Monohybrid Inheritance

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Monohybrid Inheritance
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Monohybrid Cross

• 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.

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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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Brown X White
Parental Phenotype
Parental Genotype
BB X bb
Gametes
B B b b
B B
b Bb Bb
b Bb Bb
Fertilisation
All Bb
F1 genotype
F1 Phenotype
All Brown coated mice
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Now cross the F1 generation
Brown X Brown
F1 Phenotype
F1 Genotype
Bb X Bb
Gametes
B b B b
B b
B BB Bb
b Bb bb
Fertilisation
1BB 2Bb 1bb
F2 genotype
F2 Phenotype
3 Brown coated mice 1White
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Test / Back Cross

• 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.

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The Punnet Square
B Black b White
B B
b Bb Bb
b Bb Bb
All Black shows the unknown was homozygous
B b
b Bb bb
b Bb bb
2 Black 2 White shows that the unknown was
heterozygous.
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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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Red X White
Parental Phenotype
Parental Genotype
RR X rr
Gametes
R R r r
R R
r Rr Rr
r Rr Rr
Fertilisation
All Rr
F1 genotype
F1 Phenotype
All Pink Snapdragons
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Incomplete Dominance

• If the F1 generation is self pollinated.

R r
R RR Rr
r Rr rr
1 red 2 pink 1 white
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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.

1BB 2Bb 1bb 1 Black spots 2 black and
orange 1 Orange spots
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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 21.

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Lethal Genes

• The reason for this is that YY is lethal.

¼YY (died) ½ Yy (yellow) ¼yy (grey)
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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.
• A
• B
• AB
• O

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Multiple Alleles

• Blood type is controlled by 3 alleles
• A shown by IA
• B shown by IB
• O shown by i
• 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 cant receive just any blood)

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Multiple Alleles

• A and B are equally dominant, and i is recessive
  to both.

Phenotype Genotype Antigen on cell Antibody in Serum
Type A IAIA IAi A Anti-B
Type B IBIB IBi B Anti-A
Type AB IAIB A and B None
Type O ii None Anti-A and Anti-B
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Blood Type

• If a man with type A blood (IAi) has children
  with a woman with type B blood (IBi).

IA i
IB IAIB IBi
i IAi ii
¼AB ¼ A ¼ B ¼O
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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 XC Xc Xc Xc
Y XCY XcY
¼ XC Xc ¼ Xc Xc ¼ XcY
¼ XCY ¼ Normal girl ¼ colour blind ¼
colour blind ¼ normal carrier
girl boy boy
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Haemophilia

• This is where the blood does not clot properly.
• This is famous in Queen Victorias offspring.

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Muscular Dystrophy

• This is where muscle tissue breaks down in late
  childhood.

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Other Examples

• Red / white eye colour in fruit flies
• All tortoiseshell cats are female.