Download Inheritance

Observing Patterns in
Inherited Traits
Chapter 8
Terms and Concepts
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Gene
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Heritable unit of information about traits
One gene generally codes for one protein
In a diploid cell there are pairs of genes
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One of the pair on each of the homologous
chromosomes
Locus

Location of a gene on the chromosome
Terms and Concepts
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Allele
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Different molecular forms or traits of the
same gene
Arise by mutation
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A permanent change in a gene and in the
information it carries
Diploid cells have two alleles for each gene
Terms and Concepts
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Types of alleles
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Dominant
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Its trait is always expressed
Masks the effect of a recessive allele
Represented with a capital letter in inheritance problems (A)
Recessive
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Expressed only when paired with another identical recessive
allele
Its trait is masked by dominant alleles
Represented with a lower case letter in inheritance problems (a)
Terms and Concepts
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Combinations of alleles
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Homozygous condition
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Homologs carry the same allele
A pair of identical alleles belong to a true-breeding lineage
Individuals can be either homozygous recessive (aa) or
homozygous dominant (AA)
Heterozygous condition

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Homologs carry different alleles
Individuals are referred to as heterozygous (Aa) or hybrids
(result of a cross between two different true-breeding
individuals)
Terms and Concepts

Gene expression

Process by which a gene’s information is
converted to a structural or functional part of
a cell
Transcription of DNA to mRNA
 Translation of RNA to protein

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Determines traits
Terms and Concepts

Genotype

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Particular alleles that an individual carries
Examples: AA, Aa, aa
Phenotype
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Refers to an individual’s traits
Examples: color, shape, size, texture, etc.
Terms and Concepts
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Genetic crosses
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Two individuals are crossed and the resulting
offspring are examined to determine
inheritance patterns
P stands for the parents
F1 stands for the first-generation offspring of
crossed P individuals
F2 stands for the second-generation offspring
of intercrossed F1 individuals
Questions
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Gene
Locus
Dominant allele
True-breeding
Hybrids
Genotype
Phenotype
F2 generation
An individual’s traits
Heterozygous
Second generation offspring
Homozygous
An individual’s genes
A trait that is always expressed
Heritable unit of information
Location of a gene
Genetic Crosses

The following slides will present a genetic
cross demonstrating


the use of the above terminology
and the use of punnett squares
Genetic Crosses
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Whether a person has attached or
detached earlobes depends on a single
gene with two alleles (We can name the gene
with a letter “e”)
 Dominant allele is detached ear lobes
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Referred to as E (capital for dominant)
Recessive allele is attached ear lobes
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Referred to as e (lower case for recessive)
Genetic Crosses
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Each individual inherits one allele from each parent
Depending on what combination of alleles are inherited
will determine the genotype and phenotype of the
individual
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Inherit two dominant alleles
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Inherit two recessive alleles
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Genotype = EE or homozygous dominant
Phenotype = detached earlobes
Genotype = ee or homozygous recessive
Phenotype = attached earlobes
Inherit one dominant allele and one recessive allele
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Genotoype = Ee or heterozygous
Phenotype = detached earlobes
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The dominant allele will always mask the recessive allele’s trait
Genetic Crosses
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Punnett squares can be used to determine the
probability of the genotypes and phenotypes of
offspring of any given cross such as the
following
If we crossed a homozygous dominant dad with
a homozygous recessive mom, what would the
offspring genotype(s) and phenotype(s) be?
Genetic Crosses
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First you need to determine the genotype
of the parents
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Dad = homozygous dominant = EE
Mom = homozygous recessive = ee
Genetic Crosses
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Second, determine what each parent’s gametes
will be
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Based on what we know about meiosis we can
determine what allele the gametes will carry (see
figure 10.5)
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Remember that during meiosis homologous pairs are
separated (anaphase I). One of the two alleles is on one of
the homologs, the other is on the other homolog. Therefore,
during meiosis one “E” will segregate into one gamete, while
the other “E” will segregate into the other gamete
Dad’s gametes will be E and E
Mom’s gametes will be e and e
Genetic Crosses
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Third, place the gametes in a punnett
square
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Dad’s go vertically in the first column
Mom’s go horizontally across the top
e
E
E
e
Genetic Crosses
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Fourth, determine what the possible
outcomes are if either of dad’s gametes
fuses with either of mom’s eggs
E
E
e
Ee
Ee
e
Ee
Ee
Genetic Crosses
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Fifth, determine the probability of the
genotypes and phenotypes
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Genotype possibilities are
EE, Ee, or ee
 Count up how many out of four of each
combination are in the punnett square
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Answer
EE : Ee
: ee
0 : 4 : 0
Genetic Crosses
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Fifth, determine the probability of the
genotypes and phenotypes
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Phenotype possibilities are
Detached or Attached
 Count up how many out of the four of each trait
are in the punnett square
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Answer
Detached
: Attached
4 : 0
Genetic Crosses
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Punnett Square Practice
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Cross a heterozygous dad with a homozygous
dominant mom
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Cross a heterozygous dad with a
heterozygous mom
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Ee X EE
Ee X Ee
Cross a homozygous recessive dad with a
heterozygous mom
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ee x Ee
Gregor Mendel
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Using pea plants Gregor Mendel determined
inheritance patterns
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Pea plants are self-fertilizing and so develop
“true-breeding” varieties (homozygous)
Mendel could open a floral bud of a truebreeding plant and snip out its anthers
(contains pollen grains). The buds can then
be brushed with pollen from a different truebreeding plant.
Following observable differences between
plants Mendel predicted that he would be
able to follow certain traits and see if there
were patterns in its inheritance.
Fig. 10-3, p.154
Gregor Mendel
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Theory of Segregation
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Diploid cells have pairs of genes, on pairs of
homologous chromosomes
The two genes of each pair are separated
from each other during meiosis, so they end
up in different gametes
Mendel used monohybrid crosses to
demonstrate segregation
Gregor Mendel: Monohybrid Cross
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Pea flower color
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Cross 1
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True-breeding purple flowering plants were crossed with
true-breeding white flowering plants (these are the parental
generation, P)
The offspring or F1 generation were all purple flowering
Cross 2
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The F1 generation were allowed to self-fertilize
The offspring or F2 generation had a ratio of 3 purple
flowering plants to 1 white flowering plant
Gregor Mendel: Monohybrid Cross
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Pea flower color
 Mendel was able to infer that
Both parents must have two “units” of information
 Each parent transferred one of their “units” of
information to the offspring
 The purple color dominated the white color
 The recessive white color shows up in ¼ of the F2
generation

