Mendel Genetics and Punnett Squares

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Mendel Genetics and Punnett Squares

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

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Terminology

• 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)
• Trait - characteristic that can be passed down to
  a child from their parents carried on their
  chromosome (ex. Eye color, hair, height, etc)

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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

• Why study pea plants?
• They reproduce quickly
• Easy to see their traits
• Flowers, because of their structure, are easy to
  control reproduction (by self pollinating)
• Self pollination occurs in plants that have both
  male and female reproductive structures on the
  same flower/plant. The plant is able to
  fertilize itself

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The Origin of Genetics

• Mendel discovered two types of organisms
• Purebred - they receive the same traits from each
  parent (ex. Purebred yellow Labrador)
• Hybrid - receive different traits from each
  parent (ex. Labradoodle)

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Theory of Heredity

• Based on Mendels observations
• For each trait, an individual has two copies of a
  gene- one from mom and one from dad
• There are alternative versions of genes called
  alleles
• When 2 different alleles occur together 1 of them
  may be expressed (visible) while the other may
  have no effect on appearance
• Each gamete (sperm/egg) contributes one allele

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The Origin of Genetics

• Mendels law of segregation
• States that the members of each pair of alleles
  separate when gametes are formed
• Mendels law of independent assortment
• States that pairs of alleles separate
  independently of one another during meiosis
  (gamete formation)

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• Mendels 1st Law Law of Segregation
• when any individual produces gametes, the 2
  copies
• of a gene separate so that each gamete receives
  only
• one copy
• SS ? S ss ? s Ss ? S or s

Mendels 2nd Law Genes Assort Independently
alleles of different genes assort independently
of one another during gamete formation
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The Origin of Genetics

• Mendel used monohybrid crosses to study
  characteristics of pea plants
• Monohybrid cross- a cross that involved one pair
  of contrasting traits
• Ex. Crossing a round seed of a pea plant with a
  wrinkled seed of pea plant.

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P generation- the parental generation of
true-breeding individuals
F1 generation- first filial generation offspring
of the P generation resulting from crossing of
contrasting traits
F2 generation- second filial generation
offspring from the self pollination of the F1
generation
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Another example
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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
• Allele written as a capital letter (B, R, G, etc)
• It will typically mask or cover up recessive
  traits
• Recessive - the trait that is not expressed or
  observable
• Allele written as a lowercase letter (b, r, g,
  etc)

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Problem

• If B is brown and b is blonde
• What trait would be visible for
• BB?
• bb?
• Bb?

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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
• capital letter is always written first!

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Maternal chromosome that came from the egg of
this persons mother
Heterozygous - only the dominant allele is
expressed
Homozygous dominant - both chromosomes have
identical, dominant alleles
Homozygous recessive -both chromosomes have
identical, recessive alleles
Paternal chromosome that came from the sperm of
this persons father
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Terminology

• Genotype - the set of genes or 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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Punnett 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
F
F
F
F
f
Parent 2
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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• Once a punnett square is solved you can determine
  ratios of possible outcomes
• Look at the combinations INSIDE the punnett
  square only
• Genotypic ratio asks for the combination of
  alleles/letters
• Phenotypic ratio asks for the combination of
  physical traits
• Always written homozygous dominant, heterozygous,
  homozygous recessive
• Numbers in the ratio MUST add up to the number of
  boxes in a punnett square

Genotypic Ratio Phenotypic Ratio
121
31
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Practice

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

Whats the genotype of all offspring?
Yy
Y
Y
Whats the phenotype of all offspring?
yellow
y
Y
Y
y
y
Whats the genotypic ratio?
40
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
???