Variation
This sub-topic aims to introduce the idea of the physical
basis of inheritance. Throughout, only monohybrid crosses are
considered. In referring to fertilisation, the main aim is
to reinforce the notion of gametes as links between generations.
The determination of sex also illustrates the passage of genetic information
from one generation to the next, together with the need to obtain
material from both parents.
A species is a group of organisms that can breed together
and produce fertile offspring:
- All races of humans can interbreed
- The results are always fertile
- Humanity is a single species.
Mules are the result of crossing a horse with a donkey:
They are not a species
When members of a species have differences between
them we call it variation There are two kinds of variation:
- Discontinuous variation where a
characteristic exists in two or more clearly distinct forms or another
with no intermediate stages
- An example of this might be blood groups:
- Humans fall into one of four
blood groups.
- These are: A, B, AB or O.
- There are no intermediate stages.
- Another example is ear lobes.
- Humans have either joined or free earlobes; there
are no intermediate stages.
- Continuous variation where a characteristic
changes from one extreme to the other with no observable steps:
- Height in humans is an example of continuous variation.
- Human height varies from the smallest human to
the tallest with no gaps in the height.
- Weight and handspan are also an examples.
What is Inheritance
Within the nucleus of living cells the nucleus
contains the hereditary material – the chromosomes:
Hereditary information is the set of characteristics
that can be passed on from parents to children.
Genes control each characteristic – a
gene is a short length of the chromosome that controls a single
characteristic. |
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Many characteristics in animals can be passed on this
way:
- Eye colour
- Hair colour
- Freckles
- Colour blindness
- Diabetes
Many characteristics in plants can be passed on in this
way:
- Leaf, seed and pod colour
- Height
- Flower colour and shape
When a gene exists in two or more forms we call refer
to the different forms as alleles.
- We would say “The red colour allele of the
petal colour gene in tulips”
At his stage it is also necessary to know that sex cells
(sperm, eggs and pollen) are referred to as gametes.
- Unlike the rest of the cells in an animal, gametes
have a single set of chromosomes.
- All other cells have a double set.
We refer to the effect of the genes on the organism
as its phenotype
- The phenotypes of human eyes can be blue or brown
- The phenotypes of bluebell flower’s colour can be
blue, pink or white
We refer to the genes that cause the phenotype as the
genotype.
In genetics we usually start with true breeding
strains of organisms:
- A true breeding strain is a group of organisms which
possess a characteristic.
- When they are bred together the offspring will also
possess that characteristic.
- As will their offspring and so on.
Dachshunds come in
several colour types, amongst these are black & tan and
red.
- The three pups on the left are phenotypically red, the mother
on the right is phentypically black & tan.
- The father is from a pure breeding red strain:
- If the father had been crossed with another pure breeding
red dog all the pups would have been red.
- The black & tan mother is not pure breeding. She
is crossed with the male dog which is a pure breeding
red coated dog.
- All the pups are still red.
- Even if she was crossed with another non pure breeding black
& tan dog there would be no guarantee that there would
be black & tan pups.
- There is much skill involved in breeding dachshunds to ensure
that the crossing of the strains produce healthy and attractive
animals.
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With thanks to Caldach
for permission to use this picture. |
Genetics with Peas
We start off with a true breeding population of pea
plants which always produce round seeds and we cross them with a true
breeding population of pea plants that always produce wrinkled seeds.
- This starting generation we call the P generation
(P for parental)
- The resulting offspring we call the F1
generation (F1 for first filial generation; it is English
– look it up)
- All the offspring of the parent plants produce round
seeds. This suggests that the round seeded trait is dominant and
the wrinkled seeded trait is recessive (dominant is the trait which
is present in the F1 when true breeding stocks are crossed).
- To be sure of this we need to allow all the plants
of the F1 generation to breed together freely:
- The actual ratio of 2.96:1 is very close to the expected ratio
of 3:1
- This confirms that the round seeded trait is dominant and
the wrinkled seeded trait is recessive.
