An Introduction to Genetics

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An Introduction to Genetics

• Standard Grade Biology

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The Monohybrid Cross

• Mendel cross pollinated tall and dwarf pea plants
  - the parents or P generation.
• All the offspring of this cross were tall. He
  called these plants the first filial generation
  (F1).
• He self pollinated the F1 to get the F2. These
  plants were a mixture of tall and dwarf plants in
  the ration 31.
• Mendels work was careful and systematic,he
  studied other pea characteristics and his data
  was based on results from 21,000 pea plants

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A Summary of Mendels results

• P (Parents) TALL
  DWARF
• F1(First generation) All TALL(selfed)
• F2(Second generation) TALL DWARF
  in the ratio of 3
  1

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The explanation - 1

• Mendel described the characteristic tall as
  DOMINANT - found in the F1
• The short characteristic is RECESSIVE - not found
  in the F1
• These different forms are ALLELES and are
  different forms of the same GENE.
• Mendel predicted that every organism has two
  copies of every gene only on is carried by each
  GAMETE

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The explanation - 2

• Let symbols for the tall gene be T and the dwarf
  gene be t
• A plant can either be
• HOMOZYGOUS TT or tt, or
• HETEROZYGOUS Tt
• The gametes ( male or female sex cells) can
  either carry a T or t gene

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The explanation - 3

• Parents Tall Dwarf
  
• TT tt
• Gametes All T All t
• F1 All Tall (Tt)
  -selfed
• Gametes T or t T or t
• F2 TT or Tt or Tt or tt

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The Punnet Square
- This makes the combinations easier to see
Put the genes here
T t
Put the gene combinations here to show how the
gametes fuse
T TT Tt t Tt tt
Put the genes here
This shows the probability of the out come of the
cross
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The Punnet Square can be used to predict the
outcome of any cross

• Punnet Squares can be used to predict the outcome
  of any cross. For example the BACK CROSS - the
  homozygous recessive (tt) crossed with the
  Heterozygote (Tt).

tt crossed with Tt
T t t Tt tt T Tt
tt
This give a predicted ratio of 11. NOTE that
all ratios can only be a prediction and represent
a probability - the results are rarely exact.
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Genotypes and Phenotypes

• The genotype of any organism shows the alleles
  present -it is always expressed as symbols AA
  or Aa or aa,
• The phenotype of any organism is always words
  describing the appearance of that organism - tall
  or dwarf, blue eyed or brown eyed.

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The importance of Mendels work

• Mendels Laws apply to all diploid organisms
  including man.
• His work enables humans to predict the
  inheritance of a number of inherited diseases
  like cystic fibrosis.
• His careful and systematic work forms the basis
  of all heredity

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Now try some examplesGo to S4 resources and
find the Biology web site. You will find 13
multiple choice questions on different monohybrid
crosses.Good luck!