Introduction to Genetics

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Transcript Introduction to Genetics

Observable Patterns
of Inheritance
Can you do this?
Terms to Know
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Probability
True-breeding
Hybrid
Segregation
Traits
Genes
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Homozygous
Heterozygous
Phenotype
Genotype
Dominant
Recessive
Genes
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Chemical factors that determine traits
(units of information)
Analogy: Genes are like a combination of
ingredients in a recipe. They code for a
specific food.
Passed from parents to offspring
Each has a specific location (locus) on a
chromosome
Alleles
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Different forms of a gene (back to
analogy…replacing jiffy p.b. with skippy
p.b.)
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Dominant allele (Uppercase letter)
overrules a recessive allele (lowercase
letter) that it is paired with
Allele Combinations
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Homozygous =purebred
having two identical alleles at a locus
 AA (dominant expressed) or
aa
(recessive expressed)
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Heterozygous =hybrid
having two different alleles at a locus
 Aa (dominant expressed)
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Genotype & Phenotype
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Genotype refers to particular genes an
individual carries
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Phenotype refers to an individual’s
observable traits
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Cannot always determine genotype by
observing phenotype
Tracking Generations
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Parental generation
mates to produce
P
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First-generation offspring
mate to produce
F1
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Second-generation offspring
F2
Earlobe Variation
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Whether a person is born with attached or
detached earlobes depends on a single
gene
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Gene has two molecular forms (alleles)
Earlobe Variation
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You inherited one allele for this gene from
each parent
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Dominant allele specifies detached
earlobes (E)
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Recessive allele specifies attached
earlobes (e)
Dominant & Recessive Alleles
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If you have attached earlobes, you
inherited two copies of the recessive allele
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If you have detached earlobes, you may
have either one or two copies of the
dominant allele
Early Ideas About Heredity
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People knew that sperm and eggs
transmitted information about traits
Blending theory
Problem:
Would expect variation to disappear
 Variation in traits persists
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Gregor Mendel
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Strong background in
plant breeding and
mathematics
Using pea plants, found
indirect but observable
evidence of how
parents transmit genes
to offspring
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Mendel was born in1822
Austrian monk
Studied at the Univ. of Vienna
Teacher (High School)
Figure 24–5 The Structure of a Flower
Section 24-1
Stamen
Anther
Filament
Ovule
Stigma
Style
Carpel
Ovary
Petal
Sepal
The Garden Pea Plant
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Self-pollinating
True breeding (different alleles not
normally introduced)
Can be experimentally crosspollinated
How did Mendel
fertilize the
plants?
F1 Results of One
Monohybrid Cross
F
M
Dominant
trait is
expressed
Recessive
appears
Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants
Seed Coat
Color
Pod
Shape
Pod
Color
Smooth
Green
Seed
Shape
Seed
Color
Round
Yellow
Gray
Wrinkled
Green
White
Constricted
Round
Yellow
Gray
Smooth
Flower
Position
Plant
Height
Axial
Tall
Yellow
Terminal
Short
Green
Axial
Tall
F1 Results of Mendel’s
Dihybrid Crosses
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All plants displayed the dominant form of
both traits
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We now know:
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All plants inherited one allele for each trait
from each parent
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All plants were heterozygous (AaBb)
Principle of Dominance
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Some alleles are dominant and others are
recessive.
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Mendel wanted to know if the recessive
alleles disappeared or are they still in the
f1,just hidden.
Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Principles of Dominance
P Generation
Tall
Short
F1 Generation
Tall
Tall
F2 Generation
Tall
Tall
Tall
Short
Mendel’s Theory
of Segregation
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An individual inherits a unit of information
(allele) about a trait from each parent
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During gamete formation, the alleles
segregate from each other
Independent Assortment
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Mendel concluded that the two “units” for
the first trait were to be assorted into
gametes independently of the two “units”
for the other trait
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Members of each pair of homologous
chromosomes are sorted into gametes at
random during meiosis
Independent Assortment
Metaphase I
A
A a
a
B
B b
b
OR
A
A a
a
b
b B
B
Metaphase II:
Gametes:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
B
A
B
A
1/4 AB
b
a
b
a
1/4 ab
b
A
b
A
1/4 Ab
B
a
B
a
1/4 aB
F2 Results of
Monohybrid Cross
Type of
alleles
The physical
characteristic
Impact of Mendel’s Work
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Mendel presented his results in 1865
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Paper received little notice
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Mendel discontinued his experiments in
1871
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Paper rediscovered in 1900 and finally
appreciated
Probability
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The likelihood that a particular event will
occur.
Flip a coin.
We use Punnett Squares
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D 38- Deduce the probable mode of
inheritance of traits (e.g.,
Punnett Squares of
Test Crosses
Homozygous
recessive
a
a
Homozygous
recessive
a
a
A
Aa
Aa
A
Aa
Aa
a
aa
aa
A
Aa
Aa
Two phenotypes
All dominant phenotype
Punnett Square of a
Monohybrid Cross
Female gametes
A
Male
gametes
a
A
AA
Aa
a
Aa
aa
Dominant
phenotype can
arise 3 ways,
recessive only
one
Test Cross
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Individual that shows dominant phenotype
is crossed with individual with recessive
phenotype
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Examining offspring allows you to
determine the genotype of the dominant
individual
Tt X Tt Cross
Tt X Tt Cross
Tt X Tt Cross
Genetics Practice Problem 1
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What occurs when a purple plant that is
heterozygous is fertilized by a white plant?
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Identify generations
Punnett Square
Genotypes %
Phenotype %
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Principle of Independent
Assortment
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The genes for different traits separate
independently of one another during the
formation of gametes.
Figure 11-10 Independent Assortment in Peas
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Yellow round
Green round
Yellow wrinkled
Green wrinkled
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9 : 3 : 3 : 1 Ratio
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9/16
3/16
3/16
1/16
Dihybrid Cross
Experimental cross between individuals
that are homozygous for different
versions of two traits
Straight Pinky
(Dominant)
Bent Pinky
(Recessive)
Straight Thumb
(Dominant)
Curved Thumb
(Recessive)
More Dominant Traits
Polydactylism
Achondroplastic
Dwarfism
Tay-Sachs Disease - One Wrong Letter
Dominance Relations
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Complete dominance
Incomplete dominance
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Heterozygote phenotype is somewhere
between that of two homozyotes
Codominance
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Non-identical alleles specify two phenotypes
that are both expressed in heterozygotes
Flower Color in
Snapdragons: Incomplete
Dominance
Red-flowered plant X White-flowered plant
(homozygote)
(homozygote)
Pink-flowered F1 plants
(heterozygotes)
Flower Color in Snapdragons:
Incomplete Dominance
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Red flowers - two alleles allow them to
make a red pigment
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White flowers - two mutant alleles; can’t
make red pigment
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Pink flowers have one normal and one
mutant allele; make a smaller amount of
red pigment
Figure 11-11 Incomplete Dominance in Four O’Clock Flowers
Figure 11-11 Incomplete Dominance in Four O’Clock Flowers
Flower Color in Snapdragons:
Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
(heterozygote)
(heterozygote)
White-, pink-, and red-flowered plants
in a 1:2:1 ratio
Incomplete Dominance
Neither allele
is dominant
over the other
Combination
of red and
white
flowers
Codominant
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Sickle Cell Disease
ABO Blood Types
Pleitropy
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Alleles at a single locus may have effects
on two or more traits
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Classic example is the effects of the
mutant allele at the beta-globin locus that
gives rise to sickle-cell anemia
Teachers Domain - A Mutation Story
Genetics of Sickle-Cell
Anemia
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Two alleles
1) HbA
Encodes normal beta hemoglobin chain
2) HbS
Mutant allele encodes defective chain
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HbS homozygotes produce only the
defective hemoglobin; suffer from sicklecell anemia
Pleiotrophic Effects of
HbS/HbS
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At low oxygen levels, cells with only HbS
hemoglobin “sickle” and stick together
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This impedes oxygen delivery and blood
flow
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Over time, it causes damage throughout
the body
Blood Typing
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Karl Landsteiner 1897
Worked at the Univ. of Vienna, Vienna
Austria (Sound familiar?)
