Download Genetic Crosses

GENETIC CROSSES
MONOHYBRID CROSSES
In a monohybrid cross two plants or animals, which
differ at only one gene, are bred together.
By looking at alleles of the genes that the parents have
we can tell how the offspring will turn out.
This can be very useful when carrying out selective
breeding of plants or animals.
In humans with inherited diseases it is extremely
useful to know about the alleles since we can inform
them about the likely effect if they want to have
children.
USEFUL VOCABULARY
However, like everything else in biology, it is
important to be absolutely clear about what
we are talking. So we need to sort out some
descriptive words.
Homozygous and heterozygous
Chromosomes come in pairs. Each chromosome in a
pair will have a gene at the same point on the
chromosome. There can be more than one alternative
form of the gene at that point. These alternative forms
are called alleles.
Both chromosomes in a pair have one allele for the
gene. If the two alleles are the same we say that the
individual is 'homozygous' for that gene. It they are
different the individual is 'heterozygous'.
So if 'H' is an allele for a height gene, and it produces
tall offspring, then another allele might be 'h' which
would make the individual small.
So if you had the two alleles 'HH' or 'hh' you would be
homozygous for that gene. But, if you had 'Hh' you
would be heterozygous.
DOMINANT AND RECESSIVE
With the 2 alleles we thought about above, the
allele for tallness (H) gives quite different
instructions to the plant cells than does the
allele for smallness (h).
What happens in the heterozygous case (Hh) when
they are together?
You find that the effects of the H allele would be shown more
strongly in the offspring than the h.
We say that in this case, the tallness allele H dominates the
smallness allele h.
Therefore we talk about the dominant allele (H) and the
recessive allele (h).
(To make things clear the dominant allele is always shown as
an uppercase letter).
HH = Homozygous dominant
hh = Homozygous recessive
Hh = Heterozygous
Genotype and phenotype
When you look at someone or at a plant, you can only consider
what they looklike. You can't work out which alleles they have
for a particular gene. You are considering their phenotype. This
is the outward effects of the genes - what you see.
Knowing their actual combination of alleles - for example,
whether they are homozygous recessive - is to know their
genotype. To know what genes they carry.
Think about plants with a genotype that is heterozygous (Hh).
Their phenotype will be to be tall plants - since the tallness
allele (H) dominates the smallness allele (h).
GENERATIONS
The last thing! You would soon get confused about which plants
or animals you are talking about. There are the parents, then
their offspring, and their offspring, etc. etc.
So, to make it nice and easy we give each generation a
name.
The first plants or animals bred together are called the Parental
generation, or P1 generation.
Their offspring are called the First Filial generation, or F1
generation.
Their offspring are called the Second Filial generation, or F2
generation.
AN EXAMPLE MONOHYBRID CROSS
Suppose we wanted to cross two of the tall plants we thought
about earlier.
The allele for tallness is H and is dominant to that for
smallness, h.
If the two plants are heterozygous, they will have a genotype,
which contains the alleles Hh.
Remember: the gametes of any individual contain only half of
the chromosomes. So only one of the alleles will be present in
each gamete cell.
WHAT WILL THE OFFSPRING BE LIKE?
So there will be 3 tall plants for every 1 small plant. Or to put it
another way, there is a 75% chance that each F1 (offspring)
plant will be tall.
WHAT PERCENTAGE OF TALL PLANTS WOULD
YOU GET IN THIS CROSS?
SELECTIVE BREEDING CROSSES
Knowing about genes and alleles allows you to decide which
individuals to breed together.
You can then make use of the theory to help you improve the
plants or animals.
Selective breeding
It is straight forward enough.
You choose your best animals (or plants) and breed them
together.
Then choose the best animals in their offspring (F1 generation).
Breed these ones again to give an F2 generation.
Carry this on over many generations until you have the
'perfect animal' - well, the one with the best characteristics or
traits that you wanted.
Try this exercise: Drag and drop the correct light blue boxes
over the darker blue boxes and mark your answer!
WHY SELECTIVE BREEDING IS USEFUL
You can easily imagine that one big reason
for selective breeding is money.
You can save a lot of wasted money if you
weed out weaker individuals. For example,
you could selectively breed for disease
resistance.
You can also ensure that you get the maximum output
and therefore are more efficient. More potatoes grown on
each plant means more money.
PROBLEMS WITH SELECTIVE BREEDING
There's an old expression about putting your eggs in one basket.
If you select for certain characteristics you are selecting for certain
alleles.
Over a few generations you lose those alleles from the gene pool. That is
from the alleles that individuals have and that are available for passing on.
But what happens if it turns out that those lost alleles had
an advantage.
