Transcript Slide 1

Saturday, July 18, 2015
3.4: Inheritance
Keywords:
• co-dominance
• punnet square
• pedigree chart
• sex-linked genes
Learning Objectives:
We are learning….
• How do we show the inheritance of characteristics through
diagrams?
•How does co-dominance affect the inheritance of characteristics?
•How are blood groups in humans inherited?
• How are genetic diseases like cystic fibrosis and Huntington’s
disease inherited?
• What are mutagens and how do they cause mutation?
Starter:
What do you understand by the
term ‘co-dominance?
Watch ‘The story of inheritance’ video
Genetic crosses: a history
Watch ‘Mendel’s
discoveries’ video
One of the first people to study
genetics was an Austrian monk
called Gregor Mendel in the
1850s and 1860s.
He experimented with thousands
of pea plants and established the
basic foundations of inheritance,
such as dominant and recessive
characteristics.
Mendel had no knowledge of DNA or genes but he did identify
that inheritance is particulate, i.e. it depends on the transfer of
separate (discrete) factors from parents to offspring.
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Genetic crosses: worked example
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Monohybrid cross
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Test cross
To determine whether an organism showing the dominant
characteristic of a trait is homozygous or heterozygous, a test
cross can be performed. This involves crossing the organism
with another that is homozygous recessive for the trait.
TT or Tt?
tt
×
if TT
T
T
t
Tt
Tt
t
Tt
Tt
all tall
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if Tt
T
t
t
Tt
tt
t
Tt
tt
If any of the
offspring show the
homozygous
recessive trait in the
phenotype, the
parent must have
been heterozygous.
2 tall, 2 short
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Crossing over
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Independent assortment of alleles
Independent assortment describes the
random arrangement and separation of
chromosomes (and all the alleles and
genes therein) during meiosis. It is
random chance how the chromosomes
migrate during anaphase I and II.
It was after Mendel had studied the
inheritance of two characteristics that
he described his law of independent
assortment: each of a pair of alleles
for a particular gene can combine
randomly with either of another pair
of alleles for a different gene.
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More on alleles
If the alleles for a characteristic are the same, the organism
is said to be homozygous for that characteristic. The
organism is a homozygote.
If the alleles for a characteristic are different, the organism
is said to be heterozygous for that characteristic. The
organism is a heterozygote.
An allele is dominant if it is expressed in the organism’s
phenotype, even if the organism is heterozygous.
An allele is recessive if it is only expressed in the phenotype
when the organism is homozygous.
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Representing alleles
A gene can be represented using a letter: upper case for the
dominant allele, and lower case for the recessive allele.
For example, the allele for wing length in Drosophila can
be either long (L) or short (l).
Genotype
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Phenotype
LL
homozygous dominant
long wings
Ll
heterozygous
long wings
ll
homozygous recessive short wings
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Expected offspring
The allele that controls eye colour can be expressed as;
B = Brown and b = Blue.
If two heterozygous dogs with brown eyes are bred
together, what would the ratio of brown eyed to blue eyed
offspring be?
Would this ratio always arise? Explain your answer
Gametes are not always produced in equal numbers and
the fusion of gametes at fertilisation is random.
You may expect these ratios of offspring, but with such a
small sample of dogs it may not always occur.
What is co-dominance?
Codominance results in a phenotype that shows both traits of
an allele pair. For example,
Red flower +
=> Red &
spotted flower.
An example of co-dominance occurs in the snap dragon plant,
in which one allele codes for an enzyme that catalyses the
formation of a red pigment in flowers.
The other allele codes for an altered enzyme that lacks this
catalytic activity and so does not produce the pigment.
If these alleles showed the usual pattern
of dominant and recessive, the flowers
would be just two colours: red and white.
Codominant alleles
Alleles are codominant if they are both expressed in the
phenotype of a heterozygote. They can be represented by two
capital letters superscript to the letter representing the gene.
For example, flower colour in
snapdragons Antirrhinum majus.
CR = red flowers CW = white flowers
Genotype
Phenotype
CRCR
homozygous
red flowers
CRCW
heterozygous
pink flowers
CWCW
homozygous
white flowers
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What are multiple alleles?
Multiple alleles is a type of non-Mendelian inheritance
pattern that involves more than just the typical two alleles
that usually code for a certain characteristic in a species.
With multiple alleles, that means there are more than two
phenotypes available depending on the dominant or
recessive alleles that are available in the trait and the
dominance pattern the individual alleles follow when
combined together.