Wanted to find out which red blood cells
would clot
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First found two different groups, A and B
Third group would not clot when exposed
to A or B What do you think this was?
What about the forth group?
Genetics of ABO Blood Types:
Three Alleles
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Gene that controls ABO type codes for
enzyme that dictates structure of a
glycolipid on blood cells
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Two alleles (IA and IB) are codominant
when paired
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Third allele (i) is recessive to others
ABO Blood Type:
Glycolipids on Red Cells
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Type A - Glycolipid A on cell surface
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Type B - Glycolipid B on cell surface
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Type AB - Both glyocolipids A & B
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Type O - Neither glyocolipid A nor B
ABO Blood Type:
Allele Combinations
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Type A - IAIA or IAi
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Type B - IBIB or IBi
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Type AB - IAIB
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Type O - ii
ABO and Transfusions
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Recipient’s immune system will attack
blood cells that have an unfamiliar
glycolipid on surface
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Type O is universal donor because it has
neither type A nor type B glycolipid
Codominance and Multiple
Alleles - AB or NOT AB
Codominance - both alleles are dominant
IA and IB
Multiple Alleles - genes have more than two
alleles
IA, IB, Ia
Figure 14-4 Blood Groups
Phenotype
(Blood Type
Genotype
Antigen on
Red Blood Cell
Safe Transfusions
To
From
Universal
Acceptor
Universal
Donor
Rh factor - Another Blood
Trait
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Pregnancy complications
Rh is a type of protein in the blood
If an Rh- man reproduces with an Rh +
woman complications can occur.
Polygenic Traits: Desiree’s
Baby Case Study
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More than one gene controls a trait
Skin color more than one gene,
incomplete dominance
A,B and C are dark
a,b and c are light
Sex Linked Traits - traits
that are carried on the either
the x or y chromosome
Figure 14-13 Colorblindness
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Figure 14-13 Colorblindness
Father
(normal vision)
Colorblind
Normal
vision
Male
Female
Daughter
(normal vision)
Son
(normal vision)
Daughter
(carrier)
Son
(colorblind)
Mother
(carrier)
Colorblindness
Sex-Linked Disorder
Cystic Fibrosis - Finding Cures is Hard
Male Pattern
Baldness (X
chromosome)
Hairy Pinna - long
hair on ears
Recessive Disorder
Figure 14-8 The Cause of Cystic Fibrosis
Chromosome
#7
CFTR
gene
The most common allele
that causes cystic fibrosis is
missing 3 DNA bases. As
a result, the amino acid
phenylalanine is missing
from the CFTR protein.
Normal CFTR is a chloride
ion channel in cell
membranes. Abnormal
CFTR cannot be transported
to the cell membrane.
The cells in the person’s airways
are unable to transport chloride
ions. As a result, the airways
become clogged with a thick
mucus.
Albinism
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Phenotype results when pathway for
melanin production is completely blocked
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Genotype - Homozygous recessive at the
gene locus that codes for tyrosinase, an
enzyme in the melanin-synthesizing
pathway
Human Genetics
Tracing Genes Through
Families - Human Pedigrees
Female
Partner
Male
Brothers and
Sisters
Figure 14-3 A Pedigree
A circle represents
a female.
A horizontal line connecting
a male and female
represents a marriage.
A half-shaded circle
or square indicates
that a person is a
carrier of the trait.
A completely
shaded circle or
square indicates
that a person
expresses the
trait.
A square represents
a male.
