Mendel's Law and Inheritance

© Boardworks Ltd 20041 of 36
KS4 Biology
Inheritance

Inheritance
Contents
All about alleles
Homozygous cross
Using a test cross
Co-dominance
Heterozygous cross

 the definition of certain genetic terms,
Inheritance introduction
To understand how inheritance works you need to know:
and how to use genetic crosses to determine
the characteristics of offspring.
In this unit, petal colour is used to show how characteristics
are inherited in offspring.

© Boardworks Ltd 20044 of 36 © Boardworks Ltd 2004
Homologous chromosomes
In all living things, characteristics are passed on in the
chromosomes that offspring inherit from their parents.
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?
chromosome from
female parent
chromosome from
male parent

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.
The chromosomes in a matching pair contain the same type
of genes that code for the same characteristics.
Different versions of genes
version for
yellow petals
version for
red petals
gene for
petal colour
gene for
petal colour

Pairs of alleles – homozygous
If the alleles in a matching pair are the same,
they are called homozygous alleles.
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?
(Click twice on each bud
to reveal the flower;
click again to close them.)

Pairs of alleles – heterozygous
If the alleles in a matching pair are different,
they are called heterozygous alleles.
Which characteristic is expressed if alleles are different?
allele for
yellow petals
allele for
red petals
Some alleles are dominant to other forms of a gene
and will always be expressed.
Which is the dominant allele in this heterozygous pair?
Which is the recessive allele in this heterozygous pair?
(Click twice on the bud
to reveal the flower;
click again to close it.)

Representing alleles
Letters are used to represent different alleles.
The allele pair for each characteristic is called the genotype.
What colour are flowers with the genotype Rr?
A dominant allele is always a capital letter.
A recessive allele is always the corresponding small letter.
allele for
red petals R
allele for
yellow petals r
=
=

Genotypes and phenotypes
The allele pair for each characteristic is called the genotype.
The physical expression of an allele pair is the phenotype.
What are the phenotypes of these genotypes?
genotype:
phenotype:
RR Rrrr
(Click twice on each bud
to reveal the flowers;

Genetic jargon
gene
allele
phenotype
genotype
Section of DNA that codes for a particular trait
or characteristic.
A different form of a gene that codes for a
different version of a characteristic.
A description of the pair of alleles present
for a characteristic.
The physical expression of the alleles.
What do these genetic terms mean?

homozygous
heterozygous
recessive
dominant
Pair of alleles that produce a characteristic
that are the same, e.g. HH.
Pair of alleles that produce a characteristic
that are different, e.g. Hh.
An allele that will only be expressed when
both alleles are of this type, represented
by a lower case letter.
An allele that will always be expressed even
when there is only one of these alleles present,
represented by a capital letter.
Genetic jargon
What do these genetic terms mean?

Matching pairs – genes

Matching pairs – characteristics

Contents
All about alleles
Homozygous cross
Using a test cross
Co-dominance
Heterozygous cross
Inheritance

x
phenotype:
genotype: RR rr
Homozygous means that both alleles of a gene are the same.
Red is the dominant allele for these flowers, so the alleles
for petal colour are: red = R , yellow = r.
RR x rr – crossing homozygous parents
What are the possible offspring of a cross between a
homozygous red flower and a homozygous yellow flower?

RR x rr
R R r r
parental genotype:
gametes:
rr
R
R
Rr Rr
RrRrF1 offspring
genotype:
RR x rr – F1 offspring
What are the phenotypes of the F1 offspring?
?

F1 genotypes:
RR x rr – F1 phenotypes
Rr Rr Rr Rr
F1 phenotypes:
(Click twice on the buds
The possible offspring of a cross between two homozygous
parents are always heterozygous and so the dominant
characteristic is always expressed in this generation.
RR x rrparental genotype:

Homozygous cross activity

Contents
All about alleles
Homozygous cross
Using a test cross
Co-dominance
Heterozygous cross
Inheritance

The offspring (Rr) from the first cross (RR x rr) are called
the F1 generation. What happens in a cross between
these offspring?
Both parent plants are now heterozygous, so the alleles
in each plant are different.
F1 generation
genotype:
phenotype:
X
Rr Rr
Rr x Rr – crossing heterozygous parents

Rr x Rr – F2 offspring
R r
R
r
RR Rr
Rr rr
What are the phenotypes of the F2 offspring?
?
parental genotype: Rr x Rr
R r R rgametes:
F2 offspring
genotype:

Rr x Rr – F2 phenotypes
F2 genotypes: RR Rr Rr rr
F2 phenotypes:
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.
parental genotype: Rr x Rr

Heterozygous cross activity

Contents
All about alleles
Homozygous cross
Using a test cross
Co-dominance
Heterozygous cross
Inheritance

What is 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.
For example the genotype of a red flower could be:
RR or Rr
What could you cross a red flower with to find its genotype?

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.
So the test cross is:
x
?
(RR or Rr)
rr
Carrying out a test cross

Test cross – 2 types
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.
What about the other possible cross between a heterozygous
red flower (Rr) and yellow flower (rr)?
rr
x
?
(RR or Rr)

Test cross offspring
R r r rgametes:
offspring
genotype:
r r
R
r
Rr Rr
rr rr
What are the phenotypes of these offspring?
?
Rr x rrparental genotype:

Test cross results
F2 genotypes: Rr Rr rr rr
F2 phenotypes:
A cross between a heterozygous parent and a recessive
parent yields different types of offspring in a 1:1 ratio.
Rr x rrparental genotype:

Test cross activity

Contents
All about alleles
Homozygous cross
Using a test cross
Co-dominance
Heterozygous cross
Inheritance

RR WW
What is co-dominance?
Sometimes two alleles are equally dominant to each other.
In genetics, this is called co-dominance and means that
neither allele is recessive to the other allele.
How does co-dominance affect the offspring of a cross?
For example, let’s assume that the red allele (R) and
the white allele (W) are co-dominant:
x

RR x WW
R R W W
parental genotype:
gametes:
offspring
genotype:
R
R
W W
RW RW
RWRW
The alleles are co-dominant so both are expressed.
What will the offspring flowers look like?
Co-dominance cross – offspring
?

RW RW RW RW
Co-dominance cross – phenotypes
offspring
genotypes:
offspring
phenotypes:
All the offspring flowers are pink because both the red
and white alleles are expressed.
RR x WWparental genotype:

FLASH 5 – Blood groups
Co-dominance activity

Inheritance multiple-choice quiz

Mendel's Law and Inheritance

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