Most of the time, when multiple alleles come into play for
a trait, there is a mix of types of dominance patterns that
occur.
Multiple alleles and blood groups
In humans, the inheritance of the ABO blood groups is
determined by a gene, I, which has three different alleles.
Any two of these can occur at a single locus at any one
time.
Blood group
Possible genotypes
A
IA IA or IA IO
B
IB IB or IB IO
AB
IA IB
O
IO IO
ABO blood group
Some genes have multiple alleles (i.e. more than two), but
only two can be present in an individual. For example, the
ABO blood group gene (immunoglobulin) in humans.



IA produces antigen A on the
surface of red blood cells
Genotype
Phenotype
IAIA and IAIO
blood group A
IB produces antigen B on the
surface of red blood cells
IBIB and IBIO
blood group B
I OI O
blood group O
IO produces no antigen.
IAIB
blood group AB
A and B are codominant and O is recessive to both.
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Multiple alleles and blood groups
Allele A causes the production of antigen A
on red blood cells
Allele B causes the production of antigen B
on red blood cells
Allele O causes no production of antigens on
red blood cells (universal donor)
Alleles A and B are codominant and allele O is
recessive to both.
Blood group
Possible genotypes
A
IA IA or IA IO
B
IB IB or IB IO
AB
IA IB
O
IO IO
Try a cross
between
individuals with
blood group A and
an individual with
blood group B
Diseases can be caused by a number of things, including:
• infections eg influenza
• poor diet eg scurvy
• environmental factors eg asbestosis
• spontaneous degeneration of tissues eg multiple sclerosis
Some diseases are inherited from our parents through our
genes: they are called genetic disorders.
They occur because of faulty alleles. Cystic fibrosis is an
example of a genetic disorder.
What is a carrier?
Any condition caused by a recessive allele, such as
cystic fibrosis, will only show if recessive alleles are
inherited from both parents (cc).
Parents with one dominant and
one recessive allele for a disorder
are known as carriers – they do
not suffer the symptoms of the
disorder but have the potential to
pass on the allele to their offspring.
Main Activity: Become the expert….
Working in groups of four. Each member of the group will
become an ‘expert’ on one genetic condition. It is your job to
study the condition and then report back to your group. By the
end of the activity, each person in the group needs to have
notes on the following for each of the four genetic conditions:
What is the name of the disease?
What is the disease caused by?
How common is the disease (in Australia)?
What are the symptoms of the disease?
What is the likelihood of the disease being passed on to
offspring?
Are there any symptoms for carriers of the faulty genes?
How is the disease treated?
Cystic fibrosis
A 3:1 ratio is typical of the results of a monohybrid cross
between two heterozygotes where one of the alleles is
dominant and the other recessive. This fact is very useful
when considering human genetic disorders and genetic
counselling is being given.
For example, cystic fibrosis
(CF) is a disease that occurs
due to a recessive allele of a
gene on chromosome 7.
The normal allele controls the
production of a membrane
protein essential for the proper
functioning of epithelial cells.
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Cystic fibrosis: pedigree diagram
If a person is homozygous for the recessive CF allele, thick
and sticky mucus is produced in their lungs and pancreas,
causing breathing problems and malnutrition. Very salty
sweat is also a symptom of the disease.
Pedigree diagrams show
how alleles, like the one
for CF, have been
inherited through families.
unaffected male
male carrier (heterozygote)
affected male (has CF)
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unaffected female
female carrier (heterozygote)
affected female (has CF)
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Thalassaemia
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Albinism
Albinism is an inherited
condition in which there is a
lack of the pigment melanin
in structures that are
normally coloured. Albinos
therefore have pinkish skin,
deep red pupils and pink
irises, photophobia and pale
yellow hair.
It is caused by a single recessive gene on an autosome,
resulting in a malfunctioning tyrosinase enzyme. This
prevents tyrosine from being converted into melanin.
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Dominant allele: Huntington’s disease
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Codominant crosses
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What is gene therapy?
Traditional drugs act by altering the phenotype
of the target cell, and are called phenotypic drugs.
Gene therapies deliver selected genes into a
patient’s cells and alter the genetic makeup of the cell.
They are referred to as genotypic drugs.
Initially, gene therapy was envisioned for the treatment of genetic
disorders, but it could be used to treat a wide range of diseases,
including cancer, arthritis and neurodegenerative diseases.
A person with cystic fibrosis has inherited two faulty alleles for a
certain gene on one of their chromosomes, chromosome 7.