A vertical line and a
bracket connect the
parents to their children.
A circle or square
that is not shaded
indicates that a
person neither
expresses the trait
nor is a carrier of
the trait.
Ability to roll
the tongue in
the Senator
Family
Tongue Roller - dominant, Non-Tongue Roller - recessive
White = tongue roller, Purple = non-roller
What are the genotypes of everyone? R = roller, r = non roller
George, Sam, Ann, Michael, Daniel and Alan are Rr
Arlene, Tom, Wilma, and Carla are rr
Sandra, Tina and Christopher are either RR or Rr
Case Study - Hemophilia and
the Royal Family
1. First, let’s take a look at Queen Victoria’s son Leopold’s family. His daughter,
Alice of Athlone, had one hemophilic son (Rupert) and two other children—a boy
and a girl—whose status is unknown. a) What is the probability that her other son
was hemophilic? b) What is the probability that her daughter was a carrier?
Hemophilic? c) What is the probability that both children were normal?
2. Now for the Spanish connection: Victoria’s youngest child, Beatrice, gave birth to
one daughter, one normal son, and two hemophilic sons. Looking at the pedigree of the
royal family, identify which of Beatrice’s children received the hemophilic gene; why
can you make this conclusion? Notice that Beatrice’s daughter, Eugenie, married King
Alfonso XIII of Spain and had six children, one of whom was the father of Juan Carlos,
the current King of Spain. Would you predict that Juan Carlos was normal, a carrier, or
a hemophilic?
3. Alexis did not die from hemophilia. At the age of fourteen he was executed with the
rest of the family. His four oldest sisters were also young and didn’t have children, so
we don’t know whether any of them was a carrier. But we can make an estimate. a)
What are the probabilities that all four of the girls were carriers of the allele
hemophilia? b) Supposing Alexis had lived and married a normal woman, what are the
chances that his daughter would be a hemophiliac? c) What are the chances his
daughters would be carriers? d) What are the chances that his sons would be
hemophiliacs?
Nondisjunction
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Nondisjunction
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Nondisjunction
Homologous
chromosomes
fail to separate
Meiosis I:
Nondisjunction
Meiosis II
Epistasis
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Interaction between the products of gene
pairs
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Common among genes for hair color in
mammals
Genetics of Coat Color in
Labrador Retrievers
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Two genes involved
- One gene influences melanin production
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Two alleles - B (black) is dominant over b
(brown)
- Other gene influences melanin deposition
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Two alleles - E promotes pigment deposition
and is dominant over e
Allele Combinations
and Coat Color
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Black coat - Must have at least one
dominant allele at both loci
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BBEE, BbEe, BBEe, or BbEE
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Brown coat - bbEE, bbEe
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Yellow coat - Bbee, BbEE, bbee
Comb Shape in Poultry
Alleles at two loci (R and P) interact
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Walnut comb - RRPP, RRPp, RrPP, RrPp
Rose comb - RRpp, Rrpp
Pea comb - rrPP, rrPp
Single comb - rrpp
Campodactyly:
Unexpected Phenotypes
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Effect of allele varies:
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Bent fingers on both hands
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Bent fingers on one hand
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No effect
Many factors affect gene expression
Continuous Variation
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A more or less continuous range of small
differences in a given trait among
individuals
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The greater the number of genes and
environmental factors that affect a trait, the
more continuous the variation in versions
of that trait
Human Variation
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Some human traits occur as a few discrete
types
Attached or detached earlobes
 Many genetic disorders
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Other traits show continuous variation
Height
 Weight
 Eye color
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Temperature Effects
on Phenotype
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Himalayan rabbits are
Homozygous for an allele
that specifies a heatsensitive version of an
enzyme in melaninproducing pathway
Melanin is produced in
cooler areas of body
Environmental Effects on
Plant Phenotype
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Hydrangea macrophylla
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Action of gene responsible for floral
color is influenced by soil acidity
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Flower color ranges from pink to blue