Genetics

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Genetics

• The study of how characteristics are transmitted
  from parents to offspring

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Diversity in Offspring

• The combination of genes from the parents creates
  the individual traits of each child
• Environment also plays a role, but differing
  alleles from parents are the primary reason that
  non-twin siblings are not identical

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Random Fertilization

• Gametes combine randomlywithout regard to the
  alleles they carry in a process known as random
  fertilization
• You are one out of 64 trillion genetically
  different children that your parents could produce

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Patterns of Inheritance
Eyebrow shape (separated vs joined)
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Patterns of Inheritance
Face shape (round vs square face)
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Gregor Mendel
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Mendel
Mendel crossed (bred) pea plants that differed
from one another in a single characteristic
Flower color
Purple
White
Flower position
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
He then determined whether there was some pattern
to what happened
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Dwarf
Stem length
Tall
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Terms Used in Genetics

• Genes
• Diploid Cells
• Alleles
• Homozygous
• Heterozygous

• Dominant
• Recessive
• Genotype
• Phenotype

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Terms Used in Genetics
Gene loci
Dominant allele
P
a
B
P
a
b
Recessive allele
PP
Bb
Genotype
aa
Homozygous for the dominant allele
Homozygous for the recessive allele
Heterozygous
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Genes

• Most genes are segments of DNA that carry
  information about how to make proteins
• Structural proteins for things like hair
• Functional proteins for things like breaking
  down lactose

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Genes

• Passed from parents to offspring
• Each has a specific location (locus) on a
  chromosome
• The genes are located on the chromosomes

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Genes Are on Chromosomes

• Humans have 23 pairs of chromosomes in all cells
  except sperm and egg cells.
• Each of the 23 pairs of chromosomes is a
  homologous pair that carry the same gene
• For each homologous pair, one came from mom and
  the other from dad

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Alleles

• Genes on a homologous pair are the same, but the
  exact information may not be the same
• Alleles arise by mutation
• Dominant allele masks a recessive allele that is
  paired with it

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Alleles

• Dominant alleles can mask a recessive allele
• Recessive alleles can be masked by a dominant
  allele

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Alleles

• Dominant alleles capital letter
• For example T for tall
• Recessive alleles lower case letter
• For example t for short

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Genetic Diseases in Humans

• Most alleles do not cause diseases in humans
• There are some diseases that are genetic
• Recessive, such as cystic fibrosis
• Dominant, such as Huntingtons Disease

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Allele Combinations

• Homozygous
• having two identical alleles at a locus
• AA or aa
• Heterozygous
• having two different alleles at a locus
• Aa

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Genotype Phenotype

• Genotype refers to particular genes an individual
  carries
• Phenotype refers to an individuals observable
  traits
• Cannot always determine genotype by observing
  phenotype

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Tracking Generations

• Parental generation P
• mates to produce
• First-generation offspring F1
• mate to produce
• Second-generation offspring F2

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Mendels Law of Segregation

• A pair of genes is segregated (separated) during
  the formation of gametes

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• There are 223 combinations for the way the
  homologous chromosomes could line up and separate
• This is more than 8 million combinations

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Genetic DiseasesCystic Fibrosis

• Affects 1 in 2500 individuals in European
  populations
• Recessive condition individuals have 2 copies of
  cystic fibrosis allele

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Genetic DiseasesCystic Fibrosis

• Produces nonfunctioning proteins
• Normal protein transports chloride ion in and out
  of cells in lungs and other organs
• Result thick mucus layer that is difficult to
  clear out of lungs and interferes with absorption
  of nutrients in intestines

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Cystic Fibrosis
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Punnett Squares

• Punnett squares are used to predict offspring
  phenotypes
• Uses possible gametes from parents to predict
  possible offspring

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Punnett Squares

• Punnet squares can be used to predict
  possibilities of inheriting genetic diseases

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Punnett Squares Single Gene

• A parent who is heterozygous for a trait
• Aa can produce two possible gametes
• A or a
• A parent who is homozygous for a trait
• AA can only produce gametes with A