- To explain these results and to explain the terms
dominant and recessive we need to see what is happening to the gene.
- Therep are many genes on the chromosomes
- Around 30,000 genes on 24 chromosomes in humans
(22 + X + Y – see sex determination below)
- Each gene controls the production of a single protein
that in turn controls the cell and all it does.
- In some cases a single gene controls a single characteristic
such as round or wrinkled seeds in pea plants.
- When a single gene controls a single characteristic
in this way we call it monohybrid inheritance.
- Chromosomes exist in pairs, one from the male
parent and one from the female parent.
- For each monohybrid trait there are two genes
therefore.
- We refer to genes which exist in different
forms as alleles.
- For convenience we refer to these genes by
a letter of the alphabet
- When there is a choice of different alleles on of
them will be “stronger” than the other and
will be expressed (appear as the phenotype).
- We use a capital letter for the dominant allele.
- When there is a choice of different alleles on of
them will be “weaker” than the other and
will be not be expressed (not appear as the phenotype).
- We use a lower case letter for the recessive allele.
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- So in the case of the seeds of peas we might
choose “S”:
- S is the dominant allele for round
seeds
- s is the recessive allele for wrinkled
peas
- since genes are found in pairs there are four
possibilities within a pea plant
- and since the S allele is dominant over the
s allele:
- SS – will be a round seeded pea plant
- Ss – will be a round seeded pea plant
- sS – will be a round seeded pea plant
- ss – will be a wrinkled seeded pea plant
- We refer to this type of description of genes
as the genotype of the organism.
- Several different genotypes give the same phenotype
- The genotypes SS, sS and Ss all give the
phenotype: round seeds
- The genotype ss gives the phenotype: wrinkles
seed.
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SSSs
sSss
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Now we have to think about how
the alleles will pair up when the pollen and the egg cell meet.
- A parent cell divides into 4 gamete cells.
- When gametes are formed the normal double set
of chromosomes are reduced to a single set.
- In the diagram the the pirs of similar chromosomes
have been colour coded.
- One of each type ends up in the gamete cell.
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We start off with the
cells which will prosuce the gametes:
- In the parent (P) generation the round seeded
plant can only produce gametes carrying the S allele since
they are true breeding.
- In the parent (P) generation the wrinkled seeded
plant can only produce gametes carrying the s allele since
they are true breeding.
- When the pollen and egg meet at fertilisation
the result will be a round seeded plant Ss.
When the plants of the F1 generation
are crossed the situation becomes rather more complicated: |
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- The pollen can be either s or S in a ratio
of 50:50
- The eggs can be either s or S in a ratio of
50:50
- The pollen and eggs will meet up at random
So:
- S pollen can meet S eggs to make a SS plant
- S pollen can meet s eggs to make a Ss plant
- s pollen can meet S eggs to make a sS plant
- pollen can meet s eggs to make a ss plant
- and each type will be present in equal numbers,
one quarter of the total
But the SS, Ss and sS plants all have round seeds
(S is dominant)
- So we expect ¾ to be round pea plants and ¼
to be wrinkled pea plants
- That is a ratio of 3:1
- And that is what was obtained in the experiment
above.
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However – the ratio was not exactly 3:1, it was 2.96:1
Because the pollen and eggs meet completely at random
there is always an element of chance – so the ratio we get is rarely
exactly the ratio we expect.
The pea plant cross can now be redrawn as shown below:
It is possible to calculate the
expected ratio of any cross by using a square.
- Think about crossing a male white guinea pig
with a female black guinea pig.
- We will use the letter “C” to represent the
coat colour:
- C is the dominant black coat allele
- c is the recessive white coat allele
- What would the expected ratio be of a cross
with a white-coated guinea pig and a non true breeding black
guinea pig?
- The white-coated pig must be cc (can you see
why?)
