Monohybrid inheritance

1
Monohybrid inheritance

• Phenylketonuria in humans is an example of
  monohybrid inheritance.
• Results from a lack of an enzyme in the liver and
  causes a number of effects
• mental retardation
• abnormal muscle tone
• abnormal body movement
• Early diagnosis which advises a
  phenylalnanine-free diet for children reduces the
  PKU symptoms.

2
If one parent is homozygous normal, the other is
homozygous mutant

• The homozygous normal individual is represented
  as NN, the mutant as nn since in this case normal
  is dominant to mutant.
• PARENTAL GENERATION NN nn
• at meiosis this becomes
• the GAMETES N N n n
• at fertilization, fusion of
• gametes to form a zygote Nn Nn
• These offspring are genotypically heterozygous,
  but phenotypically normal, i.e. a carrier of the
  mutant allele for phenylketonuria.

3
If both parents are carriers (i.e. heterozygous)

• Parental (P) generation Nn x Nn
• Gametes N n N n
• 1st Filial (F) generation NN Nn Nn
  nn
• NN Phenotypically and genotypically normal
• Nn Phenotypically normal but a carrier
• nn Phenotypically and genotypically mutant
• Phenotypic ratio 3 NORMAL 1 MUTANT

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3 1 RATIO

• 31 ratio is only approximate unless the number
  of offspring is very large (unlikely in humans),
  because
• Alleles may not be distributed evenly between
  viable gametes in equal number
• Fusion of gametes is completely random - it is a
  matter of chance whether one male gametes fuses
  with a particular female gamete.

5
Other examples of monohybrid inheritance in humans

• Albinism autosomal recessive
• Cystic fibrosis autosomal recessive (the most
  common lethal allele in Caucasian populations)
• Huntingtons chorea autosomal dominant
• Tay-Sachs disease autosomal recessive
  (prevalent in Jews of Eastern European origin)