Homozygous
dominant
parent
Homozygous
recessive
parent
(chromosomes
duplicated
before meiosis)
meiosis
I
meiosis II
(gametes)
(gametes)
fertilization
produces
heterozygous
offspring
Fig. 10-5, p.156
Gregor Mendel: Monohybrid Cross
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Pea flower color
 Purple is dominant = A
 White is recessive = a
 Genotypes
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True-breeding purple genotype = AA
True-breeding white genotype = aa
Punnett square for cross 1
A
A
a
Aa
Aa
a
Aa
Aa
Fig. 10-7b, p.157
Gregor Mendel: Monohybrid Cross
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Pea flower color
 F1 are allowed to self-fertilize
 Punnett square for cross 2
A
a
A
AA
Aa
a
Aa
aa
Fig. 10-7c, p.157
Fig. 10-6, p.156
Gregor Mendel
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Test cross
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A method of determining genotype
To determine the genotype of the F1 purple-flowering
plants (could be AA or Aa) Mendel could cross them with
true-breeding white-flowered plants (aa)
If the F1 is AA, then all of the flowers would be purple
If the F1 is Aa, then half of the flowers would be
purple and half white
Try the crosses on a punnett square
Gregor Mendel

Theory of Independent Assortment
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As meiosis ends, genes on pairs of
homologous chromosomes have been sorted
out for distribution into one gamete or
another, independently of gene pairs on other
chromosomes
This is due to random alignment during
meiosis
Gregor Mendel
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Theory of Independent Assortment
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Mendel used dihybrid crosses to explain how
two pairs of genes are sorted into gametes
independently
The following slides will demonstrate the type
of dihybrid crosses used
Gregor Mendel: Dihybrid Cross
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Pea flower color AND plant height
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Cross 1
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True-breeding purple flowering tall plants were crossed with
true-breeding white flowering dwarf plants (these are the
parental generation, P)
The offspring of F1 generation were all purple flowering tall
Mendel’s question was whether purple flowering would
always be linked to tall or whether purple could go with
dwarf and white with tall. Looking at the F2 generation from
cross 2 answered his question.
One of two possible alignments The only other possible alignment
a Chromosome
alignments at
metaphase I:
b The resulting
alignments at
metaphase II:
B
c Possible
combinations
of alleles in
gametes:
A
a
a
A
Aa
a
B
Bb
b
b
bB
B
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
A
A
a
a
A
A
a
a
AB
B b
ab
b
b
Ab
b B
B
aB
Fig. 10-8, p.158
Gregor Mendel: Dihybrid Cross
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Pea flower color AND plant height
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Cross 2
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The F1 generation were allowed to self-fertilize
The offspring or F2 generation had a ratio of
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9
3
3
1
purple flowering tall plants
purple flowering dwarf plants
white flowering tall plants
white flowering dwarf plant
Gregor Mendel: Dihybrid Cross
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Pea flower color AND plant height
 Mendel was able to infer that
Purple was not linked to tall and white was not
linked to dwarf
 The two different genes did in fact sort
independently
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Gregor Mendel: Dihybrid Cross
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Pea flower color AND plant height
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Purple = A
Tall = B
Genotypes
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and
and
white = a
dwarf = b
True-breeding purple tall genotype = AABB
True-breeding white dwarf genotype = aabb
Punnett square for cross 1
AB
AB
ab
AaBb
AaBb
ab
AaBb
AaBb
Gregor Mendel: Dihybrid Cross
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Pea flower color AND height
 F1 are allowed to self-fertilize
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Possible gametes for AaBb are
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AB, Ab, aB, ab
Gregor Mendel: Dihybrid Cross
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Pea flower color AND plant height
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Punnett square for cross 2
