Variation
A species is a group of interbreeding organisms whose offspring are fertile
Male horse X female horse

horses
male horse X female zebra

zebrorse
male horse X female horse

horses
male zebrorse X female zebrorse

no offspring
Horses and zebras do not belong to the same species as the offspring that they produce are not
fertile i.e. they can not produce their own offspring.
Variation can occur within a species
No two organisms of the same species are exactly alike e.g. people in your class have different
Height, weight, eye colour. They are said to show variation.
Plants can differ in leaf shape, number of leaves, height, flower colour etc.
Explain what is meant by continuous and discontinuous variation
There are 2 kinds of variation:
Continuous variation occurs when the differences are easily measured and there is a wide range of
measurements.
If you graph examples of continuous variation they usually look like this:
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Discontinuous variation
Some differences are hard to measure. People usually have or can do something which other people
have not or cannot do e.g. some people can roll their tongue while others cannot.
If you graph examples of discontinuous variation they usually look like this:
Give examples of continuous and discontinuous variation
Continuous variation
Height, weight, hand span, length of index finger, leaf length etc
Discontinuous variation
Blood groups, gender, hair type, eye colour, flower colour etc
What is Inheritance?
Certain characteristics are determined by genetic information received from the parents and give
examples, from animals and plants
Inside the nucleus of each cell there are chromosomes which carry genetic information. This
information is passed on through the generations from the parents to the offspring. Half of your
genetic information comes from your mother and half from your father. There are 46 chromosomes
in most of the cells in your body.
Many characteristics can be inherited in:
Humans
eye colour, hair type, ability to roll your tongue, blood group
Flowering plants
height, flower colour, leaf shape
Insects
wing shape, body pattern
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Identify examples of phenotypes of the same characteristic
The phenotype is the description of the appearance. Any characteristic can have several phenotypes.
E.g.
flowers can be red or white
eyes can be blue , green or brown
hair can be curly or straight
Identify examples of true-breeding, dominant and recessive characteristics from the numbers and
phenotypes of given crosses
If 2 individuals with the same phenotype are crossed and produce offspring which always have the
same phenotype, they are called true – breeding. e.g. Hamsters which are true-breeding for red
eye colour, if crossed with other true-breeding red eyed hamsters will only produce offspring with
red eyes.
In 1865 Gregor Mendel conducted some experiments on pea plants. He used a true-breeding tall
plant and a true-breeding small plant.
He grew both kinds of pea plant and when they flowered he transferred pollen from a small plant to
a tall plant and vice versa.
He then waited for the seeds to form, planted them and then observed what colour the plants
grew to. The phenotype of all of the plant offspring was tall.
From his experiments, Mendel decided that for every characteristic there were two “factors” which
carry information.
The dominant phenotype is the one which is shown in the offspring. The phenotype which is not
shown is called the recessive phenotype.
In the above cross :
the dominant phenotype is tall (T)
the recessive phenotype is dwarf (t)
Identify generations as P, F1 and F2 from given examples of crosses
The above experiment can be written as:
Parents
(P)
true-breeding
tall
X
TT
First generation
of offspring
(F1)
true-breeding
small
tt
all tall plants
He then took and F1 plant and crossed it with a different F1 plant
F1 X F1
Second generation
of offspring
(F2)
tall
X
3 tall and 1 small
3
tall
The genetic content of the offspring can be determined by drawing a punnet square.
PXP
T
T
t
Tt
Tt
t
Tt
Tt
T
t
T
t
TT Tt
Tt tt
F1 X F1
The parents in experimental monohybrid crosses are usually true-breeding and show different
phenotypes of the same characteristic
A breeding experiment where we consider only one characteristic is called a monohybrid cross.
Look at the monohybrid cross below
The dominant phenotype is purple flowers. The recessive phenotype is white.
The phenotypes of the F1 in a true-breeding cross are uniform
i.e. they all have the same phenotype.
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Predict the proportions of the phenotypes of the F2 offspring of a monohybrid cross
The predicted F2 phenotype ratio is always 3 dominant to 1 recessive.
Each body cell has two matching sets of chromosomes
In each human body cell there are 46 chromosomes. This means that each set has 23 chromosomes.
You get half of these from your mum and the other half from your dad. This means that you get 1
set from each parent.
Sex cells are called gametes
In humans:
the male gametes are called sperm. Sperm are made in the testes.
the female gametes are eggs or ova. Eggs are made in the ovaries.
The reduction of the number of chromosomes to a single set occurs during gamete formation
Each body cell has 46 chromosomes.
Gametes, however, only contain 23 chromosomes.
Each sex cell carries one set of chromosomes
One set of chromosomes is equal to 23.
Describe how a complete double set of chromosomes is achieved at fertilisation
Fertilisation is the joining of male and female gametes.
Each gamete has 1 set of chromosomes.
The chromosome set of the sperm joins with the chromosome set of the egg.
The nucleus of the fertilised egg or zygote contains a double set of chromosomes.
Female gamete
23 chromosomes
(1 set)
+
Male gamete
23 chromosomes
Fertilised egg
46 chromosomes

