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IGCSE BIOLOGY
SECTION 3 LESSON 3
Content
Section 3
Reproduction
and
Inheritance
a) Reproduction
- Flowering plants
- Humans
b) Inheritance
Content
Lesson 3
b) Inheritance
b) Inheritance
3.13 understand that the nucleus of a cell contains
chromosomes on which genes are located
3.14 understand that a gene is a section of a
molecule of DNA and that a gene codes for a
specific protein
3.15 describe a DNA molecule as two strands coiled
to form a double helix, the strands being linked by
a series of paired bases: adenine (A) with thymine
(T), and cytosine (C) with guanine (G)
3.16 understand that genes exist in alternative
forms called alleles which give rise to differences
in inherited characteristics
3.17 understand the meaning of the terms:
dominant, recessive, homozygous, heterozygous,
phenotype, genotype and codominance
3.18 describe patterns of monohybrid inheritance
using a genetic diagram
3.19 understand how to interpret family pedigrees
3.20 predict probabilities of outcomes from
monohybrid crosses
The nucleus
The nucleus of the cell,
containing all of the genetic
material. This material is
inherited from the parents.
The nucleus
The nucleus contains
chromosomes – in normal
human cells, there are 23
pairs of chromosomes. Each
chromosome is made up of a
very special molecule called
DNA.
The nucleus
The nucleus contains
chromosomes – in normal
human cells, there are 23
pairs of chromosomes. Each
chromosome is made up of a
very special molecule called
DNA.
DNA stands for deoxyribonucleic
acid, but at this stage just stick
with the initials DNA!
The nucleus
Here is an individual
chromosome ( the xchromosome). Inside,
the double helix of DNA
can be clearly seen.
Chromosomes, genes and DNA
chromosome
Chromosomes, genes and DNA
Individual sections of a chromosome are called
genes. Each gene ( a short section of DNA)
codes for a particular protein, which may control
particular characteristics, such as eye colour.
Each chromosome may contain thousands of
genes.
DNA Structure
DNA consists of
two strands,
wrapped into a
double helix.
DNA Structure
DNA consists of
two strands,
wrapped into a
double helix.
The two strands
are linked by
pairs of BASES
There are four
bases – adenine,
thymine,
cytosine and
guanine.
DNA Structure
DNA consists of
two strands,
wrapped into a
double helix.
The two strands
are linked by
pairs of BASES
There are four
bases – adenine,
thymine,
cytosine and
guanine.
Adenine + Thymine
Cytosine + Guanine
DNA Structure
DNA consists of
two strands,
wrapped into a
double helix.
A
T
C
G
The two strands
are linked by
pairs of BASES
There are four
bases – adenine,
thymine,
cytosine and
guanine.
Adenine + Thymine
Cytosine + Guanine
DNA Structure
DNA consists of
two strands,
wrapped into a
double helix.
A
T
C
G
The two strands
are linked by
pairs of BASES
There are four
bases – adenine,
thymine,
cytosine and
guanine.
Adenine + Thymine
Cytosine + Guanine
DNA Structure
The bases are ‘read’ in
threes, or triplets.
DNA Structure
The bases are ‘read’ in
threes, or triplets.
Each triplet codes for a
particular amino acid.
DNA Structure
The bases are ‘read’ in
threes, or triplets.
Each triplet codes for a
particular amino acid.
Don’t forget
that proteins
are made up of
amino acids!
DNA Structure
The bases are ‘read’ in
threes, or triplets.
Each triplet codes for a
particular amino acid.
So this triplet of bases is
cytosine – cytosine – thymine
or CCT
DNA Structure
The bases are ‘read’ in
threes, or triplets.
Each triplet codes for a
particular amino acid.
So this triplet of bases is
cytosine – cytosine – thymine
or CCT
TAT
GGA
TGT
GCT
ACC
TCG
DNA Structure
TAT
GGA
TGT
GCT
ACC
TCG
Since there are only about 20
different amino acids that
make up all the protein
chains, the different base
triplet combinations are more
than sufficient
Genetic mutations
Every time a cell divides, all
the DNA in the nucleus must
be copied exactly.
Genetic mutations
Every time a cell divides, all
the DNA in the nucleus must
be copied exactly.
Occasionally a mistake may
occur, and bases may be put
in the wrong order.
Genetic mutations
Every time a cell divides, all
the DNA in the nucleus must
be copied exactly.
Occasionally a mistake may
occur, and bases may be put
in the wrong order.