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
Fig. 10-9, p.159
Questions
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T or F: The Theory of Segregation states that the two genes of
each pair stay together during meiosis
What type of cross was used to show Mendel’s Theory of
Segregation?
What is a Punnett Square?
What genotype and phenotype ratios are seen in the F2 generation?
T or F: The Theory of Independent Assortment states that gene
pairs sort independently
What type of cross was used to show Mendel’s Theory of
Independent Assortment?
What genotype and phenotype ratios are seen in the F2 generation?
Beyond Simple Dominance
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Mendel studied traits that have clear cut
dominant and recessive forms
Some genes can have alleles that are
codominant or incompletely dominant
Beyond Simple Dominance
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Codominance
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Non-identical alleles are both fully expressed even in
heterozygotes
Blood type
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IA and IB alleles are both dominant
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They are always expressed
i is recessive
Genotype
IAIA and IAi
IBIB and IBi
IAIB
ii
Phenotype
Type A blood
Type B blood
Type AB blood
Type O blood
(codominant, they are both expressed)
Fig. 10-10, p.160
Beyond Simple Dominance
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Incomplete dominance
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One allele isn’t fully dominant over the other
allele, so the heterozygote’s phenotype is
somewhere between the two homozygotes
Snapdragon flower color
Red flowers = RR
 White flowers = rr
 Heterozygotes , Rr are pink
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Fig. 10-11, p.160
Beyond Simple Dominance
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Epistasis
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Some traits are the results of interactions of two or
more gene pairs
Labrador coat color
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Gene encoding pigment
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Gene encoding deposition of pigment
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black is dominant to brown
Dominant allele promotes deposition of pigment
Recessive allele reduces deposition
The two genes work together to determine how much of
what color pigment ends up in the coat
EB
Eb
eB
eb
EB
EEBB
black
EEBb
black
EeBB
black
EeBb
black
Eb
EEBb
black
EEbb
chocolate
EeBb
black
Eebb
chocolate
eB
EeBB
black
EeBb
black
eeBB
yellow
eeBb
yellow
eb
EeBb
black
Eebb
chocolate
eeBb
yellow
eebb
yellow
Fig. 10-13, p.161
Fig. 10-12, p.161
Beyond Simple Dominance
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Pleiotropy
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One gene can influence two or more traits
Marfan syndrome
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A mutated form of the fibrillin gene affects the
formation of connective tissues, thus its affects are
seen in several areas of the body
Linkage Groups
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Some alleles tend to be inherited as a group
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Mendel’s theory of independent assortment only
works for genes located on different chromosomes
If genes are located on the same chromosome, then
they are generally linked
In some cases crossing over during meiosis will
separate linked genes depending primarily on how
close the two genes are on the chromosome
p.162b
Fig. 10-15, p.162
Genes and the Environment
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Some genes and can be influenced by the
environment
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Temperature affects coat color on Himalayan
rabbits

Cooler body parts are dark while the main body
mass is warmer and creating a lighter coat color
Fig. 10-16, p.163
Fig. 10-17, p.163
Complex Variations in Traits

Individuals of populations can show
continuous variation in a trait if there are
multiple genes and environmental factors
that influence a trait
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Height
Eye color
Skin color
p.164
Fig. 10-19a, p.164
Fig. 10-19b, p.164
Fig. 10-19c, p.164
Summary
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Terms and Concepts
Genetic Crosses
Mendel
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Segregation
Independent Assortment
Beyond simple dominance and other
variations of inheritance patterns