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12/14/2016
1
GENETICS ESSENTIALS
Concepts and ConnectionsTHIRD EDITION
Benjamin A. Pierce
CHAPTER 4
Extensions and Modifications of
Basic Principles
© 2014 W. H. Freeman and Company
Sex Is Determined by a Number of
Different Mechanisms
• There are several different mechanisms of sex
determination
• The X and Y chromosomes pair during meiosis,
even though they are not homologous (the
genes located on each are different)
Sex Determination
• Sexual reproduction: alternates between haploid
and diploid states
• Most organisms have two sexual phenotypes,
male and female
Chromosomal Sex-Determination Systems:
Sex Chromosomes and Non-Sex
Chromosomes (Autosomes)
• XX-XO system:
• XX – female
• XO – male
• grasshoppers
• XX-XY system:
• XX – female
• XY – male
• mammals
Chromosomal Sex-Determination
Systems
• ZZ-ZW system:
• ZZ – male
• ZW – female
• Birds, snakes, butterflies, some amphibians,
and fishes
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Genic Sex-Determining System
• No sex chromosomes, only the sex-determining
genes
• Found in some plants, fungi, protozoans, and
fish
Environmental Sex Determination
• Environmental factors
• Slipper shell: position in the stack
• Crepidula fornicata
• Temperature in reptiles
• Turtles: Warmer = more females
• Crocodiles: Warmer = more males
Sex Determination in Drosophila
melanogaster
• Genic balance system
• X : A ratio (X, number of X chromosomes;
A, number of haploid sets of autosomes)
Sex Determination in Humans
XX-XY
• SRY gene on the Y chromosome determines
maleness
• Turner syndrome: XO; 1/3000 female births
• Klinefelter syndrome: XXY, or XXXY, or
XXXXY, or XXYY; 1/1000 male births
• Poly-X females: 1/1000 female births
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4.2 Sex-Linked Characteristics Are
Determined by Genes on the Sex
Chromosomes
• X-Linked characteristics
– X-linked white eye in Drosophila
– X-linked color blindness in humans
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Concept Check 4
Hemophilia (reduced blood clotting) is an X-
linked recessive disease in humans. A woman
with hemophilia mates with a man who exhibits
normal blood clotting. What is the probability
that their child will have hemophilia?
All of their male offspring will have hemophilia, and
none of their female offspring will have hemophilia,
so the overall probability of hemophilia in their
offspring is ½.
Calico and Tortoiseshell cats
Orange is X-linked
X+ = black
Xo = orange
Random inactivation
Mean some cells produce
orange some cells produce black -
Tortoiseshell
Concept Check 5
How many Barr bodies will a male with XXXYY
chromosomes have in each of his cells? What
are those Barr bodies?
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Y-Linked Characteristics
• Only present in males
• All male offspring will exhibit the trait
• Y chromosome lost DNA over time
• Important for sex determination in SRY
Additional Factors at a Single Locus Can
Affect the Results of Genetic Crosses
• Genes at the same locus—two versions of the
same gene; each version of the same gene is
defined as allele.
• Types of dominance
– Complete dominance
– Incomplete dominance
– Codominance
• Penetrance: the percentage of individuals
having a particular genotype that express
the expected phenotype.
• Expressivity: the degree to which a
character is expressed.
Additional Factors at a Single Locus Can
Affect the Results of Genetic Crosses
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Lethal Alleles
If before birth genotypes do not appear among
offspring– Ratio skewed 2:1 • Multiple alleles: for a given locus, more
than two alleles are present within a group
of individuals.
• ABO blood group
Additional Factors at a Single Locus Can
Affect the Results of Genetic Crosses
Gene Interaction Takes Place When Genes
At Multiple Loci Determine a Single
Phenotype
Gene interaction: Effects of genes at one
locus depend on the presence of genes at
other loci.
– Gene interaction that produces novel
phenotypes
– Gene interaction with epistasis: one gene
masks the effect of another gene
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• Epistasis: one gene masks the effect of
another gene.