- The other pig must be Cc
- Lay out the alleles like this:
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- Then pair them up like this:
This gives a ratio of half black (Cc – C is dominant)
and half white (cc) |
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You can
do this for any known set of alleles – for example what would
you expect the ratios to be of the offspring of two non true
breeding black guinea pigs?
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Now pair up the gametes
A ratio of three black guinea pigs (CC, Cc, cC)
to one white pig (cc) |
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Check you can do these vocabulary words:
True breeding
gene
genotype
dominant
P generation |
monohybrid cross
allele
phenotype
recessive
F1 generation |
gamete
F2 generation |
Sometimes genetic information is provided in the form
of a family tree e.g.
- Guinea pigs sometimes have a whorl of hair on their
backs
- The circles are the females – the squares are the
males
- The black shapes represent the normal haired guinea
pigs
- The white shapes represent the guinea pigs with whorls
- The P generation is true breeding
Try to answer the following questions:
- Which caracteristic is dominant and which is recessive*?
- Which pigs are definitely true breeding?
- Which pigs are definitely not true breeding?
- Which pigs can’t you be sure about?
- What ratio did you expect in the F2 generation?
- What ratio did you get in the F2 generation?
- Why don’t you get the expected ratio?
- What ratio of phenotypes would you expect if pigs
9 and 10 were crossed?
*When you look at these diagrams there is a real danger
you will think that normal hair is dominant and the whorled hair is
recessive because there are many more normal haired than
whorled – this is WRONG.
In the human population brown eyes are dominant over
blue eyes and yet there are many more blue eyed people in the UK.
Sex Determination
The sex of a human is decided by
the 23rd chromosome, the X and Y chromosomes.
- Males have an X and a Y chromosome
- Females have two X chromosomes
- Women can only produce gametes with the X chromosome
- Half of male gametes have the X chromosome
and half have the Y chromosome.
Draw out the square as we did above:
Half the offspring will be male, and half will
be female, exactly as we see in nature. |
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Genetics and Society
Humans have been breeding animals and plants for many
generations to make them better suited to our agricultural purposes.
The wild bull (auroch) was the ancestor of our modern
cattle. The aurochs lived in small herds in the forest, browsing
on vegetation, acorns and nuts; venturing into the open in the summer.
By the 1400s aurochs were hunted to extinction in most of Europe except
Poland. These small herds survived until 1627.
From the original woodland herds the best beasts suited
for dairy and beef were picked and bred. Because only the best milk
producers and best beef producers were allowed to breed slowly the
cattle improved until we have the animals familiar to us today and
the auroch is extinct.
Wheat has been cultivated for more than 9000 years.
The ancestral wild wheat had small seeds and shattered easily, that
is the seeds fell off easily and were scattered when it was harvested
By crossing the wheat with wild goat grass and selecting
the largest grains that did not shatter the modern wheat were bred.
We have done this with all our modern animals and crops.
This process is called selective breeding and it obviously
takes a long time.
Some of the things we breed to produce are:
- Increased yield
- Increased disease resistance
- Faster growth
- Increased growth.
Sometimes the genes of an organism can change suddenly
from the effects of radiation or chemicals. – This is called mutation.
- Usually mutation is harmful to the organism.
- Very occasionally a mutation can be beneficial
- Increasing the radiation to which the organism is
exposed can increase the rate of mutation.
- Increasing the mutation causing chemicals to which
the organism is exposed (such as tobacco smoke) can increase the
rate of mutation.
One such mutation in humans results in three chromosomes
in position 21.
- The result is a child with Down’s syndrome.
- Using a syringe a sample of the fluid in the mother’s
uterus can be taken
- This fluid (amniotic fluid) contains shed skin cells
of the baby
- By looking at the chromosomes doctors can tell the
mother if the unborn child has Down’s syndrome.
In plants a doubling of the number of all the
chromosomes can be of benefit to humanity.
- A doubling of the chromosome number has resulted
in larger fruits and seeds.
- Some examples are wheat, alfalfa, coffee, peanuts
and McIntosh apples
- This is called polyploidy
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