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© Boardworks Ltd 2004 1 of 36 KS4 Biology Inheritance

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InheritanceInheritance 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 2004
© Boardworks Ltd 2004
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
Pairs of alleles – homozygous
If the alleles in a matching pair are the same, they are called homozygous alleles.
What colour are the flowers with these
homozygous pairs of alleles?
to reveal the flower;
allele for
yellow petals
allele for
yellow petals
allele for
red petals
allele for
red petals
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?
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;
allele for
yellow petals
allele for
red petals
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.
=
=
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
Rr
rr
to reveal the flowers;
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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?
© Boardworks Ltd 2004
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
© Boardworks Ltd 2004
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?
© Boardworks Ltd 2004
What are the phenotypes of the F1 offspring?
?
Rr
Rr
Rr
Rr
to reveal the flowers;
RR x rr
parental genotype:
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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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
© Boardworks Ltd 2004
R
r
R
r
RR
Rr
Rr
rr
?
F2 genotypes:
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.
parental genotype:
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?
© Boardworks Ltd 2004
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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
© Boardworks Ltd 2004
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
?
to reveal the flowers;
Rr x rr
parental genotype:
A cross between a heterozygous parent and a recessive parent yields different types of offspring in a 1:1 ratio.
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What is co-dominance?
For example, let’s assume that the red allele (R) and
the white allele (W) are co-dominant:
x
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?
© Boardworks Ltd 2004
What will the offspring flowers look like?
Co-dominance cross – offspring
to reveal the flowers;
RR x WW
parental genotype:
All the offspring flowers are pink because both the red and white alleles are expressed.
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