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:
Mules are infertile
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.
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.
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.
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.
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.
SSSs
sSss
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.
We start off with the cells which will produce
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.
These F1 plants have both alleles so they are not true breeding.
When the plants of the F1 generation
are crossed the situation becomes rather more
complicated:
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.
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:
Then pair them up like this:
This gives a ratio of half black (Cc – C is
dominant) and half white (cc)
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?
Now pair up the gametes
A ratio of three black guinea pigs (CC, Cc, cC)
to one white pig (cc)
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. The reason is that – in a monohybrid cross all the F1 generation show the dominant phenotype.
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.
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 wheats
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
SS
ss
