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Contents
KS4 Biology
Inheritance
All about alleles
Homozygous cross
Heterozygous cross
Using a test cross
Inheritance
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Co-dominance
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Inheritance introduction
Homologous chromosomes
To understand how inheritance works you need to know:
z the definition of certain genetic terms,
In all living things, characteristics are passed on in the
chromosomes that offspring inherit from their parents.
z and how to use genetic crosses to determine
the characteristics of offspring.
chromosome from
female parent
chromosome from
male parent
Chromosomes are matched in pairs that contain
one chromosome inherited from each parent.
So are the genes in a matching pair of chromosomes
exactly the same?
In this unit, petal colour is used to show how characteristics
are inherited in offspring.
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Different versions of genes
Pairs of alleles – homozygous
The chromosomes in a matching pair contain the same type
of genes that code for the same characteristics.
If the alleles in a matching pair are the same,
they are called homozygous alleles.
gene for
petal colour
gene for
petal colour
version for
red petals
version for
yellow petals
Each chromosome may have a different version of a gene.
Different versions of a gene, that code for different versions
of a characteristic, are called alleles.
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allele for
yellow petals
allele for
yellow petals
allele for
red petals
allele for
red petals
What colour are the flowers with these
homozygous pairs of alleles?
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(Click twice on each bud
to reveal the flower;
click again to close them.)
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Pairs of alleles – heterozygous
If the alleles in a matching pair are different,
they are called heterozygous alleles.
allele for
red petals
Representing alleles
(Click twice on the bud
to reveal the flower;
click again to close it.)
Letters are used to represent different alleles.
A dominant allele is always a capital letter.
allele for
yellow petals
allele for
red petals
= R
A recessive allele is always the corresponding small letter.
Which characteristic is expressed if alleles are different?
allele for
yellow petals
Some alleles are dominant to other forms of a gene
and will always be expressed.
=
r
Which is the dominant allele in this heterozygous pair?
The allele pair for each characteristic is called the genotype.
Which is the recessive allele in this heterozygous pair?
What colour are flowers with the genotype Rr?
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Genotypes and phenotypes
Genetic jargon
The allele pair for each characteristic is called the genotype.
What do these genetic terms mean?
The physical expression of an allele pair is the phenotype.
gene
Section of DNA that codes for a particular trait
or characteristic.
allele
A different form of a gene that codes for a
different version of a characteristic.
What are the phenotypes of these genotypes?
RR
genotype:
rr
Rr
genotype A description of the pair of alleles present
phenotype:
for a characteristic.
(Click twice on each bud
to reveal the flowers;
click again to close them.)
phenotype The physical expression of the alleles.
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Genetic jargon
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Matching pairs – genes
What do these genetic terms mean?
homozygous Pair of alleles that produce a characteristic
that are the same, e.g. HH.
heterozygous Pair of alleles that produce a characteristic
that are different, e.g. Hh.
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dominant
An allele that will always be expressed even
when there is only one of these alleles present,
represented by a capital letter.
recessive
An allele that will only be expressed when
both alleles are of this type, represented
by a lower case letter.
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Matching pairs – characteristics
Contents
Inheritance
All about alleles
Homozygous cross
Heterozygous cross
Using a test cross
Co-dominance
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RR x rr – crossing homozygous parents
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RR x rr – F1 offspring
What are the possible offspring of a cross between a
homozygous red flower and a homozygous yellow flower?
parental genotype:
Homozygous means that both alleles of a gene are the same.
gametes:
RR
R
Red is the dominant allele for these flowers, so the alleles
for petal colour are: red = R , yellow = r.
F1 offspring
genotype:
phenotype:
x
genotype:
RR
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F1 genotypes:
RR
Rr
r
r
r
r
R
Rr
Rr
R
Rr
Rr
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Homozygous cross activity
RR x rr – F1 phenotypes
parental genotype:
R
rr
What are the phenotypes of the F1 offspring?
rr
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x
x
Rr
rr
Rr
Rr
F1 phenotypes:
(Click twice on the buds
to reveal the flowers;
click again to close them.)
