Kelso High School
Biology
Inheritance
Name……………………..... Class………..
Keep this booklet safe – you will need it for revision.
Inheritance
Use this table as a checklist for your revision. For this topic you might also
find it useful to look back over the ‘Reproduction’ topic notes.
= core
= challenge
Understood
and learnt
Level
Need to do
more work
Need to get help
- see my teacher.
You need to be able to……
State that a species is a group of interbreeding
organisms whose offspring are fertile.
State that variation can occur within a species.
Give examples of continuous and discrete variation.
Explain what is meant by continuous and discrete
variation.
State the sex cells are called gametes.
State that each gamete carries one set of
chromosomes.
State that each body cell has two matching sets of
chromosomes.
State that the reduction of the number of
chromosomes to a single set occurs during gamete
formation.
Describe how a double set of chromosomes is
achieved at fertilisation.
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State that genes are parts of chromosomes and there
are two forms of each gene.
State that certain characteristics are determined by
genetic information received from the parents and
give examples from plants and animals.
Identify examples of phenotypes of the same
characteristic.
State the meaning of the word genotype
Identify examples of homozygous, dominant and
recessive characteristics from the numbers and
phenotypes of given crosses.
Identify generations as P, F1 and F2 from given
examples of crosses.
State that the phenotypes of the F1 in a homozygous
cross are uniform.
Explain monohybrid crosses in terms of genotypes
Predict the phenotype ratios of the F2 offspring of a
monohybrid cross.
State that the sex of a child is determined by specific
chromosomes called X and Y chromosomes.
State that in humans, each male gamete may have an
X or a Y chromosome while ach female gamete has
an X chromosome.
Explain how the sex of a child is determined with
reference to X and Y chromosomes.
Date of ‘Inheritance ’ End of Unit Assessment:
………………………………….
Remember, in addition to testing your knowledge the assessment will include
problem solving questions, eg graph drawing, calculations, etc.
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Variation
Species
A species is a group of plants or animals that can breed together to produce
__FERTILE OFFSPRING________.
Factors Affecting Ecosystems
Many things can affect ecosystems.
The dogs above may all look different but they are the same species because
they can interbreed.
Variation
The differences between members of the same species is called variation.
There are two types of variation:

____________DISCRETE________________

____________CONTINUOUS________________
Discrete Variation
If the variation in a characteristic allows individuals to be divided into ___2___
or more ______DISTINCT__________ groups, the characteristic shows
discrete variation.
Examples include:
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Continuous Variation
If the variation in a characteristic can be measured (eg height in m) and it
shows a wide _______RANGE______ occurring between two extremes (eg
between the shortest and the tallest), then it is an example of continuous
variation.
Examples include:
_HEIGHT__________
__PULSE RATE
_____
SHELL DIAMETER__
_____SEED LENGTH__________
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Inheritance
Before starting this section it would be a good idea to go over your
‘Reproduction’ notes again.
Chromosomes and Genes
The sex cells (gametes) contain only one set of chromosomes. When
fertilisation takes place, the chromosome set of the sperm joins with the
chromosome set of the egg. The nucleus of the fertilised egg (zygote) now
contains two matching sets of chromosomes.
The zygote divides to produce all the other cells of the body. This means that
every cell has the same two sets of chromosomes.
When the two sets of chromosomes are examined, they can be arranged in
pairs – one of the pair will have come from the mother and the other from the
father.
(___set of chromosomes)
(___set of chromosomes)
(___sets of chromosomes)
Each chromosome carries information on tiny units called genes. It is these
genes that determine the characteristics of an organism.
All characteristics are determined by a pair of genes. The genes for each
characteristic exist in two forms. One form is usually dominant and the other
is recessive.
One example in people is tongue rolling. There are two tongue rolling genes.
One gene allows you to roll your tongue, the other gene does not. Depending
on which combination of these genes your cells contain, you either can or
cannot roll your tongue
If you get a tongue rolling gene from both your parents you will also be able to
roll your tongue.
If you get a non-tongue rolling gene from both your parents you will also be
unable to roll your tongue.
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If you get a tongue rolling gene from one parent and a non-tongue rolling
gene from the other parent the two genes will be in competition. In this case
the person will be a tongue roller because the gene for tongue rolling is more
powerful than the gene for non-tongue rolling.
