Download Sex Linked Genes

Dwarfism
Can be caused by over 300
different things!
A form of dwarfism called
Achondroplasia is caused by a
mutation to a gene (FGFR3)
that regulates bone growth.
The mutation makes the FGFR3
gene too aggressive, which
stops cartilage being turned to
bone so limbs don’t grow
properly.
GENE-GENE
INTERACTIONS
Gene-gene Interactions
Most traits are actually controlled by more than one gene
Epistasis: when the expression of one gene affects the
expression of another
E.g: Biochemical pathway with 2 steps controlled by 2 genes
Gene 1
A
Enzyme 1
Gene 2
B
Enzyme 2
C
Epistasis
Collaboration
Supplementary
Genes
Complementary
Genes
Collaboration
Two genes influence the same trait,
and interact to produce phenotypes
that could not result from the action
of either gene alone
The genes interact to produce FOUR
possible phenotypes
Combs in chickens:
Genes P and R control comb
P_R_ Walnut
ppR_ Rose
P_rr Pea
pprr Single
Complete a punnet square for a cross
between two chickens PpRr x PpRr
•How many offspring have Walnut
combs?
9
So the typical
•How many have pea?
ratio for
3
Collaborative
• How many have rose?
Genes is 9:3:3:1
3
• How many have single
1
Epistasis
Collaboration
Supplementary
Genes
Complementary
Genes
Supplementary Genes
When one gene masks or alters the effect of another gene
There are THREE possible phenotypes
In mice, coat colour is controlled by gene A and gene B
Gene A makes melanin for coat colour
Gene B deposits the melanin
For black coat colour, a dominant A allele is needed (causes lots of
melanin produced) AND a dominant B is needed to deposit the melanin
(A_B_)
For brown coat colour, no dominant A allele is present (causes only a
small amount of melanin produced) AND a dominant B is needed to
deposit the melanin (aaB_)
If no dominant B allele is present, the melanin is not deposited and so
the mouse will be white (_ _ bb)
Complete a punnet square for a cross
between two mice AaBb x AaBb
•How many offspring are Black?
9
•How many are Brown?
3
•How many are White?
4
So the typical ratio for Supplementary
Genes is 9:3:4
Epistasis
Collaboration
Supplementary
Genes
Complementary
Genes
Complimentary Genes
Some genes can only be expressed in the presence of other genes.
These genes are said to be complimentary; both genes need a
dominant allele for the final phenotype to be produced.
There are TWO possible phenotypes
Sweet pea flower colour involves gene P and gene C. For the flower
to be purple, dominant alleles for BOTH genes must be present.
Otherwise the flower will be white.
Flower Colour
Gene P
Colourless Enzyme A
starting
chemical
Purple:
P_C_
Gene C
Enzyme B
Colourless
(white)
intermediate
White:
P_cc, ppC_, ppcc
PURPLE
PRODUCT
Complete a punnet square for a cross
between two flowers PpCc x PpCc
•How many offspring are Purple?
9
•How many are White?
7
So the typical ratio for
Complementary Genes is
9:7
Worksheet
RECAP
Match the definition with the type of gene
interaction…
Collaboration
Genes that can only be expressed in
the presence of other genes (e.g.
sweet pea flower colour)
Supplementary
Genes
Two genes influence the same trait,
and interact to produce phenotypes
that could not result from the action
of either gene alone (e.g. chickens
combs)
When one gene masks or alters the
effect of another gene (e.g. mouse
coat colour)
Complementary
Genes
SEX LINKED GENES
What do you remember?
•The human sex chromosomes are….
X and Y
•The genotype of a female is…..
XX << This is called Homogametic (2 of the same)
•The genotype of a male is….
XY << This is called heterogametic (2 different)
•Is it the same for all species??
Nope!! In birds, butterflies and moths, the female is
heterogametic and the male is homogametic. And in some
insects (grasshoppers) there is no Y chromosome at all, so:
XO = male and XX = female.
A sex linked gene is….
A gene that is only found on only one of
the sex chromosomes – either the X or
the Y
These are your sex
chromosomes:
Which sex chromosome
(X or Y) do you think has
the most sex linked
genes and why?
A good way to visualise sex-linked genes is to think
of the Y as being like an X with on bottom cut off.
Imagine the sex-linked genes are the ones that are
carried on the the bottom right hand bar of the X,
which the Y is missing.
Sex linked genes generally show up more in one
sex than the other
What are some examples of conditions or
characteristics that show up more regularly in females
or males?
• Red-green colour blindness
• Haemophilia (blood disorder where clotting factors
aren’t produced so blood can’t clot properly)
• All tortoiseshell cats are female!
Red Green Colour Blindness
X – Linked
Recessive Disorder.
XcXc or XcY to have
the disorder
Haemophillia
A disease that impairs the body’s
ability to form blood clots
X-linked recessive
disorder.