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Punnett Squares

• The possible gametes are listed along the top and
  side of the square
• The predicted offspring genotypes are filled in
  the center boxes of the square

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Punnett Squares

• The offspring can be homozygous or heterozygous
• It all depends on the parents and the possible
  gametes

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Genetic DiseasesCystic Fibrosis

• Carriers have one cystic fibrosis allele but do
  not have cystic fibrosis
• Carriers can pass the allele to children

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Testcross

• Individual that shows dominant phenotype is
  crossed with individual with recessive phenotype
• Examining offspring allows you to determine the
  genotype of the dominant individual

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Mendels Law of Independent Assortment

• Factors for individual characteristics are
  distributed to gametes independently.
• (only true for genes located on separate
  chromosomes or that are far apart on the same
  chromosome)
• Members of each pair of homologous chromosomes
  are sorted into gametes at random during meiosis

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Independent Assortment

• Another independent assortment analogy
• A pair of shoes is comparable to a homologous
  pair of chromosomes
• Meiosis separates the members of one pair
  independently of other

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Independent Assortment
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Punnett Squares Multiple Genes

• You can also use Punnett squares to predict the
  offspring with multiple genes
• It is more significantly more difficult as the
  number of genes being studied increases

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Dihybrid Cross

• Experimental cross between individuals that are
  homozygous for different versions of two traits

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Impact of Mendels Work

• Mendel presented his results in 1865
• Paper received little notice
• Mendel discontinued his experiments in 1871
• Paper rediscovered in 1900 and finally
  appreciated

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Dominance Relations

• Complete dominance
• Incomplete dominance
• Heterozygote phenotype is somewhere between that
  of two homozyotes
• Codominance
• Non-identical alleles specify two phenotypes that
  are both expressed in heterozygotes

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Incomplete Dominance

• Straight, Wavy, Curly Hair
• Snapdragon Flower Color
• Hypercholesterolemia

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Straight, Wavy, and Curly Hair
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Snapdragon Flower Color
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Incomplete Dominance
P generation
P generation
White rr
Red RR
?
White rr
Red RR
?
R
r
Gametes
R
r
Gametes
F1 generation
F1 generation
Pink Rr
Pink Rr
1 2
1 2
R
r
Gametes
1 2
1 2
R
r
Gametes
Sperm
Sperm
1 2
1 2
R
r
1 2
1 2
R
r
Red RR
Pink rR
2
R
Red RR
Pink rR
1 2
R
F2 generation
Eggs
F2 generation
Eggs
1 2
Pink Rr
White rr
r
1 2
Pink Rr
White rr
r
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Flower Color in Snapdragons Incomplete Dominance

• Red flowers - two alleles allow them to make a
  red pigment
• White flowers - two mutant alleles cant make
  red pigment
• Pink flowers have one normal and one mutant
  allele make a smaller amount of red pigment

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Incomplete Dominance
LE 9-12b
Genotypes
HH Homozygous for ability to make LDL receptors
Hh Heterozygous
hh Homozygous for inability to make LDL receptors
Phenotypes
LDL
LDL receptor
Cell
Normal
Mild disease
Severe disease
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Hypercholesterolemia LDL receptor is mutated
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Genetics of ABO Blood Types Three Alleles

• Gene that controls ABO type codes for enzyme that
  dictates structure of a glycolipid on blood cells
• Two alleles (IA and IB) are codominant when
  paired
• Third allele (i) is recessive to others

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LE 9-13
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
Blood Group (Phenotype)
Antibodies Present in Blood
Genotypes
O
A
B
AB
Anti-A Anti-B
O
ii
IAIA or IAi
A
Anti-B
IBIB or IBi
B
Anti-A
AB
IAIB
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ABO Blood TypeAllele Combinations

• Type A - IAIA or IAi
• Type B - IBIB or IBi
• Type AB - IAIB
• Type O - ii

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ABO Blood Type Glycolipids on Red Cells

• Type A - Glycolipid A on cell surface
• Type B - Glycolipid B on cell surface
• Type AB - Both glyocolipids A B
• Type O - Neither glyocolipid A nor B