It is hoped that it may one day be possible to repair the faulty
alleles using gene therapy, perhaps by putting the normal allele
into the cells of the lungs.
“Genetic disorders should not be
corrected using gene therapy.
Some people are born weaker
than others and consequently
die....it’s called survival of the
fittest.”
A couple of other Inherited
conditions…
Progeria
• Progeria is caused by a single tiny defect in a child's genetic code.
• On average, a child born with this disease will be dead by age 13.
• Symptoms include premature baldness, heart disease, thinning bones
and arthritis.
• Progeria is rare, only around 48 people living with it in the world.
• However, there is a family that has five children with the disease
A couple of other Inherited conditions…
Human Werewolf Syndrome (Congenital
Hypertrichosis Lanuginosa)
• Inerited – a genetic
mutation occurs
spontaneously.
• People who suffer from it
are completely covered in
hair except in the palms
of their feet and hands.
• No treatment
Sex determination
Sex in mammals is determined by two chromosomes, known
as the sex chromosomes or heterosomes.
The X chromosome is
larger and contains about
2000 genes, whereas the
Y chromosome contains
fewer than 100.

Females (XX) are the homogametic sex.

Males (XY) are the heterogametic sex.
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How is sex determined in other organisms?
Sex Chromosomes X-O:
Grasshoppers, roaches, and other insects have a similar
system for determining the sex of an individual. Adult males
lack a Y sex chromosome and have only an X chromosome.
They produce sperm cells that contain
either an X chromosome or no sex
chromosome, which is designated as O.
The females are XX and produce egg cells that contain an X
chromosome.
If an X sperm cell fertilises an egg, the resulting zygote will be
XX or female. If a sperm cell containing no sex chromosome
fertilises an egg, the resulting zygote will be XO or male.
How is sex determined in other organisms?
Sex Chromosomes Z-W:
Birds, insects like butterflies, frogs and some
species of fish have a different system for
determining gender.
In these animals it is the female gamete that determines the
sex of an individual.
Female gametes can either contain a Z chromosome or a W
chromosome.
Male gametes contain only the Z
chromosome. Females of these
species are ZW and males are ZZ.
How is sex determined in other organisms?
Parthenogenesis:
What about animals like most kinds of wasps, bees, and ants
that have no sex chromosomes?
How is sex determined?
In these species, fertilisation
determines sex.
If an egg becomes fertilised it will develop into a female. A
non-fertilised egg may develop into a male.
The female is diploid and contains two sets of chromosomes,
while the male is haploid. This development of an unfertilized
egg into an individual is called parthenogenesis.
Caster Semenya is a South African
middle-distance runner and world
champion. Semenya won gold in the
women's 800 metres at the 2009
World Championships with a time of
1:55.45 in the final.
Following her victory at the 2009
World Championships, it was
announced that she had been
subjected to gender testing…..
Gender testing
While it would seem a simple case of
checking for XX vs. XY
chromosomes to determine whether
an athlete is a woman or a man, it is
not that simple.
Fetuses start out as undifferentiated,
and the Y chromosome turns on a
variety of hormones that differentiate
the baby as a male.
Sometimes this does not occur, and
people with two X chromosomes can
develop hormonally as a male, and
people with an X and a Y can
develop hormonally as a female.
Women who test in the
male range for
testosterone, and whose
bodies respond to the
hormone, may not be
eligible to compete as
females.
Hyperandrogenism
Back to slides
The presence of male and female sex hormones influences the
sexual development of a person.
While both sexes have male and female sex hormones, there is a
predominance of male sex hormones (androgen) in men and female
sex hormones (oestrogen and progesterone) in women.
Women, therefore, should have more oestrogen and progesterone
than androgens in their ovaries.
Androgens are related to masculine traits like large muscles and a
low-sounding voice. When a woman's body produces more
androgens than estrogens, the disturbance in her hormonal balance
may lead to a condition known as hyperandrogenism.
Hyperandrogenism usually occurs from over production of male
hormones in a woman's ovary and is associated with other health
problems, including cysts in the ovaries.
Sex linkage
Human cells contain 23 pairs of chromosomes for a total of 46. There
are 22 pairs of autosomes and one pair of sex chromosomes. The sex
chromosomes are the X chromosome and the Y chromosome.
Sex linkage refers to the carrying of
genes on the sex chromosomes.
These genes determine body characters
and have nothing to do with sex.
The X chromosome carries many such genes,
the Y chromosome has very few.
Features linked on the Y chromosome will only arise in the
heterogametic (XY) sex, i.e. males in mammals, females in birds.