• Can look similar to dominance/recessive, but
not the same gene
‒ Recessive epistasis
‒ Dominant epistasis
‒ Duplicate recessive epistasis
Gene Interaction Takes Place When Genes
At Multiple Loci Determine a Single
Phenotype
Dominant epistasis in squash
BBEE X bbee
Black Lab Yellow Lab
Recessive Epistasis
BbEe
Black Lab
BbEe
Black LabX
9/16 B_E_ Black
3/16 bbE_ Brown
3/16 B_ee
1/16 bbeeYellow
ee is epistatic to B and b
Gene Interaction Takes Place When Genes
At Multiple Loci Determine a Single
Phenotype
Blue eye color hypothesis: Duplicate recessive epistasis.
Could two blue eyed parents have a brown eyed child?
Concept Check 8
A number of all-white cats are crossed, and they
produce the following types of progeny. 12/16
all-white, 3/16 black, and 1/16 gray. What is the
genotype of the black progeny?
a. Aa
b. Aa Bb
c. A_ B_
d. A_ bb
Sex Influences the Inheritance and
Expression of Genes in a Variety of Ways
Sex-influenced and sex-limited
characteristics
‒ Sex-influenced characteristics
‒ Sex-limited characteristics
Both sex-influenced and sex-limited characteristics are
encoded by autosomal genes whose expression is affected
by the sex of the individual who possesses the gene. Sex-
linked characteristics are encoded by genes on the sex
chromosomes.
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Cytoplasmic Inheritance
Some DNA is found in the mitochodria
(or in plants the chloroplasts).
Inheritance of mitochodria is almost
always just from the mother. Different
phenotypes exists mostly due to the
uneven distribution of the DNA (no
mieosis).
Position Effect
Offspring’s phenotype determined by
phenotype of branch where the pollen
germinated.
Maternal Effect
Offspring’s phenotype determined by
mother’s genotype.
• Sex-influenced and sex-limited characteristics
– Genetic maternal effect
– Genomic imprinting: differential expression of genetic
material depending on whether it is inherited from the
male or female parent
Epigenetics: phenomena due to alterations to DNA that
do not include changes in the base sequence; often
affect the way in which the DNA sequences are
expressed
Sex Influences the Inheritance and
Expression of Genes in a Variety of Ways
Genomic Imprinting
• Gene expressed only if
given by male parent
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The Expression of a Genotype May Be
Influenced By Environmental Effects
• Temperature-sensitive allele: an allele whose
product is functional only at certain
temperature.
The Inheritance of Continuous
Characteristics
• Discontinuous characteristics: relatively few
phenotypes
• Continuous characteristics: continuous
distribution of phenotypes; occurs when genes
at many loci interact
• Polygenic characteristics: characteristics
encoded by genes at many loci
• Pleiotropy: one gene affects multiple
characteristics
GENETICS ESSENTIALS
Concepts and ConnectionsTHIRD EDITION
Benjamin A. Pierce
CHAPTER 5
Linkage, Recombination, and
Eukaryotic Gene Mapping
© 2014 W. H. Freeman and Company
Linked Genes Do Not Assort
Independently
• Principle of segregation: alleles separate
during meiosis
• Independent assortment: alleles at one
locus sort independently from alleles at
another locus
• Recombination: alleles sort into new
combinations
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Linked Genes Segregate Together and
Crossing Over Produces Recombination
Between Them
• Complete linkage leads to nonrecombinant
gametes and nonrecombinant progeny
• Crossing over with linked genes leads to
recombinant gametes and recombinant progeny
Concept Check
For single crossovers, the frequency of recombinant gametes is half the frequency of crossing over because
a. a testcross between a homozygote and heterozygote produces ½ heterozygous and ½ homozygous progeny.
b. the frequency of recombination is always 50%.
c. each crossover takes place between only two of the four chromatids of a homologous pair.
d. crossovers occur in about 50% of meiosis.
