Mendel Genetics and Punnett Squares

1
Mendel Genetics and Punnett Squares

• Origin of Genetics
• Steps of a Monohybrid cross
• Terminology
• Practice
• Punnett Squares
• Determining Unknown Genotypes

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The Origin of Genetics

• Heredity- the passing of characteristics from
  parents to offspring
• Genetics- the branch of Biology that focuses on
  heredity
• Cross- combining traits from two parents by
  breeding (sexual reproduction)

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The Origin of Genetics

• Gregor Mendel
• Austrian monk
• Studied theology, science and mathematics
• Noticed patterns in the pea plants he was growing
• Mendel crossed pea plants and studied the genetic
  turn-outs
• Was the first to develop rules that accurately
  predict patterns of heredity
• Father of genetics

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The Origin of Genetics

• Monohybrid cross- a cross that involved one pair
  of contrasting traits
• Ex. Crossing a white flower with a purple
  flower.

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Steps of a Monohybrid cross

• Ensure each variety is true-breeding for a
  particular characteristic
• Ex. A true-breeding purple flower will produce
  only purple flowers
• These are the 1st two individuals crossed
• - P generation (parental generation)

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Steps of a Monohybrid Cross

• 2. Cross-pollinate two P-generation plants
• Ex. A true breeding purple flower crossed with a
  true-breeding white flower
• The offspring are called F1 generation (filial
  generation)

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Steps of a Monohybrid Cross

• 3. Self-pollinate the F1 generation to create the
  F2 generation (second filial generation)
• This creates a 31 ratio

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Terminology

• Alleles- alternative versions of genes
• The gene for hair color can have multiple alleles
  (blonde hair allele, brown hair allele, etc)
• Dominant- the expressed (visible) trait
• Recessive- the trait that is not expressed or
  observable

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Terminology

• Homozygous- when two alleles of a particular gene
  are the same
• (PP), (pp), (RR), (rr), (VV), (vv), etc
• Heterozygous- when two alleles of a particular
  gene are different
• Only the dominant allele is expressed while the
  recessive is present, but not expressed
• (Pp), (Rr), (Vv), etc

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Terminology

• Genotype- the set of alleles an individual has
  for a trait
• think genes and letters
• (Rr), (RR) or (rr)
• (Vv), (VV) or (vv)
• Phenotype- the physical appearance of a trait
• Think face and colors
• White, purple, red, blonde, freckles, blue eyes,
  etc

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Practice

• (assume that P is purple and p is white)
• (Pp)
• What is the genotype?
• What is the phenotype?
• Is it homozygous or heterozygous?

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Practice

• (assume that P is purple and p is white)
• (pp)
• What is the genotype?
• What is the phenotype?
• Is it homozygous or heterozygous?

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Practice

• (assume that P is purple and p is white)
• (PP)
• What is the genotype?
• What is the phenotype?
• Is it homozygous or heterozygous?

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Punnett Squares

• Punnett square- is a diagram that predicts the
  outcome of a genetic cross by considering all
  possible combinations

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Punnet Squares

• Ex. Having freckles (F) is dominant to not having
  freckles (f). Parent 1 is heterozygous for
  freckles and Parent 2 is also heterozygous

Parent 1
f
F
Phenotype ratio?
F
F
F
F
f
Parent 2
31
F
f
f
f
f
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Punnett Squares

• Box 1 is homozygous for.
• Box 2 and 3 is..
• Box 4 is.

1
2
3
4
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Punnett Squares

• Ex. A homozygous yellow pea plant (Y) is dominant
  over a homozygous green pea plant (y).

Whats the genotype of all offspring?
Y
Y
Yy
y
Y
Y
y
y
Whats the phenotype of all offspring?
yellow
y
Y
Y
y
y
Whats the phenotypic ratio?
40
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Determining Unknown Genotypes

• When a phenotype is dominant but the genotype is
  not known, a test cross is performed.
• You cross the unknown genotype with a homozygous
  recessive individual

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Determining Unknown Genotypes

• What does that mean?
• You can physically see that the pea plant is
  yellow, but you dont know if they are (YY) or
  (Yy).
• So, you cross the yellow pea plant with a
  recessive pea plant (yy) that is green.
• If the offspring are all yellow the genotype is
  (YY)
• If the offspring are ½ yellow and ½ green the
  genotype is (Yy)

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Determining Unknown Genotypes

• Doesnt make sense? Use the Punnett Square and
  work backwards!

Known, recessive genotype
y
y
Y
Unknown genotype
???