Features linked on the X chromosome
may arise in either sex.
Hyperandrogeny
Sex linkage
Genes are located on the sex chromosomes are described as
sex linked. The study of their inheritance involves examining
both the sex of the offspring and the genetic trait of interest.
X-linked diseases
Haemophilia
Duchenne muscular
dystrophy
Red–green colour blindness
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Y-linked diseases
Rare and debatable!
It is argued that there is little
room on the Y chromosome
for anything other than genes
controlling testes formation
and function.
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Sex linkage: X linkage
X linkage is more common because:

the X chromosome is larger

part of it does not have a homologous section on the Y
chromosome, therefore only one allele of a gene will be
present and so will always be expressed.
homologous regions
do not carry sex-determining genes
non-homologous regions
carry sex-determining genes
and other genes
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H – allele for production of clotting protein
h – allele for non-production of clotting protein
Xh
XH
(carrier
female)
Xh
Y
(sufferer
male)
How does sex-linkage
work?
Because the X chromosome
is much longer than the Y
chromosome, for much of the
length of the X chromosomes
there is no equivalent
homologous portion of the Y
chromosome.
Those characteristics
controlled by recessive alleles
on the X chromosome appear
more frequently in the male –
there is no homologous
portion on the Y chromosome
that might have the dominant
allele. The recessive allele will
always be expressed.
Haemophilia – the royal disease
Haemophilia figured prominently in the history
of European royalty in the 19th and 20th centuries.
Britain's Queen Victoria, through two of her five
daughters (Princess Alice and Princess Beatrice),
passed the mutation to various royal houses across the continent,
including the royal families of Spain, Germany and Russia.
Victoria's son Prince Leopold, Duke of Albany suffered from the
disease.
For this reason, haemophilia was once popularly called "the royal
disease".
Tests of the remains of the Romanov imperial family show that the
specific form of haemophilia passed down by Queen Victoria was
likely the relatively rare Haemophilia B.
Sex linkage: haemophilia
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Haemophilia in the Royal Family
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Treatment of haemophilia
Though there is no cure for haemophilia, it can be
controlled with regular infusions of the deficient clotting
factor, i.e. factor VIII in haemophilia A or factor IX in
haemophilia B.
Factor replacement can be either isolated from human
blood serum, recombinant, or a combination of the two.
Some haemophiliacs develop antibodies (inhibitors)
against the replacement factors given to them, so the
amount of the factor has to be increased or non-human
replacement products must be given, such as factor VIII
from pigs.
The long-term outlook for haemophiliacs?
Like most aspects of the disorder, life expectancy
varies with severity and adequate treatment.
People with severe haemophilia who don't receive adequate, modern
treatment have greatly shortened lifespans and often do not reach
maturity.
Prior to the 1960s when effective treatment became available, average
life expectancy was only 11 years.
By the 1980s the life span of the average haemophiliac receiving
appropriate treatment was 50–60 years.
Today with appropriate treatment, males with haemophilia typically
have a near normal quality of life with an average lifespan
approximately 10 years shorter than an unaffected male.
Haemophiliacs are more likely to die from HIV/AIDS and hepatitis than
severe heamorrhaging due to contaminated blood products.
Sex linkage and colour blindness
Did you know? One
person in twenty is
colour blind
Red/green colour blindness is a common hereditary condition
which means it is usually passed down from your parents.
Colour blindness is usually passed from mother to son on the
23rd chromosome, which is the sex chromosome.
The X chromosome is the sex chromosome: males have an
X chromosome and a Y chromosome and females have two
X chromosomes.
For a male to be colour blind the faulty colour blindness gene
only has to appear on his X chromosome.
For a female to be colour blind it must be present on both of
her X chromosomes. This is why red/green colour blindness
is far more common in men than women.
Inheritance of colour
blindness allele –
complete the family trees
In the back of your books
1.
2.
Hint
3.
5.
4.
6.
Pedigree charts
Pedigree charts are used
to trace the inheritance of
sex-linked characteristics
such as haemophilia and
red-green colour
blindness.
See if you can make sense
of this one.
Female sufferer
Male
Female
Could you add anything to the
key for this diagram?
Male sufferer
The pedigree chart explained
I
II
III
Complete the pedigree charts exam
questions
So what are mutations?
Any change in the structure or the amount of DNA of an
organism is called a mutation.
Most mutations occur in somatic (body) cells and are not
passed from one generation to the next.
Only those mutations which occur in the formation of gametes
can be inherited.
