Learning Intentions

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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.

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True Breeding, Dominant and Recessive
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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.
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(No Transcript)
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Crossing True-breeding White Mice
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Crossing True-breeding Black Mice
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Generation Example 1 Colours of Mice (phenotypes) Example 2 Colours of Mice (phenotypes)
P
F1
F2
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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.
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When the same characteristic is passed repeatedly
from generation to generation then we say the
organism is.
True Breeding
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An animal is not always crossed with another
animal of the same phenotype. Look at the next
slide.
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Crossing two different true breeding animals
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Generation Phenotype(s) Colour(s)
P
F1
F2
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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
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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.
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The Monohybrid Cross
This involves looking at a single characteristic
to see how it is passed from generation to
generation.
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A lot of important work in genetics has been done
by studying inheritance in fruit flies.
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One characteristic which can be used to
illustrate inheritance is wing type in Fruit
Flies.
These can be NORMAL or VESTIGIAL.
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Fruit fly with normal wings
Fruit fly with vestigial wings
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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.
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There are three possible genotypes but only two
possible phenotypes.
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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.
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Always circle the allele found in the gamete
Genotype all Nn
Phenotype all Normal winged
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It is more difficult to work out the results of a
cross between members of the F1 generation but it
can be done.
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We can work out the chances of each type of
offspring using a punnett square.
NN
Nn
Nn
nn
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F2 Generation
Normal
Normal
Normal
Vestigial
Genotypes NN , Nn and nn
Phenotypes Normal and Vestigial
Phenotype Ratio 31
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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.

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• 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?

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1. 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?

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• 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?

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• 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

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• 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

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Family Tree Diagrams
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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
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The pattern of inheritance can be traced using a
family tree diagram, where
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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.
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tt
Tt
Tt
tt
tt
tt
Tt
Tt
Family X
Family Y
tt
tt
TT
?
Tt
Tt
Family Z