XhXh or XhY to have the
disorder
Tortoiseshell Cats – They’re all Girls!
Red-Green Colour Blindness
An X-linked RECESSIVE disorder
Who gets it more, men or women?
Colour blindness is an X-linked recessive disorder. If it’s
found on the X chromosome, why do you think that more
males get it than females?
If the trait is recessive, males only need to
receive one recessive allele to show the
trait because they have nothing to mask
it. Females need two recessive alleles to
receive the trait, so they are less likely to
show it than males .
Checkpoint:
If an X-linked gene for a disorder is recessive,
why is it more common for males to get it than
females?
Either write in in your book or explain it to the
person next to you.
In humans, red-green colour blindness is a sex
linked trait. Having the disorder (Xc) is recessive
to not having the disorder (XC)
What are the sexes and phenotypes (colour
blind, normal, normal but carrier) of humans
with with following genotypes?
Female, Normal but carrier
X CX c
X CY
Male, Normal
X cX c
Female, colour blind
X CX C
Female, Normal
X cY
Male, colour blind
Red-Green colour blindness is an X linked
recessive trait (c)
A woman with normal vision but who is a carrier of the red-green
colour blindness gene marries a man with normal vision. Give the
predicted genotype and phenotype of their children.
Woman (carrier)
Genotype:
Gametes:
x
Man (normal)
XCXc
XC
XC
XC
Xc
Xc
XC
XCXC
XCXc
Y
XCY
XcY
Y
Don’t put
anything
XCYthe Y!
above
It’s genetically
empty!
Genotype: 25% XCXC 25% XCXc, 25% XCY, 25%
XcY
Phenotype: 25% girl normal vision
25% girl normal but carrier
25% boy normal vision
25% boy with colour blindness
Haemophillia is an X linked recessive trait (h)
A woman with normal blood but who is a carrier of the haemophillia
gene marries a man who has haemophillia. Give the predicted
genotype and phenotype of their children.
Woman (carrier)
Genotype:
Gametes:
x
Man (has haemophillia)
XHXh
XH
Xh
XH
Xh
Xh
XHXh
XhXh
Y
XHY
XhY
Xh
Y
Don’t put
hY
X
anything above
the Y! It’s
genetically
H empty!
h
H
Genotype: 25% XHXh, 25% X X , 25% X Y,
25% XhY
Phenotype: 25% girl normal but carrier
25% girl with haemophillia
25% boy with haemophillia
25% boy normal
Worksheet…
If you’ve finished..
A normal sighted woman whose father
was colour blind marries a man with
normal vision. Give the genotypes and
phenotypes, plus their ratios, that could
be expected among the children of such a
marriage
Test Yourself:
•What is a sex-linked Gene?
A gene found on only ONE of sex chromosomes
•Why can’t a male be a carrier for an X-linked
recessive trait like colour blindness?
Because he only has ONE copy of the allele, so
he will either HAVE the disorder, or NOT HAVE
the disorder
LINKED GENES
Where are genes located?
CHROMOSOMES!
How are genes arranged on a
chromosome??
Genes are arranged in a linear order
on the chromosome, so one
chromosome has many genes.
Linked Genes are:
Genes that are located on the same
chromosome.
A
a
B
b
written
AB
ab
OR
A
a
b
B
written
Ab
aB
Genes A
and B are
LINKED
Linked Genes result in fewer
possible combinations of
alleles in offspring…
Normally…
If genes are located on
different chromosomes,
each allele will separate
during meiosis (in this
case, the A allele will be
able to separate from the
B allele) so that new
combinations can be
produced.
But if the genes are
linked, then the alleles
don’t separate – and so
the traits are inherited
together.
Checkpoint:
Why do linked-genes result in fewer
combinations of alleles in offspring?
Because when genes are linked, the
alleles aren’t separated during meiosis
so they move into the gametes
together.
This means that the inheritance patterns we
would expect to see aren’t always seen….
Flies have different phenotypes for wings and body colour.
They can have:
Grey body (B)
Normal wings (W)
Black body (b)
Curly wings (w)
What genotype and phenotype ratios would you expect if a fly
heterozygous for body colour and wing type mated with a fly with
a black body and curly wings?
BW
1 BbWw : 1 bbWw : 1 Bbww : 1 bbww
1 Grey body, normal wings : 1 black body
normal wings : 1 grey body curly wings :
1 black body curly wings.
bw
Bw
bW
bw
BUT when the cross was actually done, the ratios
that were observed were very different….
BW bw Bw
bw
bW
What
connection can
you make
between the
parental
phenotypes and
the phenotype
ratio of the
offspring??
Much higher ratio
Black Body, Grey body, Black body, of offspring that
Grey Body,
Normal wings Curly wings curly wings normal wings are of the
parental type
Why are most
offspring of the
parental type??
Think back to this….
If the genes are linked,
then the alleles don’t
separate – and so the
traits are inherited
together.