As a result, there will be a
different sequence of amino
acids, and therefore a
different protein will be
made.
Genetic mutations
Every time a cell divides, all
the DNA in the nucleus must
be copied exactly.
Occasionally a mistake may
occur, and bases may be put
in the wrong order.
As a result, there will be a
different sequence of amino
acids, and therefore a
different protein will be
made.
This change in the order
of the bases is called a
MUTATION
Genetic mutations
Causes
Effects
Mutations occur naturally
but ……
• there is an increased risk
if
• individuals are exposed to
mutagenic agents
• such as ionising radiation
(UV, X-rays)
• radioactive substances and
certain chemicals.
• the greater the dose, the
greater the risk.
Most mutations are harmful
and in …
• Reproductive cells can
cause death or abnormality
• in body cells they may
cause cancer
• some mutations are neutral
and some may increase the
survival chances of an
organism
• and its offspring who
inherit the gene
Alleles
Let’s just
recap a
second!
Alleles
Let’s just
recap a
second!
Alleles
Let’s just
recap a
second!
Genes control
specific
characteristics,
such as eye
colour
Alleles
How many different
eye colours are
there?
Alleles
How many different
eye colours are
there?
Blue
Hazel
brown
Grey
Alleles
There are
different forms
of the same gene,
all coding for
different eye
colours.
How many different
eye colours are
there?
Blue
Hazel
brown
Grey
Alleles
There are
different forms
of the same gene,
all coding for
different eye
colours.
These different
forms of the same
gene are called
alleles.
How many different
eye colours are
there?
Blue
Hazel
brown
Grey
Alleles
There are
different forms
of the same gene,
all coding for
different eye
colours.
These different
forms of the same
gene are called
alleles.
How many different
eye colours are
there?
Blue
Hazel
So there are alleles for blue
eyes, brown eyes, etc.
brown
Grey
Remember that
we inherit
specific genes
from both
parents.
So we will inherit one eye
colour gene from our
mother, and another eye
colour gene from our father
(remember that different
forms of the same gene are
called alleles)
It’s
definition
time!
DOMINANT
-when a pair of alleles (or genes) are
present, each coding for a particular
characteristic, the dominant allele is the
one that shows. For example, the brown
eye colour allele is dominant over the
blue allele, so an individual with both blue
and brown alleles will have brown eyes.
RECESSIVE
- the recessive allele will only have an
effect when the dominant allele is
missing. For example, if you inherit the
blue allele from your mother and the blur
allele from your father, then you would
have blue eyes (there is no other allele
present to ‘dominate’ the blue allele).
Dominant alleles are shown using capital
letters.
For example, the brown eye allele is ‘B’
Dominant alleles are shown using capital
letters.
For example, the brown eye allele is ‘B’
Recessive alleles are lower case.
For example, the blue eye allele is ‘b’
Sperm
+
Egg
Zygote
Sperm
+
Egg
Contains half the
chromosome number
of normal body cells
Eg . 23 in humans
Contains half the
chromosome number
of normal body cells
Eg. 23 in humans
Zygote
Contains the full
chromosome number
Eg. 46 in humans
Sperm
+
Egg
b
B
Zygote
Bb
Homozygous.
If both chromosomes in a pair contain
the same allele of a gene then the
individual is homozygous for that gene or
condition.
eg. BB or bb
Heterozygous.
If the chromosomes in a pair contain
different alleles of a gene then the
individual is said to be heterozygous for
that gene or condition.
eg. Bb (bB)
Inheritance terminology
Homozygous
dominant
Heterozygous
Homozygous
recessive
Tongue rolling
TT (can roll)
Tt (can roll)
tt (can’t roll)
Eye colour
BB (brown)
Bb (brown)
bb (blue)
Ear lobes
EE (free
lobes)
Ee (free
lobes)
ee (attached
lobes)
Phenotype
The phenotype describes the outward
appearance of an individual.
eg. BB or Bb individuals will both have
brown eyes.
Genotype
The genotype describes the actual genes
present in an individual
eg. BB, Bb or bb
Co-dominance
This refers to a situation when both
alleles are clearly visible and do not
overpower each other in the phenotype.
eg. the ‘A’ and ‘B’ alleles are co-dominant
in producing the ‘AB’ blood group
phenotype.
OK, let’s move on
now and consider
monohybrid
inheritance
OK, let’s move on
now and consider
monohybrid
inheritance
Monohybrid
what …???