These mutations produce sudden and distinct differences
between individuals.
Michael Berryman is an American actor who has
appeared in many horror films. He was born with a
rare genetic condition which prevents him from
developing hair, sweat glands or fingernails and
claims to have had "twenty-six birth defects."
An example of how mutagens cause damage to the
genes controlling the normal cell cycle is shown below.
After exposure to UV light, adjacent thymine bases in
DNA become cross-linked to form a ‘thymine dimer’
(lesions form between bases).
This disrupts the normal base pairing and throws the
controlling gene’s instructions into chaos.
Changes in gene structure – point mutations
Changes in the structure of DNA which occur at a single
locus on a chromosome are called gene
mutations or point mutations.
Any change in the sequence
of nucleotides will produce
the wrong sequence of amino
acids in the protein it makes.
This protein is often an enzyme.
Why might this be a problem?
Complete the card sort to find out how sickle cell
anaemia is caused.
Types of gene
mutations – the causes
Chernobyl - The accident
In the early hours of 26 April 1986, one of four nuclear reactors at the
Chernobyl power station exploded.
Moscow was slow to admit what had happened, even after increased
radiation was detected in other countries.
The lack of information led to exaggerated claims of the number killed
by the blast in the immediate area.
Contamination is still a problem,
however, and disputes continue
about how many will eventually
die as a result of the world's worst
nuclear accident.
Chernobyl - The environment
The disaster released at least 100 times more radiation than the atom
bombs dropped on Nagasaki and Hiroshima.
Much of the fallout was deposited close to Chernobyl, in parts of Belarus,
Ukraine and Russia.
More than 350,000 people resettled away from these areas, but about
5.5 million remain.
Contamination with caesium and strontium is of particular concern, as it
will be present in the soil for many years.
After the accident traces of radioactive deposits
were found in nearly every country in the northern
hemisphere.
But wind direction and uneven rainfall left some
areas more contaminated than their immediate
neighbours.
Scandinavia was badly affected and there are still
areas of the UK where farms face post-Chernobyl controls.
Chernobyl – The present day
The sarcophagus encasing Chernobyl was built in haste and is crumbling.
Despite strengthening work there are fears it could collapse, leading to
the release of tonnes of radioactive dust.
Work has begun on a £600m replacement shelter designed to last 100
years. This New Safe Confinement will be built on site and then slid over
the sarcophagus.
Video - Chernobyl: The catastrophe that never ended
Despite the lasting contamination of the area, scientists have been
surprised by the dramatic revival of its wildlife.
Wild horse, boar and wolf populations are thriving, while lynx have
returned to the area and birds have nested in the reactor building
without any obvious ill-effects.
Quizzes and activities
Quick quiz – Use the whiteboards to answer
the following questions (draw punnet squares)
1 mark = correct genotype
1 mark = correct phenotypes
1 mark = ratio of phenotypes
Question 1:
A homozygous dominant green plant is crossed
with a homozygous recessive yellow plant
Question 2:
Two heterozygous carriers of cystic fibrosis
want to know the possibility of their children
inheriting the disease.
Question 3:
Two pea plants are crossed – green and
yellow. One is a pea plant which is
homozygous for green peas. The other is a
heterozygous pea plant.
Question 4:
Huntington’s disease is caused by the
presence of a dominant allele. Two parents,
one of whom is a sufferer and the other who is
normal, want to know what the likelihood is of
their children inheriting the disease is.
Plenary:
1. If a female is a carrier for haemophilia X H X h and is married to a
man with haemophilia X h Y , what is the probability that she will
have a daughter with haemophilia?
25% of the offspring are daughters with genotype X h X h
2. A normal female marries a man who has haemophilia . (You’ll
need to figure out the genotypes). What percentage of their sons will
have haemophilia?
0 sons will have haemophilia, all inherit the normal allele from mother
3. If a female has haemophilia and is married to a normal man.
What percentage of her sons will have haemophilia?
4. What percentage of her daughters will have haemophilia?
Plenary:
Plenary:
Autosome means
the 22 non sex
determining
chromosomes.
Plenary:
Suggest a genetic explanation for the difference
in life expectancy of females and males (3)
Males have XY / Females have XX
Males have only one allele for some genes – the
homologous portion is missing
These alleles are expressed
Harmful alleles increase the chance of early death
/ genetic conditions
Males develop testes due to XY chromosomes
Testes produce testosterone
Testosterone causes males to take more risks
Glossary
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What’s the keyword?
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Multiple-choice quiz
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