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DNA and Inheritance
Hayley Siddons
• What is DNA
– Deoxyribonucleic Acid
– The unit of inheritance for all living organisms
• DNA is packaged into Chromosomes in the cell
– To make it more compact to fit into the cell
– Easy of replication and equal division of the DNA
during cell division (reproduction)
Chromosomes in cells
• Same species of organisms always have the
same amount of chromosomes
– Humans have 46
– Fruit Fly have 8
– Corn has 20
Human (female) Karyotype
• Chromosomes which are not the sex
chromosomes are called Autosomes
Chromosomes in cells
The X shape of the chromosome
is because during cell division
(only time you can see defined
Chromosomes down a
microscope) each chromosome
is duplicated and the pairs are
joined in the middle at the
Mitosis Video
• Cell duplication
• 1 parent cell into to 2 identical daughter cells
• Stages of Mitosis
• Interphase
– Prophase
– Metaphase
– Anaphase
– Telophase
• cytokinesis
• Cell in normal functioning mode.
• Signal is given to enter into Mitosis
and the cell replicates its DNA
• DNA condenses down into the X
shape that we recognise as a
• Nuclear membrane breaks down
• Chromosomes line up in the
middle of the cell (equator)
• The X shape of the chromosomes
gets pulled apart
• One of the identical chromatids
goes to one side (pole) of the cell
• The other identical chromatid
goes to the other side (pole) of
the cell
• Chromosomes are now on each
side of the cell
• Nuclear membranes start to form
around each of the
DNA/chromosome groups
• Cells start forming cell
membranes between the groups
of DNA (looked pinched)
• Cell membranes ‘pinch off’ in
between the two nuclei
• Forming 2 genetically identical
daughter cells
Worksheet Activity
Making sex cells
• Sex cells (sperm and eggs)
have (and need to have)
half the amount of
chromosomes as the
nuclei of all other cells
• This is because when the
sperm and egg cells
combine during
fertilisation the resulting
cells (offspring) have the
required amount of
chromosomes and no
Pg 59
Pg 59
Meiosis Video
• Meiosis 1
– Separation of the ‘X’ shaped
chromosome pairs (one from
mum, one from dad)
– Results in half the amount of
genetic material (eg now 23 pairs)
– Chromosomes get randomly
separated, so will get a mixture of
mum and dad chromosomes going
into the 2 cells
• Meiosis 2
– Separation of the sister chromatids
• 1 cell going through meiosis
results in 4 cells, all with half the
amount of genetic material
Mitosis vs Meiosis Worksheet
Determining Sex of the offspring
• In humans only 2 chromosomes of the 46
chromosomes contribute to determining the sex
(gender) of the offspring
– The other 22 pairs of chromosomes have no effect
Sex Determination
Activity Pg 61
Check Questions Pg 62
Extracting DNA Practical
• Deoxyribonucleic Acid
• DNA made up of
– Sugar Phosphate Backbone
– Nitrogen-containing substances
called bases
Adenine (A)
Guanine (G)
Thymine (T)
Cytosine (C)
• The 2 stands of DNA bind
together through the bases –
forming base pairs
– A with T
– G with C
• This holds the DNA
molecules in the well known
double helix structure
DNA Strands
Write the complementary strand
Write the complementary strand
• Organisms are different because the proteins in
their cells are different
• The DNA provided the information about which
amino acids make up proteins
• It is the arrangement of the base pairs (A-T, C-G)
which will determine the type of protein made
• Sections of DNA which ‘code’
for a protein are called Genes
• Determines the organisms
The DNA Code
• If the two strands in the DNA molecule of DNA
separated, part of the strand would look like this:
It is the sequence of the DNA bases which form the
• Any three of these bases form a triplet code for an
amino acid.
– Eg GAT codes for Aspartic Acid
– TCG codes for Serine
The DNA Code
• With the 4 bases
– 4x4x4 = 64 triplets which can be coded
• This is more than the 20 amino acids used by cells
to make proteins
• Many amino acids have more than one coding
The DNA Code
• Different sequences code for different amino acids,
which make up different proteins
• the number of amino acids in a protein can be less
than a dozen and more than 50 000.