What can you remember about
linked genes?
A recap:
Linked genes are genes that are found on the
same chromosome.
Linkage results in fewer genetic combinations
of alleles in offspring, because traits that are
linked tend to be inherited together
Most offspring are of the parental type
But why were there some
flies that were a mixture of
the parents phenotypes??
• What happens during meiosis to
reshuffle alleles?
• CROSSING OVER!!
• When crossing over occurs
between linked alleles, then
they will be separated into
different gametes and so you
get different combinations of
alleles together (recombinants)
Practice…
Red Hair - b Brown hair - B
Freckles - f No Freckles - F
A woman with red hair and freckles (bbff) marries
a man with brown hair and no freckles (BbFf).
They have 12 children.
6 have red hair and freckles, 5 have brown hair and no freckles and
1 has red hair and no freckles.
1. Is this the ratio you would expect from this cross? Support your
answer with a punnet square, and state the expected ratio.
2. Explain in detail a possible reason why the expected ratio is not
seen.
3. Explain how one of the children had red hair and no freckles
Nature vs Nurture
One of the great debates in life is whether you are the
way you are because of your genes or because of the
environment you were brought up in.
What do you think?? Does nature or nurture have the
biggest effect??
Its very difficult to determine this in reality, we would
need about 1000 sets of identical twins, separated at
birth, exposed to different environments and then
compared later in life.
Research…
There are many things in the environment that can
influence genes, such as:
• Drugs (alcohol, cigarettes, medications)
• Nutrition
• Temperature
• Ionising Radiation
• Polluting Chemicals
Drugs
When taken by pregnant woman these can affect the unborn
child.
Thalidomide
•This was a mild sedative, given to mothers
for morning sickness.
•This drug caused severe deformities,
particularly unformed limbs, in unborn
babies.
Thalidomide Babies
Alcohol and Cigarettes
• Babies of smokers have low birth weights
• Alcohol can have a very bad effect on a child, leading to foetal
alcohol syndrome.
• This can be identified by certain facial traits, and an unusual
behaviour pattern that seems to fail to connect actions with
consequences.
• The babies of chronic alcoholics are often severely retarded in
their development, leading to:
deformed joints
imperfectly formed heart
lowered IQ.
FAS
Starvation or Malnutrition
• This can effect the proper development of both
plants and animals.
• The effect of severe malnutrition in humans is
stunted growth and mental retardation, especially
if it is due to a protein deficiency.
• Malnutrition has an especially bad effect on the
brain if it occurs under the age of about 8.
Starvation or Malnutrition
• In many third world countries whole populations
are affected.
• This will lead to a future population of mentally
slow adults.
• In these circumstances genes do not get a chance
to show through.
Temperature
• This can affect enzymes and fur colour.
• In Himalayan rabbits and Siamese cats, the fur colour can be
affected by temperature.
• These animals are normally light-coloured with black feet, ears,
nose and tail
All cats are born with a mutation to the enzyme in charge of
depositing melanin in the fur. The mutated enzyme is heatsensitive; it fails to work at normal
body temperature, but becomes
active in cooler areas of the skin.
The extremities are colder than the
rest of the body, which results in dark
colouration in the face, ears, feet
and tail.
Temperature
• If an area of the rabbit is shaved and a cool pack is placed on
that area, the low temperature causes the growth of a patch of
black hair.
• Coldness affects the expression of the gene.
Temperature
The temperature that certain reptile eggs are
incubated in can determine the sex of the
offspring.
Temperature
• In the fruit fly Drosophila, there is a condition that causes the
wings to curl.
• If flies with this gene are raised at a temperature of 25ºC, the
wings curl.
• If the flies are raised at a temperature of around 16ºC, the trait
appears only rarely; most of the flies have straight wings.
• If their offspring are raised at 25ºC, the curly wings reappear.
Ionising Radiation
Ionising Radiation causes changes to the DNA sequence
(mutations), which has a huge affect on genes…
• A large increase in the incidence of thyroid cancer has occurred
among people who were young children and adolescents at the
time of the Chernobyl accident
• Also increases in cardiovascular disease, leukaemia and other
cancers
• If mutations from radiation occur in gametes, problems can be
passed to the next generation and stillbirths, cancer and death
can occur.
Polluting Chemicals in the Environment
• The chemical 2.4.D was part of agent orange, used in the
Vietnam war to defoliate the forests.
• It was full of a toxic substance called dioxin, which got into the
bloodstream of those exposed and caused mutations to their
DNA sequence and chromosomes
• Many died
• Caused miscarriages, stillbirths and birth defects in babies of
those exposed - Cleft palate, mental disabilities, hernias, facial
distortion, limb distortion and extra fingers and toes!!
Light
• Lack of light can prevent the formation of chlorophyll or
development of seedlings.
• Light also affects skin colour. Some people can tan, as the sun
affects the deposition of melanin.