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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.
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.
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.
One characteristic which can be used to illustrate inheritance is wing type in Fruit
Flies.
These can be NORMAL or VESTIGIAL.
Normal wings
Wing type is controlled by a pair of genes.
Each gene can be one of two types or ALLELES.
N = normal wingsn = 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.
NN X nnParent (P)
N nGametes
Genotype all Nn
Phenotype all Normal winged
All NnF1 generation
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.
Nn X NnF1 generation
N or n N or nGametes
We can work out the chances of each type of
offspring using a punnett square.
Nn
N nNN NnNn nn
NN NnNn nn
NN
n
nNorma
lNorma
l
Normal
Vestigial
Genotypes NN , Nn and nnPhenotypes Normal and VestigialPhenotype Ratio 3:1
F2 Generation
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 non-tongue rolling allele.
T = tongue rolling
t = 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
tttttt
Tt
Tt Tt
Tt TtTT
TtTtTtTt
?
Tt
First identify the Homozygous
recessive individuals.
These must be
tt
We can now work out the
genotype of the male parent
The tongue rollers in the F1 generation must
be Tt
We can now work out the
genotypes of the children in family X.
Family X
Before we work out the
genotypes of family Y we can work out family
Z
Family Y
Family Z
We cannot work out the the
genotypes of the rest for certain.
However, we can make an educated
guess of the genotype of one of
them.
The genotype of the last person is impossible to
work out.