• Therefore a DNA molecule can code for thousands
of different proteins
Pg 66
Animals which are classes as
Albino are born without any
skin or hair colouring
This is because the gene in
each cell does not produce any
colour pigments
Because there has been a
detrimental alternation in the
original gene which codes for
colour pigmentation
• Are Alterations in genes or important parts of the
DNA molecule
• Depending on where the mutation is and how it
effects the gene/gene products/proteins it can be
bad/fatal for the organism or it can be good for the
• Can be either a single base change in the DNA
• Or a whole deletion or insertion of a segment of DNA
into the gene
• Or a rearrangement of the DNA segments on
Depending on the base change in a DNA sequence, you
could just change one amino acid or you could change it
into a stop codon and thus shorten the protein. Either
one could have a drastic effect depending on where it is
located in the protein
How do Mutations occur
• Occur naturally through normal DNA replication
– 1 mutation in every 30 million bases
– Very slow rate of mutation
• Mutation rates can be increased by exposure to
– High energy radiation
• Nuclear radiation
• X-rays
– Chemicals
– UV in sunlight
Pg 68
A genome is an organism's complete set of DNA,
including all of its genes. Each genome contains
all of the information needed to build and
maintain that organism.
Human Genome Project
When did the Human Genome Project begin?
Who was involved in the Human Genome Project?
What is the Human Genome Project?
What was the first step of the Human Genome
5. How has advances in technology aided the Human
Genome Project?
6. What are the benefits of the Human Genome
7. What problems could arise from the Human
Genome Project?
Genotype vs Phenotype
• An organism’s genotype is the set of genes that it
• An organism’s phenotype is all of its observable
characteristics—which are influenced both by its
genotype and by the environment.
For example, differences in the genotypes can
produce different phenotypes. In these house
cats, the genes for ear form are different,
causing one of these cats to have normal ears
and the other to have curled ears.
A change in the environment also can affect
the phenotype. Although we often think of
flamingos as being pink, pinkness is not
encoded into their genotype. The food they
eat makes their phenotype white or pink.
DNA detective work
Why can understanding a persons genome or even a large
segment of their DNA be important:
1. Matching donor organs with recipients for transplants
2. Identifying suspects from DNA in evidence left at a crime
3. Exonerating people wrongly accused of crimes
4. Identifying victims in major accidents or natural disasters
5. Establishing paternity of a child
Check Questions Pg 72
Dominant vs Recessive Genes
The colour of the fish on the previous slide is controlled by a particular gene
The gene comes in 2 forms:
1. Codes for black
2. Codes for red
Alleles- Different
versions of the
same gene
Even though the
parents have the gene
for red as well as
black, the parents are
still black
This is because the
black masks the red
The gene for the black
colour is said to be the
dominant gene
The gene for the red
colour is said to be the
recessive gene
Homozygous (pure breed)
Two copies of the same allele
Heterozygous (hybrid)
Two different copies of the
same allele
• Represents the dominant gene
• Upper case
• Preceeds the recessive gene
• Represents the recessive gene (not r)
• Lower case
• Proceeds the dominant gene
Always use the first letter of the dominant gene
use the capital letter to represent the dominant allele
use the lowercase letter to represent the recessive allele
Predicting crosses
It is possible to predict the type of offspring produced when 2 organism mate
One of the easiest ways of predicting crosses is to use a Punnett square
By studying family histories a pattern of inherited characteristics can be developed
This pattern can be seen on diagrams called pedigrees
Pedigrees can be used to work out the genotypes of individuals
X-linked genes
There are a number of genes on the X chromosome that have no equivalent on the Y
These genes are X-linked
The allele for normal colour vision is dominant
A female who has an allele for colour-deficient
vision on one X chromosome and an allele for
normal colour vision on the other, will have normal
colour vision. However she is called a carrier
because she carries the affected gene for colour
deficient vision
A male who has an allele for colour-deficient vision
on one X chromosome will be colour deficient
Incomplete Dominance
• When you have two alleles which are neither
dominant or recessive
• Instead they results as a mixture or a blend of
the characteristic
• When both allele or gene copies are produced
AA or Ao
BB or Bo
Blood type
Alleles A and B are co-dominant, allele o is recessive
Check Qs Pg 80-81
Genetics Problems worksheet
Pedigree Worksheet
Review Qs Pg 83-84