Genetics

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Genetics
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Review

• Chromosomes are paired (homologous) in a 2N
  organism.
• Each chromosome contains many genes
• Homologous chromosomes carry the same genes.

Homologous pair of chromosomes
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Review

• Genes can have different versions of the same
  trait. These versions are called alleles.
• Homologous chromosomes separate into different
  gametes.

alleles
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Alleles - Dominant and Recessive

• Each parent will donate one allele.
• Alleles can be physically apparent (dominant) or
  physically hidden (recessive).
• Mendel worked with traits that had only two
  alleles of each trait, one dominant and one
  recessive.

Alleles
Mother
Father
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Mendels Laws

• The Law of Segregation
• each pair of alleles separates during meiosis
  (gamete formation)
• The Law of Dominance
• When two alleles differ,
• one can control the trait - Dominant
• the other can be hidden - Recessive
• The Law of Independent Assortment
• Pairs of genes segregate into gametes randomly
  and independently (due to random separation of
  homologous pairs.

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How Can We Demonstrate these Laws?

• Mendel recorded phenotypes ( form of the trait
  that he saw)
• Based on outcomes from matings (called crosses)
  over several generations, he could determine the
  genotypes (the actual genetic makeup for that
  trait) of previous generations
• Homozygous - both alleles for a trait are the
  same
• Heterozygous - alleles are not identical.

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Monohybrid Crosses

• Studying the inheritance of a single trait while
  ignoring all other traits
• Start with 2 pure parents
• P Generation
• GG (only green)
• gg (only yellow)
• F1 generation
• All green heterozygous

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Determining Genotype (1 Trait)
PARENT
green
yellow
F1
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All green
Cannot see the yellow in this generation
Self- fertilized
F2
Self fertilization of this generation produces a
different pattern for homozygous vs. heterozygous
Yellow returns
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Using a Punnett Square instead

• Grid for Organizing Genetic Information
• Shows Probability that a combination will occur

g g
Parent 2
Gg
G G
Gg
1) Determine Parent alleles 2) Place alleles on
edge of square 3) Combine alleles inside the
boxes 4) Determine genotypes and phenotypes
Gg
Gg
Possible offspring genotypes for F1
Parent 1
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Heterozygous cross

• Phenotypes vs. Genotypes
• The recessive trait will show ONLY in the absence
  of the dominant (G) trait.

G g
GENOTYPE
G g
GG Gg or gG gg 1 2 1
GG
Gg
PHENOTYPE
Green Yellow 3 1
gg
gG
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Test Crosses

• Purpose -
• To distinguish between a homozygous dominant and
  a heterozygous dominant
• Cross the unknown with a known homozygous
  recessive.
• Create two Punnett squares.
• Cross a homozygous dominant with a homozygous
  recessive
• Cross a heterozygous dominant with a homozygous
  recessive
• How are the patterns different?

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Heterozygous Homozygous Recessive

• Phenotypes vs. Genotypes
• The recessive trait will show ONLY in the absence
  of the dominant (G) trait. What will happen?

g g
GENOTYPE
GENOTYPE
G g
GG Gg or gG g


PHENOTYPE
Green Yellow


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Predicting any single outcome

• Given Mendels Laws, the probability of an
  outcome for any one combination can be estimated.
• Can Use coins to estimate probability
• See Exercise on Probability.

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Probabilities and Proportions

• Should approximately match.
• Are only estimates of what SHOULD happen.
• Can be easily changed If randomness is
  compromised.

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Dihybrid crosses

• Allow the study of two traits at a time.

Make a P cross (RRYY x rryy) Cross F1 plants
among themselves (RrYy x RrYy) List all gametes
on a 4 x 4 Punnet Square
RY RY
ry ry




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Intermediate Inheritance

• Mendels study
• Dominant and recessive
• Many genes have intermediate inheritance
• Heterozygous phenotype not like either homozygous
  phenotype

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Types of Intermediate Inheritance

• Incomplete Dominance
• Heterozygote phenotype is inbetween the two
  homozygous phenotypes
• EXAMPLE Red White Pink Flowers
• BUT Gametes will contain either Red or White!
• No dominant phenotype so only capital letters
  with superscripts are used.
• Red FrFr White FwFw Pink FrFw

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Types of Intermediate Inheritance

• Codominance
• Both alleles express themselves fully
• EXAMPLE Blood types
• A, B, AB

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Polygenic Traits

• Trait is controlled by two or more gene pairs
• Eye color, skin, hair
• Tone amount, and distribution of pigments are all
  factors. Each is controlled by a different gene
  pair, for each attribute (color, skin, hair)

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Multiple alleles

• Three or more alleles in a trait
• Blood type
• IA IB and i
• Both IA and IB are dominant over I
• IA and IA are codominant.
• NOT THE SAME as polygenic
• A single pair of genes with gt 2 alleles.

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Environmental Stress on Genetics

• Seasonal Changes( external changes)
• Turning on beard growth (internal changes)
• Genes can be turned on and off.