Without the allele all your best crops or animals could die!
Many domestic dogs are pedigree dogs. They are
Alsatians, or Red Setters, etc.
Over many generations their characteristics have been
chosen. However many breeds of dog show specific
weaknesses and bad health problems.
So adopt a healthy mongrel instead! They have a good
range of alleles to choose from!
INHERITED DISEASES
As humans we don't consciously go in for selective
breeding. We just follow our romantic feelings.
Usually this works out fine. However, there are occasions
when people discover that one of their genes actually gives
rise to an inherited disease.
It is helpful for a couple to know how likely they are to pass
on these 'bad' alleles to any children they might have.
Let's look at some of the common diseases that can be
inherited.
CYSTIC FIBROSIS
Sufferers of this disease produce a thick, sticky mucus which coats
their airways and lungs. If it is not cleared by daily massage and
physiotherapy, and treated with antibiotics, the person can get
serious chest infections.
The cause of the disease was discovered in 1989 as being a
recessive allele. This allele is carried by about 1 in 20 of people.
Let's call the recessive allele c and the dominant, normal allele C.
If a person is heterozygous (Cc) then they are a carrier but have a
normal phenotype, they don't develop cystic fibrosis but can pass
it on to their children.
What happens if a carrier (Cc) and a 'normal' person (CC)
want to have children?
It works out that half of the children will be 'normal' and
half will be 'carriers' of the cystic fibrosis gene. So none of
the children will actually develop the disease.
But what happens if another two carriers wanted to
have children?
There is a 1 in 400 chance that two carriers will meet and
have children.
Would that make a difference to their children's chances
of developing the disease?
One in four children (25%) would be cystic fibrosis sufferers and half
(50%) would be carriers. Not great odds. But potential parents can be
warned of them.
If the chances of two carriers having children is 1 in 400. And the chances
of a child having cystic fibrosis is 1 in 4.
HAEMOPHILIA
Haemophilia is a famous blood disease. Its fame comes
from the children of Queen Victoria and their offspring.
The symptoms are that blood fails to clot. The smallest
wound or tooth extraction can prove fatal. A bump will not
lead to a bruise but large, internal bleeding.
Nowadays sufferers are treated with a protein extracted from
the blood of donors. Regular injections of clotting factor 8
can allow the patient's blood to clot normally.
The genetic basis of the disease is that the damaged allele
occurs on the X sex chromosome.
Therefore it is sex-linked.
The normal blood-clotting allele is dominant and is shown as H. The
recessive allele that causes Haemophilia is shown as h. (It doesn't
matter really what letter is used, just as long as everyone is clear about
what you are describing).
There are five possible combinations of the defective allele with the sex
chromosomes. They are shown together like this for clarity:
In 2 cases the 'h' allele is not involved and so the people
have normally clotting blood.
When a woman is heterozygous for the allele she will be
an unaffected 'carrier'.
A man who has the 'h' allele on his single X chromosome
will have the disease.
A woman who was homozygous for the 'h' allele would
have the disease, but they never develop.
What happens if a female carrier had children with a
normally clotting man?
Therefore a woman carrier has a 1 in 4 chance of an affected
child. Risky odds!
SICKLE CELL ANAEMIA
This inherited disease causes the red blood cells to change
from their usual round shape to become pointed like a
sickle.
This shape change means that they get stuck in blood vessels and
cannot pick up oxygen properly from the lungs.
The allele responsible for it is a recessive one just as in haemophilia.
Again you get carriers but this time the 'wrong' allele is not on a sex
chromosome.
What happens if two carriers were to have children?
They would have a 1 in 4 chance of a child being a sickle cell
sufferer.
Sufferers usually die young, before they can reproduce.
Why doesn't the allele just die out?
By a strange quirk being heterozygous (Ss)
for the gene gives some protection against
malaria.
Since the carriers escape suffering from this
serious and possibly lethal disease of malaria
they live and reproduce, therefore passing on
the sickle cell allele to another generation.
HUNTINGTON'S CHOREA
This is also known as Huntington's Disease.
Chorea means dancing, that's where we get
the word choreography.
The symptoms of Huntington's chorea are a
series of uncontrolled, dance-like movements
which do not appear until the sufferer is in their
forties. There is also a severe mental damage
which gets worse with increasing age.
Unusually the disease is caused by a dominant allele.
So, if you look at the case of a sufferer who has children
with an unaffected person:
There is a 50% chance that any children will suffer with this disease. Very bad
odds!
Any person with one gene will be a sufferer and a carrier.
If two of these had children 75% of them would get the disease! Horrible odds!
DOWN'S SYNDROME
Unlike the previous examples, Down's syndrome is caused by having
an extra whole chromosome.
