Learning Intentions
• I understand genetic terminology and can identify vital
information for a monohybrid cross.
• I can identify if parents are true breeding or
homozygous by carrying out a back cross.
• I can carry out a monohybrid cross to the F2 generation
and state the genotype, and phenotypic ratio produced.
• I can state why the observed and predicted results of
inheritance are different.
Success Criteria
• I can use the layout effectively to show a step-by-step
account of inheritance.
• I can confidently determine homozygous and
homozygous alleles.
• I can highlight gametes by circling them.
The patterns of inheritance have been worked out using
breeding experiments.
Breeding experiments involve the mating together or
CROSSING of two organisms in order to study their
offspring.
The next three slides shows three generations of white
and three generations of black mice.
P
Represents the parents
F1
Represents the first generation
F2
Represents the second generation
Crossing True-breeding White Mice
P
X
parents
F1
first
generation
F2
second
generation
X
Crossing True-breeding Black Mice
P
X
parents
F1
first
generation
F2
second
generation
X
Generation
P
F1
F2
Example 1
Colours of Mice
(phenotypes)
Example 2
Colours of Mice
(phenotypes)
The offspring of the white parents were all white.
When members of this generation were crossed with
others from this generation then all of their offspring
would also be white.
The offspring of the black parents were all black.
When members of this generation were crossed with
others from this generation then all of their offspring
would also be black.
When the same characteristic is passed
repeatedly from generation to generation
then we say the organism is.
An animal is not always crossed with another
animal of the same phenotype.
Look at the next slide.
Crossing two different true breeding animals
P
X
parents
F1
first
generation
F2
second
generation
X
Generation
P
Phenotype(s)
Colour(s)
Black
And
White
F1
F2
All Black
Black
And
White
Perhaps you would expect that when you
cross a true-breeding white mouse with a
true-breeding black mouse you would get
something in between, eg. a GREY mouse.
However, all of the mice in the F1
generation are the same colour – black.
Black is said to be DOMINANT
White is said to be RECESSIVE
From this is clear that the black mice in
the F1 generation are not true-breeding.
Since the black mice in this generation
produce both Black and White offspring.
The Monohybrid Cross
This involves looking at a single
characteristic to see how it is passed
from generation to generation.
A lot of important work in genetics has been done by
studying inheritance in fruit flies.
One characteristic which can be used to
illustrate inheritance is wing type in Fruit Flies.
These can be NORMAL or VESTIGIAL.
Normal wings
Fruit fly with
normal wings
Fruit fly with
vestigial wings
Wing type is controlled by a pair of genes.
Each gene can be one of two types or ALLELES.
N = normal wings
n = vestigial wings
Normal wing is dominant over vestigial wing.
Vestigial wing is said to be recessive.
There are three possible genotypes but only two
possible phenotypes.
NN = normal wings
Nn = normal wings
nn = vestigial wings
If we cross true breeding normal winged flies
with true breeding vestigial winged flies it is
possible to work out the genotypes and
phenotypes of the offspring.
Parent (P)
NN X nn
N
Gametes
F1 generation
n
All
Nn
Genotype all Nn
Phenotype all Normal winged
Always
circle the
allele found
in the
gamete
It is more difficult to work out the results of a
cross between members of the F1 generation but
it can be done.
F1
generation
Gametes
Nn X Nn
N or n
We can work out
the chances of each
type of offspring
using a punnett
square.
N or n
N n
N NN Nn
n Nn nn
F2
Generation
N n
N NN
Nn
Normal Normal
n Normal
Nn nn
Vestigial
Genotypes
NN , Nn and nn
Phenotypes
Normal and Vestigial
Phenotype Ratio
3:1
Observed ‘v’
Predicted Results
• When a monohybrid cross is carried out
the actual results that you obtain are not
always exactly the results you predicted.
• This is because fertilisation is a random
process involving the element of chance
• Producing large numbers of offspring
gives more reliable results.
1. Brown eyes is dominant to blue eyes. A brown eyed
man (BB) marries a blue eyed woman (bb). What
colour of eyes would you expect their children to
have? Why?
2. A brown eyed man (Bb) marries a brown eyed woman
(Bb) and have 4 children. How many should have
brown eyes and how many blue eyes? Would all the
brown eyed children have the same genes?
3. Tongue rolling is dominant and non-rolling is recessive.
A tongue rolling woman (Tt) marries a non-rolling man
(tt). What fraction of their children should be able to roll
their tongues?
4. Two tongue rollers (Tt) get married. If they have 4
children how many should be able to roll their tongue
and how many should not be able to roll their tongue?
Write this as a ratio ? Rollers
:
? Non-rollers
5. In flies normal wings are dominant and short
wings are recessive.
A normal winged fly (NN) is mated with a short
winged fly.
What genes should their offspring inherit and what
should they look like?
If two of these offspring are mated what fraction
of their offspring would you expect to have
normal wings and what fraction short wings?
Write this as a ratio of ? Normal wings :
? Short wings
Tongue Rolling
Tongue rolling is an inherited characteristic which is
controlled by a single gene.
The tongue rolling allele is dominant over the nontongue rolling allele.
T
t
=
tongue rolling
=
non tongue rolling
The pattern of inheritance can be traced using a family
tree diagram, where……
Tongue rolling female
Non tongue rolling female
Tongue rolling male
Non tongue rolling male
It is possible to work out the genotypes of
individuals by knowing details a number of
generations of a family.
This is demonstrated in the following example.
Tt
Tt
Tt
tt
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