Genes which are more powerful in a pair are said to be the dominant gene.
Genes which are the least powerful in a pair are said to be the recessive
gene.
The Monohybrid Cross
The two forms of a gene can be represented by letters. The dominant gene is
always represented by a capital letter and the recessive gene by a small
letter.
Tongue rolling is dominant to non-tongue rolling. These genes can be
represented as follows:
T = tongue roller
t = non-tongue roller
Each characteristic of an organism is determined by two forms of the gene.
The two forms of the gene present in an organism is known as its genotype.
A tongue roller has the genotype TT or Tt
A non-tongue roller has the genotype tt.
If the two genes are the same (TT or tt) for a characteristic the genotype is
said to be homozygous.
If the two genes are different (Tt) for a characteristic the genotype is said to
be heterozygous.
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The simplest genetic cross involves one characteristic and is called a
monohybrid cross.
In this example colour in green coloured pea pods is dominant to yellow
colour.
G = Green pod
g = Yellow pod
X
Parents phenotype
Green pea pod
Yellow pea pod
Parents genotype
GG
gg
G
g
Gametes
Fertilisation
F1 genotype
Gg
F1 Phenotype
All green pod
The next step involves crossing two individuals from the F1 generation:
X
F1 phenotype
Green pea pod
Green pea pod
F1 genotype
Gg
Gg
G or g
G or g
Gametes
A punnet square is then used to show the possible ways in which these two
sets of gametes could combine during fertilisation:
Female gametes
G
g
G
F2 genotypes
and
phenotypes
F2 phenotype ratio
Male
gametes
GG
Green pod
Gg
Green pod
g
Gg
gg
Green pod
Yellow pod
3 Green pod : 1 Yellow pod
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Sex Determination
A person’s sex is determined by a pair of chromosomes, the sex
chromosomes.
They are called the X and Y chromosomes.
A female has the genotype XX
A male has the genotype XY
The diagram below shows how sex is inherited:
Parents phenotype
Male
Parents genotype
XY
XX
X or Y
X
Gametes
X
Female
Fertilisation
F1 genotype
F1 Phenotype
F1 Phenotype Ratio
XX
XY
Female
1
Male
:
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1
Glossary
You should know and be able to use all of the following terms:
Variation
Species
Variation
Offspring
Fertile
Continuous Variation
Discrete Variation
Term
Definition
Continuous
variation
A characteristic which varies from one extreme to
another, eg from very small to very tall.
Discrete variation
A characteristic which allows individuals to be divided
into two or more distinct groups, eg earlobes attached or
earlobes unattached.
fertile
Able to produce offspring.
offspring
New individuals produced when parents reproduce.
species
A group of organisms that can interbreed to produce
fertile offspring.
variation
Differences which occur between members of the same
or of different species.
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Inheritance
Genes
F1
Homozygous
Dominant
Chromosomes
Heterozygous
Genetic Information
Monohybrid Cross
Inherited Characteristics
P
Fertilisation
Term
Recessive
Gametes
Phenotype
F2
Genotype
Definition
chromosomes
Thread-like structures made up of genes and found in
the nucleus of a cell.
dominant
The form of a gene which always shows itself in the
phenotype; usually given a capital letter as its symbol.
F1
The first generation of a cross
(first filial generation)
F2
The second generation of a cross
(second filial generation)
fertilisation
Occurs when two different gametes fuse to form a
zygote.
gametes
Sex cells, eg sperm, egg, pollen, ovules.
gene
Small parts of a chromosome which control the
inherited characteristics.
Genetic
information
Information which determines hair colour, tongue rolling
and many other characteristics and is passed on from
one generation to the next in the genes.
genotype
The set of genes an organism possesses; usually
written as a formula, eg Tt
homozygous
An individual that possesses two identical forms of a
gene, eg TT or tt.
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heterozygous
An individual that possesses two different forms of a
gene, eg Tt
Inherited
characteristics
Characteristics which are passed on from one
generation to the next.
monohybrid
A cross between two parents which have only one
difference, eg flower colour.
P
Parents
phenotype
The outward appearance of an organism
Recessive
The form of a gene which is masked by a dominant
gene or is only expressed when the dominant gene is
absent, eg tt.
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