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Linked Genes Segregate Together and
Crossing Over Produces Recombination
Between Them
• Calculating Recombination Frequency
– Recombination frequency = (No. recombinant
progeny/Total no. of progeny) x 100%
• Coupling and Repulsion Configuration of Linked Genes
– Coupling (cis configuration): wild-type alleles are found on one
chromosome; mutant alleles are found on the other chromosome
a+ b+
a b
a+ b
a b+
Coupling or cis Repulsion or trans
Testing for independent
assortment
• Our Friend Chi Square
• Gene Mapping with Recombination
Frequencies
– Genetics maps are determined by
recombinant frequency
– Map unit and centiMorgans
• Constructing a Genetic Map with Two-
Point Testcrosses?
Linked Genes Segregate Together and
Crossing Over Produces Recombination
Between Them
Genetic vs. Physical Maps
Two-point crosses – issues
1.Cannot distinguish between genes far apart on one chromosome &
on different chromosomes. Both may exhibit 50% recombination.
2. Testcross for two genes that are linked but not close to each other
will underestimate the true distance because of double
recombination
3. Double crossover will produce non-recombined phenotype
Genetic maps are based on rates of
recombination; physical maps are based on
physical distances.
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A Three-Point Testcross Can Be Used to
Map Three Linked Genes
• Constructing a Genetic Map with the
Three-Point Testcross
– Determining the gene order
– Determining the location of crossovers
Recombination Freq 50%
• On separate
chromosomes
• Or very far apart
Recombination Freq less than 50%
• In same linkage group
Basic idea
• % recombinants = distance
Genetic Mapping
Linkage groups
Start of worked problem:
a and b, 10% recomb. – 10m.u. apart – linkage group 1
a and c, 50% recomb. – assorted independently, c in another linkage group
a and d, 14% recomb. In linkage group 1,
b and d, 4% recomb, so b is closer to d than to a
Map so far is a__10___b__4_d
Three-point cross
Concept Check
Write the genotypes of all recombinant
and nonrecombinant progeny expected
from the following three-point cross:
Concept Check
A three-point testcross is carried out
between three linked genes. The
resulting nonrecombinant progeny are
s+r+c+ and s r c, and the double-
crossover progeny are s r c+ and s+r+c.
Which is the middle locus?
The c locus
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1. Determine gene order.
Based on double recombinants
Least number of progeny
2. Determine location of crossovers.
Identify number and location
of single crossovers.
Identify non-recombinant progeny.
3. Calculate recombination frequencies.
single recombinants + double recombinants
total number of offspring
Three-point cross
4. Coefficient of coincidence =
number of observed double crossovers
number of expected double crossovers
Number of expected double crossovers =
probability of single crossovers X probability of single crossovers X total number
between genes 1 & 2 between genes 2 & 3 progeny
5. Interference – the degree to which one crossover interferes with additional
crossovers.
Interference = 1 – coefficient of coincidence
When no double crossovers occur, interference is complete
(interference = 1). When it is between 0 and 1, one crossover
is interfering with additional crossovers. When it is 0, one
crossover does not interfere with additional crossovers. When
it is negative, a crossover increases the probability of another
crossover.
• Calculating the Recombination Frequencies
• Sum of all single and double
crossovers/Total progeny
• Interference and Coefficient of Coincidence
– Coefficient of coincidence = Number of observed
double crossovers/Number of expected double
crossovers
– Interference = 1n – Coefficient of coincidence
A Three-Point Testcross Can Be Used to
Map Three Linked Genes
P.127
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Mapping Techniques
• New Techniques – more closely look at
DNA variations
– Mapping with Molecular Markers
• RFLPs
• Genome-wide Association Studies
– Associations within populations
– Haplotype
CHAPTER 6
Chromosome Variation
Bananas have multiple
sets of chromosomes.