The possible offspring of a cross between two homozygous
parents are always heterozygous and so the dominant
characteristic is always expressed in this generation.
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Rr x Rr – crossing heterozygous parents
Contents
The offspring (Rr) from the first cross (RR x rr) are called
the F1 generation. What happens in a cross between
these offspring?
Inheritance
All about alleles
Both parent plants are now heterozygous, so the alleles
in each plant are different.
Homozygous cross
F1 generation
Heterozygous cross
phenotype:
X
Using a test cross
Co-dominance
genotype:
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Rr x Rr – F2 offspring
parental genotype:
gametes:
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Rr x Rr – F2 phenotypes
Rr
R
Rr
Rr
x
r
parental genotype:
Rr
R
F2 genotypes:
r
Rr
RR
x
Rr
Rr
Rr
rr
F2 phenotypes:
R
F2 offspring
genotype:
r
R
RR
Rr
r
Rr
rr
(Click twice on the buds
to reveal the flowers;
click again to close them.)
?
In the F2 generation, 3 of the 4 possible offspring are red.
Only one offspring shows the recessive phenotype.
When two heterozygous parents are crossed, the possible
offspring will always show a 3:1 ratio in favour of the
dominant phenotype.
What are the phenotypes of the F2 offspring?
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Heterozygous cross activity
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Contents
Inheritance
All about alleles
Homozygous cross
Heterozygous cross
Using a test cross
Co-dominance
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What is a test cross?
Carrying out a test cross
A test cross allows you to find out if an organism showing
a dominant characteristic is homozygous or heterozygous
for the dominant allele.
A test cross is carried out between the flower of unknown
genotype and another flower whose genotype is known.
For example, a yellow flower can only have the genotype rr
because it’s recessive.
For example the genotype of a red flower could be:
So the test cross is:
Rr
or
RR
x
?
(RR or Rr)
What could you cross a red flower with to find its genotype?
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rr
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Test cross – 2 types
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Test cross offspring
If the red flower is homozygous (RR) then the cross is the
same as the first cross (RR x rr). All of the offspring will be
heterozygous and have red petals.
parental genotype:
gametes:
Rr
R
x
r
rr
r
r
What about the other possible cross between a heterozygous
red flower (Rr) and yellow flower (rr)?
offspring
genotype:
x
?
(RR or Rr)
rr
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R
Rr
Rr
r
rr
rr
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Test cross results
F2 genotypes:
r
?
What are the phenotypes of these offspring?
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parental genotype:
r
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Test cross activity
Rr
Rr
x
Rr
rr
rr
rr
F2 phenotypes:
(Click twice on the buds
to reveal the flowers;
click again to close them.)
A cross between a heterozygous parent and a recessive
parent yields different types of offspring in a 1:1 ratio.
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Contents
What is co-dominance?
Sometimes two alleles are equally dominant to each other.
Inheritance
In genetics, this is called co-dominance and means that
neither allele is recessive to the other allele.
All about alleles
How does co-dominance affect the offspring of a cross?
Homozygous cross
For example, let’s assume that the red allele (R) and
the white allele (W) are co-dominant:
Heterozygous cross
x
Using a test cross
Co-dominance
RR
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Co-dominance cross – offspring
RR
parental genotype:
gametes:
R
x
R
parental genotype:
offspring
genotypes:
W
W
W
R
RW
RW
R
RW
RW
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WW
RW
RW
RW
All the offspring flowers are pink because both the red
and white alleles are expressed.
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Co-dominance activity
FLASH 5 –
x
(Click twice on the buds
to reveal the flowers;
click again to close them.)
The alleles are co-dominant so both are expressed.
What will the offspring flowers look like?
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RW
RR
offspring
phenotypes:
?
offspring
genotype:
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Co-dominance cross – phenotypes
WW
W
WW
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Inheritance multiple-choice quiz
Blood groups
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