Transcript B - NCEA Level 2 Biology

Monohybrid
Inheritance
Monohybrid Cross
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Assign letters to the alleles and write these at the
top right hand corner of the page.
Usually a capital letter for the dominant trait and a
small letter for the recessive trait.
Write the phenotype of each parent.
Write the genotype of each parent.
Write the gametes.
Draw a punnet square.
Give the genotype and phenotypes of the offspring.
Monohybrid Cross
E.g. a homozygous brown (dominant) mouse
is crossed with a homozygous white mouse.
What are the likely phenotypes of the
offspring)
 B = brown
 b = white
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Parental Phenotype
Parental Genotype
Gametes
B
F1 Phenotype
X
BB
X
B
White
bb
b
B
B
b
Bb
Bb
b
Bb
Bb
Fertilisation
F1 genotype
Brown
All Bb
All Brown coated mice
b
Now cross the F1 generation
F1 Phenotype
F1 Genotype
Gametes
B
F2 Phenotype
X
Bb
X
b
Brown
Bb
B
B
b
B
BB
Bb
b
Bb
bb
Fertilisation
F2 genotype
Brown
b
1BB : 2Bb : 1bb
3 Brown coated mice: 1White
Test / Back Cross
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This can be used to determine the genotype of an
individual showing the dominant characteristic;
since these individuals may be homozygous
dominant or heterozygous.
Assume that in guinea pigs black coat colour is
dominant to white coat colour.
To work out the genotype of a black individual, we
do a test cross.
This is done by crossing the unknown individual
with a homozygous recessive individual.
The Punnet Square
B = Black
b
b
B
Bb
Bb
2 Black : 2 White
shows that the
unknown was
heterozygous.
B
Bb
Bb
All Black
shows the
unknown was
homozygous
b = White
b
B
Bb
b
bb
b
Bb
bb
Incomplete Dominance
In some cases one allele is not completely
dominant over the other, and the
heterozygote is a mixture of the 2 alleles.
 E.g. in snapdragon flowers, a red flowered
plant is crossed with a white flowered plant.
 This gives a pink heterozygote.
 R= red
Rr = Pink
r = white
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Parental Phenotype
Red
Parental Genotype
RR
Gametes
R
F1 Phenotype
White
X
R
rr
r
R
R
r
Rr
Rr
r
Rr
Rr
Fertilisation
F1 genotype
X
All Rr
All Pink Snapdragons
r
Incomplete Dominance
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If the F1 generation is self pollinated.
R
r
R
RR
Rr
r
Rr
rr
1 red: 2 pink: 1 white
Codominance
This is the same as incomplete dominance,
except that both traits show up in the
heterozygote.
 E.g. a black-spotted cat crossed with an
orange-spotted cat would give offspring
which were black and orange spotted.
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1BB
1 Black spots
:
2Bb
2 black and orange
:
1bb
1 Orange spots
Lethal Genes
There are certain genes which are essential
for life.
 Any allele that does not produce the required
gene will be fatal.
 If a missing allele is in the homozygous state,
it is lethal to the animal.
 E.g. in a certain form of mice, yellow fur, Y, is
dominant to grey, y. If 2 yellow mice are
crossed the result is always 2:1.
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Lethal Genes
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The reason for this is that YY is lethal.
¼YY (died) :
½ Yy (yellow) : ¼yy (grey)
Multiple Alleles
Many genes have more than 2 alleles that
can fit at a locus on a chromosome.
 When there are 3 or more alleles for a single
characteristic, this is called a Multiple allele.
 E.g. the ABO blood group system in humans.
 There are 4 phenotypes for blood type.
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A
B
 AB
O
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Multiple Alleles
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Blood type is controlled by 3 alleles:
A shown by IA
 B shown by IB
 O shown by i
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The letter I shows the allele, and the antigen
(a glycoprotein on the red blood cell) is
shown by the superscript.
 Humans make antibodies against antigens.
(this is why you can’t receive just any blood)
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Multiple Alleles
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A and B are equally dominant, and i is recessive
to both.
Phenotype
Genotype
Antigen on
cell
Antibody in
Serum
Type A
IA IA ; IA i
A
Anti-B
Type B
IBIB ; IBi
B
Anti-A
Type AB
IA IB
A and B
None
Type O
ii
None
Anti-A and
Anti-B
Blood Type
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If a man with type A blood (IAi) has children with a
woman with type B blood (IBi).
¼AB
:
IA
i
IB
IAIB
IBi
i
IAi
ii
¼A
:
¼B
:
¼O
Sex-Linked Genes
Genes located on one sex chromosome but
not the other are called Sex-linked genes.
 The traits that show this type of inheritance
are almost always on the X chromosome.
 The X chromosome is much larger than the Y
chromosome.
 Any gene carried on the X chromosome that
has no matching part on the Y chromosome
is sex-linked, as the Y chromosome is
genetically empty for that characteristic.
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Red-Green Colour Blindness
This is an inability to distinguish between the
colours red and green.
 First identified in a young boy who could not
pick ripe cherries and always came home
with a mixture of red and green ones.
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Red-Green Colour Blindness
XC
XC Xc
XCY
Xc
Y
¼ XC Xc
:
¼ Xc Xc :
¼ Normal girl
¼ colour blind
carrier
girl
Xc
Xc Xc
XcY
¼ XcY
:
¼ XCY
¼ colour blind
¼ normal
boy
boy
Haemophilia
This is where the blood does not clot
properly.
 This is famous in Queen Victoria’s offspring.
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Muscular Dystrophy
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This is where muscle tissue breaks down in
late childhood.
Other Examples
Red / white eye colour in fruit flies
 All tortoiseshell cats are female.
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