Autopolyploid: 3n
33 chromosomes
Chromosome Mutations Include
Rearrangements, Aneuploids, and
Polyploids
• Chromosome morphology (position of the centromere on the chromosome):
‒ Metacentric
‒ Submetacentric
‒ Acrocentric
‒ Telocentric
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• Types of Chromosome Mutations:
– Rearrangements:
• Aneuploidy, change in # of individual chromosome: 2n +1
• Polyploidy, addition of whole set: 3n
Chromosome Mutations Include
Rearrangements, Aneuploids, and
Polyploids
Chromosome Rearrangements Alter
Chromosome Structure
• Four types of chromosomal rearrangements:
– Duplication, Deletion, Inversion, Translocation
• Unbalanced gene dosage:
• Tandem Duplication – directly next to duplicated region
• Displaced Duplication – away from duplicated region
• Reverse Duplication – ABCDEFFEDCBA
Specific Duplications
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Bar phenotype
• Fewer eye facets
• Bar shape
• X-linked duplication
Unbalanced gene dosage• Developmental abnormalities
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Chromosome Rearrangements Alter
Chromosome Structure
• Deletions: loss of a chromosomal segment
• Large deletions easily detected; during pairing,
normal chromosome loops out
• Effects of deletions
‒ Imbalances in gene product
‒ Pseudodominance: ‒ Expression of a normally recessive gene in a heterozygous due to a
deletion on one chromosome
‒ Haploinsufficiency‒ When two copies are needed to produce wild type effect. A deletion
results in only one normal copy of the gene; Notch: Drosophila
needs two copies of gene to form on wings
Chromosome Rearrangements Alter
Chromosome Structure
• Inversion (depends on the involvement of the
centromere in the inversion):
‒ Paracentric inversion
‒ Pericentric inversion
Translocations
• Nonreciprocal translocation
• Reciprocal translocation
• Robertsonian translocation
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Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Variations in copy number: aneuploidy and
polyploidy
• Causes of aneuploidy
– Deletion of centromere during mitosis and
meiosis
– Robertsonian translocation
– Nondisjunction during meiosis and mitosis
Types of Aneuploidy
• Nullisomy: loss of both members of a
homologous pair of chromosomes. 2n − 2
• Monosomy: loss of a single chromosome.
2n − 1
• Trisomy: gain of a single chromosome. 2n + 1
• Tetrasomy: gain of two homologous
chromosomes. 2n + 2
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Concept Check 5
A diploid organism has 2n = 36 chromosomes.
How many chromosomes will be found in a
trisomic member of this species?
2n + 1 = 36 + 1 = 37
Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy
– In humans
• Sex-chromosome aneuploids
– Turner syndrome. XO
– Klinefelter syndrome. XXY
Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy:
– In plants: mutants could actually be trisomics
– In humans
• Autosomal aneuploids:
– Trisomy 21: Down syndrome
» Primary Down syndrome, 75% random
nondisjunction in egg formation
» Familial Down syndrome, Robertsonian
translocation between chromosomes 14 and
21
Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy:
– In humans
• Autosomal aneuploids
– Trisomy 18: Edward syndrome, 1/8000 live
births
– Trisomy 13: Patau syndrome, 1/15,000 live
births
– Trisomy 8: 1/25,000 ~ 1/50,000 live births
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Aneuploidy Is an Increase or Decrease in
the Number of Individual Chromosomes
• Effects of Aneuploidy:– In humans
• Autosomal aneuploids
– Aneuploidy and maternal age:
– Possible interpretation of this connection?
• Uniparental disomy: both chromosomes are inherited from
the same parent.
– Mosaicism and nondisjunction in mitotic division
Polyploidy is the Presence of More
Than Two Sets of Chromosomes
• Autopolyploidy
– From single species
• Allopolyploidy
– From two species
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Allopolyploid Allopolyploid
Allopolyploid Allopolyploid
• The significance of polyploidy
– Increase in cell size
– Larger plant attributes
– Evolution: may give rise to new species
Polyploidy is the Presence of More
Than Two Sets of Chromosomes
Polyploidy and Sterility
No seeds - Bananas & Watermelon
Wheat is hexploid
• Originally 3 species
Allopolyploidy
between species
Autopolyploidy same
species
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Concept Check
Species A has 2n = 16 chromosomes and species
B has 2n = 14. How many chromosomes would be
found in an allotriploid of these two species?
a. 21 or 24
b. 42 or 48
c. 22 or 23
d. 45
Symbols
• 2n = diploid
• 3n, 4n = polyploid
• 2n + 1 = trisomy (type of aneuploidy),+2
tetrasomy
• 2n – 1 = monosomy
– (X_ unk or XO indicates no second sex
chromosome