When a characteristic is
determined by a single pair
of alleles, then a simple
genetic diagram can be
shown. This type of
inheritance is referred to as
monohybrid inheritance.
As an example, let’s look at a
genetic cross for two
parents – one is homozygous
for brown eyes (BB) and the
other is homozygous for
blue eyes (bb)
Brown eyes
Parents
BB
x
x
Blue eyes
bb
Gametes
B
B
b
b
Offspring
Bb
Bb
Bb
Bb
Brown
Brown
Brown
Brown
All four offspring are heterozygous (Bb) for
brown eyes
Heterozygous brown eyed mother
Heterozygous brown eyed
father
B
B
b
Punnett Square
b
Heterozygous brown eyed
father
Heterozygous brown eyed mother
B
b
B
BB
Bb
b
Bb
bb
3 brown eyed offspring and one
blue eyed offspring. 3:1 ratio
Heterozygous brown eyed
father
Homozygous blue eyed mother
b
b
B
Bb
Bb
b
bb
bb
2 brown eyed offspring and two
blue eyed offspring. 1:1 ratio
Golden rules of monohybrid crosses
Monohybrid cross
Outcome
Parent 1: homozygous dominant
(eg. TT)
Parent 2: heterozygous recessive
(eg. tt)
All offspring will be
heterozygous and show the
dominant characteristic.
(eg. Tt)
Golden rules of monohybrid crosses
Monohybrid cross
Outcome
Parent 1: homozygous dominant
(eg. TT)
Parent 2: heterozygous recessive
(eg. tt)
All offspring will be
heterozygous and show the
dominant characteristic.
(eg. Tt)
Parent 1: heterozygous dominant
(eg. Tt)
Parent 2: homozygous recessive
(eg. tt)
50% of offspring will be
heterozygous dominant (Tt)
and 50% will be homozygous
recessive (tt) Ratio 1:1
Golden rules of monohybrid crosses
Monohybrid cross
Outcome
Parent 1: homozygous dominant
(eg. TT)
Parent 2: heterozygous recessive
(eg. tt)
All offspring will be
heterozygous and show the
dominant characteristic.
(eg. Tt)
Parent 1: heterozygous dominant
(eg. Tt)
Parent 2: homozygous recessive
(eg. tt)
50% of offspring will be
heterozygous dominant (Tt)
and 50% will be homozygous
recessive (tt) Ratio 1:1
Parent 1: heterozygous dominant
(eg. Tt)
Parent 2: heterozygous dominant
(eg. Tt)
25% of offspring will be homozygous
dominant (TT), 50% will be
heterozygous dominant (Tt), and 25%
will be homozygous recessive (tt).
Phenotype ratio 3:1
Content
Lesson 3
b) Inheritance
b) Inheritance
3.13 understand that the nucleus of a cell contains
chromosomes on which genes are located
3.14 understand that a gene is a section of a
molecule of DNA and that a gene codes for a
specific protein
3.15 describe a DNA molecule as two strands coiled
to form a double helix, the strands being linked by
a series of paired bases: adenine (A) with thymine
(T), and cytosine (C) with guanine (G)
3.16 understand that genes exist in alternative
forms called alleles which give rise to differences
in inherited characteristics
3.17 understand the meaning of the terms:
dominant, recessive, homozygous, heterozygous,
phenotype, genotype and codominance
3.18 describe patterns of monohybrid inheritance
using a genetic diagram
3.19 understand how to interpret family pedigrees
3.20 predict probabilities of outcomes from
monohybrid crosses
Understand how
to interpret
family pedigrees.
Pedigree charts are
normally used to show
disease down a family
tree. For example, the
inheritance of sickle cell
anaemia.
Doctors can use a
pedigree analysis chart
to show how genetic
disorders are inherited
in a family.
They can use this
analysis to work out the
probability (chance)
that someone in the
family will inherit the
condition.
Pedigree analysis
Male with sickle cell disease
Normal male
Female with sickle cell disease
Normal female
Pedigree analysis
Pedigree analysis
Both parents are sufferers
Pedigree analysis
Both parents are sufferers
Of their 4 children, 3 have sickle cell disease
Pedigree analysis
Both parents are sufferers
Of their 4 children, 3 have sickle cell disease
If one of the affected offspring marries a normal
male, what are the chances that their children
will inherit the disease?
End of Section 3 Lesson 3
In this lesson we have covered:
Chromosomes, genes and DNA.
Genetic mutations.
Alleles.
Genetic inheritance.
Monohybrid crosses.