Therefore, Down's syndrome is a mutation in which an extra
chromosome 21 is passed into the same egg cell during meiosis.
(The other egg cell created during the same meiotic division will have
no chromosome 21 at all).
If the egg cell with two chromosomes 21 becomes fertilised, the zygote
will end up with three chromosomes 21. It will have a total of 47
chromosomes instead of the usual 46. This causes Down's
syndrome.
The main effects of this syndrome are that the person has a lower
mental ability and are more susceptible to certain diseases. Sadly they
tend to die quite young at about thirty years old.
CLONING AND GENETIC ENGINEERING
We now know a lot about genes, chromosomes and the
inherited diseases that can occur.
We also know how to use genetics to plan our selective
breeding programmes to be most effective.
Cloning techniques and genetic engineering allow many
new opportunities but also carry challenges.
CLONING PLANTS
There is both natural and artificial cloning. Both produce
clones, plants that are genetically identical to the parent
plants.
Natural cloning
Clones occur naturally in plants. It can occur when plants such as the
strawberry send out runners, which establish a whole new plant.
If you plant an old potato it will grow into a clone of the original.
Yet another example is plants such as daffodils, which produce
bulbs. Quite often they split into two bulbs with each plant becoming a
clone of the other.
The cloning process occurs through cell division mechanism of
mitosis. It therefore allows them to undergo this form of asexual
reproduction.
However, these plants can also reproduce using sexual reproduction
(that is releasing gametes). This is important as it allows for genes to
be shared between different individuals and then on to their offspring.
This avoids the loss of genetic variation, which is the main problem
of cloning.
Artificial cloning
For many years, gardeners have sat hunched in potting
sheds around the world taking cuttings. A small piece of
branch or stem is cut from a larger plant and is perhaps
dipped into an auxin rooting powder. In a few weeks a
new plant develops.
In a few weeks a new plant develops. Little do these
humble gardeners realise that they are carrying out a form
of micropropagation. This is a high-tech version of the
traditional cutting approach.
In micropropagation, cuttings are taken from a stem and cut into
smaller sections. Each section is sterilised first before adding them to
a growth medium containing rooting hormones. After each develops
roots it grows intoa plantlet. Finally, they are hardened up by being
grown in a greenhouse.
Tissue culture is another new technique that has been
used for cloning plants. Here, only a few plant cells are
needed. These are then added to the growth medium and
hormones. They will develop into a new plant.
The advantages: the new systems are that they can
produce new plants very quickly, in large numbers and in a
small space. Also, they can grow inside all year round and
within a controlled, disease-free environment.
The disadvantages: are that using cloned plants you face
reducing the gene pool and therefore increasing the
vulnerability to diseases.
Cloning animals
While cloning does occur naturally within animals, it is less
common. Cloning is usually restricted to cells dividing by
mitosis, and cells splitting as is the case of identical twins.
However, a new technique of embryo transplantation has
been used, especially in farming and zoos.
The eggs and sperm are checked to ensure that they are
free from genetic defects. Also, it is possible to separate
the sperm into those that contain a Y chromosome from
the slightly heavier ones containing X chromosomes.
By doing this, the final sex of the offspring can be chosen.
(Y-containing sperm will give a bull calf, an X-containing
one will give a female).
The advantages: your best cow and bull can artificially
produce hundreds of 'perfect' calves every year rather
than the usual single one. The original 'best cow' can
have her eggs implanted into other cows all year round.
The disadvantages: the usual reduced gene pool and
disease vulnerabilit
Genetic engineering
The most radical and potentially useful new technique is
genetic engineering.
This technique has already been used to produce large
quantities of human insulin using bacteria. It has been
a great help to sufferers of diabetes.
It has been a great help to sufferers of diabetes.This
process avoids the older method of extracting insulin
from dead bodies or pigs. That wasn't a very attractive
idea if you had to inject yourself with insulin every day!
In genetic engineering the gene that you want is cut out of
a human chromosome using special enzymes.
The gene is then fitted ('spliced') into a length of DNA from
a bacterial cell and then reintroduced back into the
bacterial cell.
The bacteria is tricked into carrying out the instructions on
the human gene and producing the protein, insulin.
Once the bacteria has been cultivated so that it multiplies
many times, enough insulin is produced so that it can be
filtered off and collected.
This whole process is carried out on an industrial scale so
that masses of insulin is produced in a continuous process.
There is a big debate at the moment over whether we
should introduce human genes into animal embryos.
This would allow us to make things like other hormones
and collect them from the animals milk.
This whole area is open to a big debate, covering scientific,
social and ethical issues.
It's a good idea for you to understand all this so that
you can join the debate!