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ABO and Transfusions

• Recipients immune system will attack blood cells
  that have an unfamiliar glycolipid on surface
• Type O is universal donor because it has neither
  type A nor type B glycolipid

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Pleiotropy
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Polygenic Inheritance
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Polygenic Inheritance
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Environmental Influences
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Sex-Linked Traits and Disorders
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Y chromosome
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Hemophilia
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Dominant and Recessive Traits
Dimples (dominant)
Bent little finger (dominant)
Hitchhikers thumb (recessive)
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Dominant and Recessive Traits
Blaze (dominant)
Mid-digital hair (dominant)
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Recessive Traits
Dominant Traits
Freckles
No freckles
Widows peak
Straight hairline
Free earlobe
Attached earlobe
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Genetics of Coat Color in Labrador Retrievers

• Two genes involved
• - One gene influences melanin production
• Two alleles - B (black) is dominant over b
  (brown)
• - Other gene influences melanin deposition
• Two alleles - E promotes pigment deposition and
  is dominant over e

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Allele Combinations and Coat Color

• Black coat - Must have at least one dominant
  allele at both loci
• BBEE, BbEe, BBEe, or BbEE
• Brown coat - bbEE, bbEe
• Yellow coat - Bbee, BbEE, bbee

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Albinism

• Phenotype results when pathway for melanin
  production is completely blocked
• Genotype - Homozygous recessive at the gene locus
  that codes for tyrosinase, an enzyme in the
  melanin-synthesizing pathway

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Continuous Variation

• A more or less continuous range of small
  differences in a given trait among individuals
• The greater the number of genes and environmental
  factors that affect a trait, the more continuous
  the variation in versions of that trait

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Describing Continuous Variation
(line of bell-shaped curve indicates continuous
variation in population)
Number of individuals with some value of the trait
Number of individuals with some value of the trait
Range of values for the trait
Range of values for the trait
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Dominant Mutations
Huntingtin tangles in Huntingtons Disease
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Huntingtons Disease

• Dominant condition
• Fatal condition
• Only one Huntingtons allele needed
• Produces abnormal protein that clumps up in cell
  nuclei especially nerve cells in the brain

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Temperature Effects on Phenotype

• Himalayan rabbits are Homozygous for an allele
  that specifies a heat-sensitive version of an
  enzyme in melanin-producing pathway
• Melanin is produced in cooler areas of body

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Pea plant is self-pollinating
Petl
Stamen
Carpel
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Mendel cross-fertilized the pea plant
Removed stamens from purple flower
White
Stamens
Carpel
Transferred pollen from stamens of white flower
to carpel of purple flower
Parents (P)
Purple
Pollinated carpel matured into pod
Planted seeds from pod
Offspring (F1)
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Monohybrid Crosses

• Use F1 offspring of parents that breed true for
  different forms of a trait(AA x aa Aa)
• The experiment itself is a cross between two
  identical F1 heterozygotes, which are the
  monohybrids (Aa x Aa)

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Monohybrid Crosses
Homozygous
Homozygous
dominant parent
recessive parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis
II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
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F1 Results of One Monohybrid Cross
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F2 Results of Monohybrid Cross
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Mendels Theory of Segregation

• An individual inherits a unit of information
  (allele) about a trait from each parent
• During gamete formation, the alleles segregate
  from each other

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A Dihybrid Cross - F1 Results
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F1 Results of Mendels Dihybrid Crosses

• All plants displayed the dominant form of both
  traits
• We now know
• All plants inherited one allele for each trait
  from each parent
• All plants were heterozygous (AaBb)

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Explanation of Mendels Dihybrid Results

• If the two traits are coded for by genes on
  separate chromosomes, sixteen gamete combinations
  are possible

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Phenotypic Ratios in F2
AaBb X
AaBb

• Four Phenotypes
• Tall, purple-flowered (9/16)
• Tall, white-flowered (3/16)
• Dwarf, purple-flowered (3/16)
• Dwarf, white-flowered (1/16)