Monohybrid Inheritance

1
Monohybrid Inheritance

• Section 11

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Monohybrid Inheritance

• Gregor Mendel (1822-1884)
• - Used varieties of pea plant
• - Ensured large numbers were used
• - Studied only 1 characteristic
• - Observed 31 phenotypic ratio
• Principle of segregation the alleles of a gene
  exist in pairs but when gametes are formed, the
  numbers of each pair pass into different gametes.
  Thus each gamete contains only one allele of each
  gene.

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• Homozygous (TT) or (tt) 2 of the same allele
• Heterozygous (Tt) 2 different alleles

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Law of probability

• Most monohybrid crosses give roughly 31
  phenotypic ratio
• 1 in 4 chance of a recessive allele being
  expressed
• 200 animals roughly 150 have 1 phenotype, 50
  will have another
• Since fertilisation is random rarely 31 exactly

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Monohybrid Inheritance in humans

• Family trees used to predict genetic trends
• E.g. tongue rolling
• Rhesus D antigen
• D-antigen found on some blood cells
• Present Rhesus positive blood (Rh)
• Absent Rhesus negative (Rh-)
• Rh- people produce anti-D antibodies
  sensitised
• Antigen-D present (DD/Dd)
• Antigen-D absent (dd)

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Monohybrid Inheritance in humans

• Albinism
• Inability to make melanin pigment for skin,
  eyes, hair etc.
• Is a recessive trait
• AA or Aa normal
• aa albino
• Other examples
• - Cystic Fibrosis (mucus secretion)
• - Phenylketonuria (PKU)
• - both are recessive traits

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Monohybrid Inheritance in humans

• Huntingdons Chorea
• Determined by a dominant allele
• Only expressed after average of 38 years old
• 50 chance of children of Huntingdons sufferer
  getting the disorder
• 1 in 20,000 cases

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Incomplete dominance sickle cell anaemia

• Mutation of haemoglobin gene haemoglobin S
  synthesised
• Homozygous (SS) Sickle cell anaemia
• - reduced oxygen carrying capacity
• - blood cells malformed (sickle-shaped)
• Heterozygous (HS) Sickle cell trait
• - both types of haemoglobin present
• - neither H or S are dominant (incomplete
  dominance)
• - sickle cell malaria resistance

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Incomplete / Co-Dominance

• Incomplete both alleles expressed in the
  phenotype, but unevenly
• - sickle-cell more H than S expressed
• Co-Dominance - both alleles equally expressed in
  the phenotype
• - AB blood exactly 5050 split
• - MN blood - another blood type antigen
• - no antibodies produced
• - equal expression (equal
  dominance

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Multiple Alleles

• 3 or more alleles for a characteristic
• 3 alleles 6 possible genotypes
• No. of phenotypes depend on the type of dominance
  
• complete, incomplete or co-dominance
• E.g ABO blood 6 genotypes, 4 phenotypes