Fundamentals of Genetics

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Fundamentals of Genetics
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Patterns of Inheritance

• The History of Genetics
• Genetics - scientific study of heredity

• Trait - characteristic that can be passed from
  parents to offspring

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Blending of Traits

• Once accepted hypothesis
• did not explain appearance of unexpected traits
  in offspring

Oopsblond baby boy. Where did that come from?
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Gregor Mendel - 1860s

• self-fertilization - cross of the same plant
  offspring receives genetic information from one
  parent only

• cross-fertilization - cross of 2 different
  plants offspring receives different forms of
  genetic trait from each parent

• Studied pea plants

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• Purebred - organism receives the same genetic
  traits from both of its parents
• Hybrid - organism receives different forms of a
  genetic trait

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Mendels Observations

• Crossed two different purebreds (P generation
  parent generation)
• resulting offspring is F1 (1st filial) -offspring

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• crossed these, resulting in F2 (2nd filial)
  generation
• Found that there were always two types of traits
  - Dominant and Recessive

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Mendels Conclusions

• Disproved blending hypothesis
• offspring receives one of two factors from each
  parent - factors are genes
• genes - sections of a chromosome that code for a
  trait

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• Allele - distinct form of a gene

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• dominant allele - expressed when two different
  alleles are present represented with a capital
  letter A
• recessive allele - form of a gene that is not
  expressed when paired with a dominant allele
  represented with a lower case letter a

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• Mendel is the Father of Modern Genetics

Gregor Mendel
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Cellular Basis of Inheritance

• Chromosome Theory of Heredity states that the
  material of inheritance is carried by the genes
  in the chromosomes

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Genes represent traits

• Genotype - genes that make up an organism
• includes both genes in a homologous pair
• Phenotype - outward expression of the trait

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• homozygous - two alleles are identical (AA or aa)
• also known as purebred organism
• heterozygous - two alleles are different (Aa)
• also known as a hybrid organism

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Mendels Laws

• Law of Segregation - gene pairs separate when
  gametes form
• one gamete carries one gene and the other carries
  the other gene

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• Law of Independent Assortment - gene pairs
  segregate into gametes randomly and independently
  of each other

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• Law of Dominance - dominant allele is expressed,
  recessive allele will be hidden unless in
  homozygous form

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Genetics and Prediction
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Predictions for One Trait

• Probability - predict likelihood of an event or
  outcome

tails
heads

• Heads is one possible outcome out of a total of
  2 possible outcomes.

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• Punnett square - grid for organizing genetic
  information
• can be used to make predictions about a cross
  between two organisms
• monohybrid cross - cross between two parents
  involving one trait

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AA x aa (A normal, a albino)
A
A
A
a
A
a
a
A
a
a
A
a
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• Genotypic ratio AA 0/4 or 0
  Aa 4/4 or 100
  aa 0/4 or 0
• Phenotypic ratio Normal 4/4 or 100
• Albino 0/4 or 0

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Predictions for Two Traits

• Two trait cross cross between two parents and
  two traits
• Dihybrid cross - cross between two heterozygous
  parents
• use a 4 X 4 punnett square
• sixteen possible outcomes

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HhTt x hhtt (H long hair, h short hair T
tail, t no tail)
FOIL!
Ht
hT
ht
HT
ht
HhTt
Hhtt
hhTt
hhtt
Hhtt
hhTt
hhtt
ht
HhTt
ht
HhTt
Hhtt
hhTt
hhtt
ht
HhTt
Hhtt
hhTt
hhtt
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• Genotypic ratio
• HHTT 0/16 or 0
• HHTt 0/16 or 0
• HHtt 0/16 or 0
• HhTT 0/16 or 0
• HhTt 4/16 or ¼ or 25
• Hhtt 4/16 or ¼ or 25
• hhTT 0/16 or 0
• hhTt 4/16 or ¼ or 25
• hhtt 4/16 or ¼ or 25

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• Phenotypic ratio
• 25 Long hair, tail
• 25 Long hair, no tail
• 25 short hair, tail
• 25 short hair, no tail

JK!
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Test Cross
G? GG or Gg

• Test cross - breed an organism whose genotype is
  questionable with a homozygous recessive organism

• results determine genotype of questionable
  organism

then its Gg
If all offspring
gg
X
G?
then its probably GG
If any offspring
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Incomplete Dominance

• color in snapdragons, hypercholestolemia, instead
  of white or red color is pink

• heterozygous offspring show a phenotype that is
  in-between the phenotypes of the two homozygous
  parents
• blending of traits

X
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r
w
r
rr
rw
w
rw
ww
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Codominance

• HA normal
• HS sickle
• both are dominant and equally expressed both
  normal red blood cells and sickle-shaped blood
  cells present in heterozygote

• Both alleles are expressed equally
• Ex. Sickle Cell Disease

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Polygenic Trait (Epistasis)

• Trait controlled by more than one gene
• ex. Eye color

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Multiple Alleles

• Three or more alleles for a trait in a human
  population
• ex. Blood types IA, IB, i (type O)
• Karl Landsteiner discovered antigens which are
  responsible for different blood types

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Multiple Alleles - Blood Types

• IA - contains antigen A
• IB - contains antigen B

• IAIB - contains antigens for both A B
• i - contains neither A nor B

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Blood Groups
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• Universal donor O
• Universal recipient AB
• Dominant Trait in Blood Groups

• Rh antigen named after Rhesus Monkey
• people who have the Rh antigen Rh (positive)
• people who do not have it Rh- (negative)
• Rh-, Rh person positive

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Pleiotropy

• Single gene affects more than one trait
• ex. Sickle cell anemia - effects include blood
  cell shape, anemia, weakness, brain damage,
  spleen damage, heart damage

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Environmental Effects

• Phenotype is a combination of genetic and
  environmental influences, epigenetics

• Ex. Identical twins share the same DNA but may
  look and act differently

ex. Himalayan rabbit - fur color depends on body
temperature dark where cooler, white where
warmer
Yes, they are identical!
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Issues in Biology

• Up to a 3 inch height difference observed in
  studies where twins were raised in two different
  countries/environments

• Nature vs. Nurture Issue
• studies have been done on identical twins that
  have been separated at birth

Twins raised in different adoptive homes showed
similar interests and careers when they
reconnected as adults
VS