(1 set)
5
(2 sets)
Genes are part of chromosomes
A chromosome contains a large number of genes
One chromosome
Each gene controls one characteristic.
A characteristic is controlled by two forms of a gene
Genes can have different forms. E.g. blue eye colour and brown eye colour are both forms of the
gene for eye colour.
Different forms of a gene are called alleles
E. g.
Alleles for:
Hair colour
brown, black, white, blonde
Flower colour
red, pink, white, orange, purple etc
Tongue rolling
roller, non-roller
Each parent contributes one of the two forms
Organisms inherit one gene for each characteristic from each of their parents.
Each individual therefore has 2 alleles for any one characteristic.
The two forms of the gene can be the same (homozygote) Blue eye colour is recessive to brown so a
blue eyed individual must have 2 copies of the blue eye colour allele. This would be expressed as bb.
Dwarfism in plants is recessive to tall so dwarf plants would be represented by tt.
The two forms of the gene might not be the same but can be different (heterozygote) e.g. a
heterozygotic:
tall plant
would be
Tt
Brown eyed individual
would be
Bb
Heterozygotic individuals exhibit the dominant phenotype.
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Each gamete carries one of the two forms of the gene
Each set of chromosomes contains one form of the gene.
This means that body cells contain 2 forms of the gene but a gamete will only contain 1 form of the
gene.
State the meaning of the word genotype
The genotype of an organism tells us which forms of the gene are present.
Single letters are used as symbols in genotypes.
e.g.
TT or Tt indicate that a plant is tall whereas tt represents a dwarf plant.
A plant with the genotype TT has 2 copies of the dominant allele T. A plant with the
genotype Tt has both alleles T and t.
A plant can be homozygotic for height if it is TT or tt
Explain monohybrid crosses in terms of genotypes
The capital letter is usually the first letter of the characteristic which is dominant.
E.g. Brown eye colour is dominant to blue.
A true-breeding individual with brown eyes would have the genotype BB. The genotype of all their
gametes would be B.
A true-breeding individual for the recessive eye colour would have the genotype bb. All of their
gametes would have the genotype b.
A member of the F1 generation from a cross with the 2 individuals above would have the genotype
Bb. Some of their gametes would have the genotype B others would have the genotype b.
The F2 generation will have 2 different phenotypes but 3 different genotypes: BB, Bb and bb.
Explain differences between observed and predicted figures in monohybrid crosses
The predicted ratio of the F2 generation in any monohybrid cross is 3 dominant: 1 recessive. This
ratio is very rarely observed in real crosses. Gamete formation is a continuous process. Only 1 of
the two alleles can go into a gamete, and this is a random choice.
You can get much nearer the predicted ratio by dealing with a large number of offspring.
A good answer to remember is that fertilisation is a random process.
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The sex of a child is determined by specific chromosomes called X and Y chromosomes
Each human cell contains 46 chromosomes or 23 pairs. 22 pairs are the same in both sexes, but one
pair is different. This pair decides if the baby is a boy or a girl.
Males have one X chromosome and one Y chromosome whereas females have two X chromosomes.
In humans, each male gamete may have an X or Y chromosomes, while each female gamete has an X
chromosome
Sperm can contain either an X or a Y chromosome but eggs will only ever contain an X
chromosome.
Explain how the sex of a child is determined with reference to X and Y chromosomes
When an egg (X) joins with a sperm containing a Y chromosome, the zygote will have the genotype
XY and will be a boy.
When an egg joins with a sperm containing an X chromosome, the zygote will only contain X
chromosomes. It will have the genotype XX and be a girl as it does not contain a Y chromosome.
The gamete of the father determines the sex of the child.
Genetics and Society
Give two examples of an improved characteristic resulting from selective breeding, e.g. increased
yield, increased disease resistance, or increased growth
Many animals and plants have particular characteristics which are useful to man. By breeding from
the most useful individuals man can improve and modify organisms for his own benefit. This is
known as selective breeding.
Describe two examples, one plant, one animal, of the enhancement of a characteristic through
selective breeding
By selective breeding, we have produced many new varieties of crop plants which:
grow more quickly, produce higher yields and are more resistant to disease.
Animals have also been selectively bred to produce higher yields of meat and milk.
In addition they have been selectively bred for other qualities e.g. dogs have been selectively bred as:
Guard dogs e.g. Alsatian
Shepherds e.g. Border Collie
Tracker dogs e.g. retriever
“Cute” pets e.g. poodle
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Describe one example of a human condition caused by a chromosome mutation, e.g. Down’s
syndrome
Sometimes chromosomes can change. These changes can occur naturally and are called
mutations. A mutation can cause changes in characteristics.
Down’s syndrome is a human condition which occurs when the child inherits an extra copy of a
particular chromosome (chromosome 21).
Amniocentesis can be used to detect chromosome characteristics before birth
Amniocentesis is the removal of amniotic fluid for medical examination.
The fluid contains cells from the developing embryo. The chromosomes in these cells can
be examined under a microscope to see if they are normal.
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Give an example of a chromosome mutation, advantageous to man, in a plant or animal of economic
importance.
Not all chromosome mutations are harmful to humans.
1
Mutations in some crops result in fruit without seeds e.g. bananas, water melons and
grapes.
2
Some crop plants have extra sets of chromosomes
apples produce more vitamin C
Give an example of a factor which can influence the rate of mutation in an organism
Natural mutations are quite rare. The rate of mutation in a population can be increased if they are
exposed to certain factors. Such factors are called mutagenic agents.
Various types of chemicals can do this, and radiation (x-rays and ultraviolet).
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