Punnet Squares

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Punnet Squares
Pedigree Charts
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• How do you tell the sex of a chromosome?

Pull down its genes!
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• Assume eyes can be blue or brown
• Assume that brown dominates blue
• B brown, b blue
• Assume each parent gives one gene relating to eye
  colour
• B or b
• If parent is BB (brown eyes)
• Can only give B
• If parent is bb (blue eyes)
• Can only give b
• If parent is Bb (brown eyes)
• Can give B or b

Phenotype
Genotype
homozygous
heterozygous
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b
B
B
b
Punnet Squares
b
B
B
b
b
Eye Colour
B
b
B
b
Which is the dominent gene?
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B
Punnet Squares
B
b
B
b
Eye Colour
B
b
b
B
b
B
b
Which is the dominant gene?
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• Phenotype
• The actual characteristic that the organism shows
• Eg. brown eyes

• Genotype
• The code given to represent the genes that an
  organism carries
• Eg. BB or Bb

heterozygous brown
homozygous brown
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Pedigree Charts
Copy
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• Bill (BB) and Margaret (bb) decide to reproduce,
  Bill can only give a B and Margaret can only give
  a b, therefore their children can only be Bb (B
  from Bill and b from Margaret) which means they
  will have brown eyes but carry a gene for blue
  eyes.
• John (Bb) and Sarah (bb) also reproduce, John can
  give a B or a b, and Sarah can only give a b,
  therefore the children could be Bb (brown eyes)
  or bb (blue eyes).

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QUESTIONS

• If L Long legs, and l Short legs, what is the
  ratio of offspring with Long and Short Legs if
  parents have
• LL and ll
• Ll and ll
• Ll and Ll

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• Explain the difference between phenotype and
  genotype

• Punnet squares can be used to estimate the
  characteristics of offspring based on the
  Genotypes of the parents
• Pedigree charts are family trees which include
  phenotypes /or genotypes

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Punnet Squares
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