SG Biology
Summary notes
Inheritance
Sub-topic a: Variation
A group of organisms which are able to interbreed and produce fertile offspring is
called a species
Differences exist between organisms. This is known as variation
Variation is usually easy to see when comparing different species (eg: tigers with
lions). However, variation also occurs between individuals belonging to the same
species (eg: coat colour, stem height, seed size)
There are 2 types of variation:
- continuous variation
(eg: height, hand span, weight, seed mass, leaf size)
- discontinuous variation
(eg: eye colour, tongue rolling, flower colour,
fingerprint characteristics, ear lobes)
© Continuous variation: this describes differences that can be readily measured. It
shows a gradual range from one extreme to the other. Therefore there are a wide
range of variables.
Discontinuous variation: this describes differences that cannot be readily measured,
and are therefore discrete from each other. It shows distinct differences.
Therefore usually only has about 2 -4 variables (a set number).
Discontiouous Variation
(e.g. tongue rolling)
Numbers
Numbers
Continuous Variation
(e.g. height histogram)
Dwarf
tall
Height
SG Biology: Summary notes- Inheritance
can’t
can
Tongue rolling
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Sub-topic b: What is inheritance?
All organisms produced through sexual reproduction will inherit characteristics from
each parent.
Animals can inherit characteristics such as size, hair colour, skin colour, sex.
Plants can inherit characteristics such as flower colour, stem height, seed mass.
Such characteristics may display several possible traits / versions (eg: hair colour
may be brown, black or red).
Each of these is called a phenotype for that characteristic. Words are used to
describe phenotypes
- the phenotypes for stem height in a plant may be tall or dwarf
The phenotype depends on the information (genes) inherited from each parent.
Genes represent the coded instructions necessary to control the development of
organisms. The genes are carried on the chromosomes
All the members of a species possess the same number of chromosomes (unless a
mutation had occurred)
There are 2 sets of chromosomes in every body cell; 1 set inherited from each
parent
Each characteristic is therefore controlled by 2 copies of each gene (1 from each
parent)
© The different versions of a gene are called alleles
Special cells called gametes are produced for sexual reproduction. Animals produce
ova and sperm, whilst plants produce ovules and pollen grains
The cells which develop into gametes divide in a special way so that only 1 member of
each pair of chromosomes goes into each gamete; each gamete therefore contains
only 1 set of chromosomes (and therefore only 1 copy of each gene)
When 2 gametes combine at fertilisation, the new cell (zygote) will now have 2 sets
of chromosomes; and therefore 2 copies of each gene (the new individual can now
begin its development)
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In some individuals the 2 copies of a gene will be the same, in others they may be
different
- if the 2 copies are the same, the individual is true-breeding for that
characteristic. (© homozygous)
- if the 2 copies are different, then that individual will not breed true for that
characteristic. (© heterozygous)
One of the copies may cancel out the effect of the other and can be said to be
dominant, the one which is cancelled out is recessive.
The description of the versions of a gene which are present for a particular
characteristic is called a genotype (symbols/letters are used to describe genotypes)
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Inheritance of stem height in pea plants can be used to illustrate this.
Letters used for this: T = tall, and t = dwarf
-The genotype of a true-breeding tall plant would be shown as TT
-The genotype of a true-breeding dwarf plant would be shown as tt
© As only 1 characteristic is being studied, this is known as a monohybrid cross
A cross between 2 plants, a true-breeding tall plant and a true-breeding dwarf
plant would be shown as:
Parents (P) Phenotype
Parents (P) Genotype
Gametes (G)
Tall
TT
x
x
T
First generation (F1) genotype
First generation (F1) phenotype
dwarf
tt
t
all Tt
all Tall
(remember: each gamete can
only receive 1 set of
chromosomes and therefore
only 1 of the symbols/letters)
The tall version is dominant and so all the F1 will be tall. However, unlike their tall
parent, they will not be true-breeding (like their tall parent) because they contain 2
different versions of the stem height gene
A cross between 2 of the F1 generation (both are heterozygous) would be shown as:
F1
Gametes (G)
Tt
x
T and t
F1 Gametes
T
t
Second generation (F2) genotype
Second generation (F2) phenotype
Tt
T and t
T
TT
Tt
t
Tt
tt
1 TT: 2 Tt: 1 tt
3 tall: 1 dwarf
In the F2, because their parents were not true-breeding, 3 different genotypes can
be produced: TT, Tt and tt -this results in 2 different phenotypes
© The 2 phenotypes resulting from this type of cross are always in the ratio of
approximately 3 dominant to 1 recessive; in this case 3 tall : 1 dwarf
An exact ratio of 3 : 1 is hardly ever obtained in practice as not all of the gametes
will have been fertilised, and not all of the resulting zygotes will develop.
This is because fertilisation is a random process.
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The sex of an individual is controlled by special sex chromosomes
-there are 2 types of sex chromosome: X and Y
In humans 2 X chromosomes (XX) produce a female; whilst an X and a Y (XY) produce
a male
A female can therefore only produce gametes containing an X chromosome; whilst a
male produces gametes containing an X chromosome, and gametes containing a Y
chromosome.
Humans have 46 chromosomes, 44 “normal” chromosomes plus 2 sex chromosomes.
Male 44 + XY
Female 44 + XX
Sub topic c: genetics and society
It is possible to select individuals from a species which possess a particularly useful
characteristic (eg: fruit size, milk yield, growth rate etc:) with a view to breeding
only from such plants or animals
If several varieties are available it may be possible to cross breed.
Any offspring which show an improvement in the characteristic/s being sought can be
used for continued breeding. If this process is continued for several generations, the
result will be organisms which will always produce offspring which show such
improvement (true-breeding) - this process is called selective breeding.
© All of our modern cereal crops (eg: rice, wheat, barley, oats) have been produced
as a result of selective breeding.
A variety of British wheat with soft grains was crossed with a Canadian variety with
hard grains but low yield. A few of the plants produced had hard grains coupled with
a high yield. These were then used for further breeding, and after about 12 years a
variety of wheat had been bred which consistently gave a high yield and hard grains.
Cattle have been selectively bred over many years to produce improved milk yields in
dairy cows; and faster growth rates in beef cattle.
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Mutations
Occasionally, part of a chromosome may become damaged or altered in some way.
This is called a chromosome mutation.
© Mutations will always occur naturally, but some agents (eg: radiation, such as UV
light, X-rays, atomic radiation and certain chemicals such as mustard gas) increase
the chances of mutations occurring. These agents are called mutagenic
Down's syndrome in humans is an example of a condition caused by a chromosome
mutation (an extra chromosome number 21; giving 3 of these chromosomes instead of
just 2). Humans have 46 chromosomes in every cell, an individual with Down’s
syndrome will have 47 chromosomes.
It is possible to obtain some information about an embryo's chromosomes before it is
born. The embryo is surrounded by amniotic fluid (in the amniotic sac) whilst it is in
the uterus. As some of its cells become detached and float about in the fluid, taking
a sample of this fluid will provide a sample of embryonic cells.
This can be done by using a large syringe/needle, inserted through both the
abdominal and uterine walls, to draw off some of the fluid; a process called
amniocentesis.
The cells obtained are then examined, and the chromosomes identified and counted
to determine whether the embryo has Down's syndrome (or various other inherited
disorders). It is also possible to tell if the embryo is male or female.
© Most mutations are harmful but some mutations produce changes which are useful
to man; such as larger fruits or increased resistance to disease in plants, or faster
growing breeds of domestic livestock.
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