example of multiple alleles and codominance.

Sex-Linked Characteristics – Traits on Sex Chromosomes

What is a Sex-Linked Characteristic?

A sex-linked characteristic is a trait controlled by a gene that is located on a sex chromosome – usually the X chromosome.

🔍 Why Does This Matter?

Males (XY) and females (XX) have different combinations of sex chromosomes.
A gene on the X chromosome affects males and females differently.
Males have only one X, so any gene on it will show — even if it’s faulty.
Females have two Xs, so a normal copy can often hide a faulty one.

🧪 Result:

Sex-linked traits (especially recessive ones) are often more common in males than in females.

🧾 Example:

Haemophilia and colour blindness are classic X-linked recessive disorders.
A male with one faulty X → shows the condition.
A female needs two faulty Xs to be affected; otherwise, she is a carrier.

🧠 Key Point:
A sex-linked characteristic is caused by a gene on a sex chromosome, and it often affects one sex (usually males) more than the other.

Red-Green Colour Blindness – A Sex-Linked Trait

What Is It?

Red-green colour blindness is a condition where a person has difficulty telling the difference between red and green colours.
It is a classic example of a sex-linked genetic disorder.

🧬 Where Is the Gene?

The gene responsible is found on the X chromosome.

This makes it X-linked recessive.

👨 Why Is It More Common in Males?

Males (XY) have only one X chromosome.
If their X has the faulty gene → they will be colour blind.
Females (XX) need two faulty copies to be affected.
If only one X is faulty → they are carriers, not affected.

🧾 Example Genotypes:

Genotype	Individual	Effect
XᴺXᴺ	Female	Normal vision
XᴺXᶜ	Female carrier	Normal vision (carrier)
XᶜXᶜ	Female	Colour blind
XᴺY	Male	Normal vision
XᶜY	Male	Colour blind (affected)

🧠 Key Point:
Red-green colour blindness is sex-linked because the gene is on the X chromosome, and this makes the condition much more common in males than in females.

Monohybrid Crosses: Codominance & Sex Linkage

1. Codominance Example

Trait: Coat colour in cattle

R = Red coat
W = White coat
RW = Roan coat (red & white hairs seen together)

🧪 Cross: RW × RW (2 roan cattle)

	R	W
R	RR (Red)	RW (Roan)
W	RW (Roan)	WW (White)

🔢 Phenotypic Ratio:
1 Red : 2 Roan : 1 White → 1 : 2 : 1

2. Sex Linkage Example

Trait: Red-green colour blindness

Xᴺ = Normal vision
Xᶜ = Colour blindness
Males = XY, Females = XX

🧪 Cross: Carrier female (XᴺXᶜ) × Normal male (XᴺY)

	Xᴺ	Y
Xᴺ	XᴺXᴺ (Normal girl)	XᴺY (Normal boy)
Xᶜ	XᴺXᶜ (Carrier girl)	XᶜY (Colour-blind boy)

🔢 Phenotypic Ratio:
Girls: 1 normal : 1 carrier
Boys: 1 normal : 1 colour-blind
→ Overall: 1 normal girl : 1 carrier girl : 1 normal boy : 1 affected boy

🎯 Final Tips:
Use capital/lowercase letters consistently
Codominance → both alleles expressed equally (not blended)
Sex-linked → pay attention to X and Y chromosomes
Always label phenotypes clearly after the Punnett square
Then count each type to get the phenotypic ratio

CIE iGCSE Biology-17.4 Monohybrid inheritance- Study Notes

CIE iGCSE Biology-17.4 Monohybrid inheritance- Study Notes- New Syllabus