Genes

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Genes
(EdExcel)
W Richards The Weald School
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Genes, Chromosomes and DNA
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How genes work
Some facts - Made up of paired bases A links
with T and C links with G to form a double
helix - Contain instructions on what a cell
does, how the organism should work etc - The
instructions are in the form of a code - The
code is made up from the four bases that hold the
strands together - The bases represent the order
in which amino acids are assembled to make
proteins - The Human Genome Project (completed
in 2000) successfully mapped the human DNA,
enabling gene therapy and DNA testing
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Gene Therapy
Gene therapy is when diseases are treated by
modifying a persons genome. For example, take
cystic fibrosis
1) Take a disease-causing virus
2) Remove the genetic material that causes
excessive mucus to be produced
3) Insert the genetic material that stops cells
producing excess mucus
4) The virus then works on the cells in the lung
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Gene Therapy
Gene therapy is when a disease is treated by
modifying a persons genome (genetic makeup).
Consider the example of cystic fibrosis, a
disease causing lung problems
The patient is anaesthetised
They inhale a vector virus carrying a normal
gene
The normal genes enter lung cells
Patients feel better for around 4 weeks
Gene therapy can also be used to treat breast
cancer but it is very expensive.
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Uses of Genetic Engineering

• With genetic engineering I can produce milk that
  contains
• Extra protein
• Lower levels of cholesterol
• Human antibodies
• I am called a transgenic animal

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Sexual Reproduction
We have similar characteristics to our parents
due to genetic information being passed down in
genes through gametes
The human egg and sperm cell (GAMETES) contain
23 chromosomes each.
When fertilisation happens the gametes fuse
together to make a single cell called a ZYGOTE.
The zygote has 46 chromosomes (23 pairs) and
contains information from each parent.
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Sexual vs. Asexual reproduction

• Sexual reproduction
• 2 parents are needed
• Offspring will have pairs of chromosomes
• This will cause genetic variation
• Asexual reproduction
• Only 1 parent needed
• Offspring are GENETICALLY IDENTICAL to parent
  (clones)

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Cloning Plants
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Cloning Animals
Method 1 Embryo transplants
A developing embryo is split before the cells
specialise and the identical embryos are
implanted into host mothers.
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Cloning Animals
Method 2 - Fusion
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Designer Babies
Should we be able to design our children?
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Variation
Variation is the name given to differences
between individuals of the SAME
species. Variation is due to GENETIC or
ENVIRONMENTAL causes. For example, consider dogs

1. Ways in which they are different due to genes
2. Ways in which they are different due to the
   environment

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Plant Minerals

• Nitrates
• Used to make proteins
• Lack of it leads to a small plant, yellow leaves

• Phosphates
• Used to provide phosphorus to help photosynthesis
  and respiration
• Lack of it leads to small roots and purple leaves

• Potassium
• Helps the enzymes that are needed for
  photosynthesis
• Lack of it leads to yellow leaves with dead bits

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Eye colour
In eye colour the brown eye allele is dominant,
so we call it B, and the blue eye is recessive,
so we call it b
bb
BB
Bb
Homozygous brown-eyed parent
Heterozygous brown-eyed parent
Blue-eyed parent
What would the offspring have?
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Eye colour
Example 1 A homozygous brown-eyed parent and a
blue-eyed parent
Example 2 2 heterozygous brown-eyed parents
X
X
Parents
Gametes
(FOIL)
Offspring
All offspring have brown eyes
25 chance of blue eyes
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Eye colour
Example 3 A heterozygous brown-eyed father and a
blue-eyed mother
Equal (50) chance of being either brown eyed or
blue eyed.
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Another method
Example 3 A heterozygous brown-eyed father and a
blue-eyed mother
B b
b
b
B b
b Bb bb
b Bb bb
Father
Mother
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Example questions
1) In mice, white fur is dominant. What type of
offspring would you expect from a cross between a
heterozygous individual and one with grey fur?
Explain your answer with a genetic diagram.
2) A homozygous long-tailed cat is crossed with
a homozygous short-tailed cat and produces a
litter of 9 long-tailed kittens. Show the
probable offspring which would be produced if two
of these kittens were mated and describe the
characteristics of the offspring (hint work out
the kittens genotype first).
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Inherited diseases
1) Cystic fibrosis a disease that causes thick
and sticky mucus to coat the lungs, gut and
pancreas. Its caused by recessive alleles
2) Huntingdon's disease a disease of the
nervous system that causes shaking and eventually
dementia. Its caused by a dominant allele
3) Sickle cell anaemia a disease that alters
the shape of red blood cells, thereby reducing
their oxygen capacity, causing weakness and
anaemia. Its caused by recessive alleles
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Key words
Gamete Zygote Allele Dominant Recessive Homoz
ygous Heterozygous

• This allele determines the development of a
  characteristic
• This is formed when an egg is fertilised by a
  sperm
• This allele will determine a characteristic only
  if there are no dominant ones
• This word refers to a pair of chromosomes being
  made of two different alleles of a gene
• An egg or a sperm are called this
• This word refers to a pair of chromosomes being
  made of two of the same alleles of a gene
• An alternative form of a gene