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Chapter 16
Lecture Outline
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Chapter 16 Simple Patterns of Inheritance
Mendel’s Laws of Inheritance
Chromosome Theory of Inheritance
Pedigree Analysis of Human Traits
Sex Chromosomes and X-linked Inheritance
Molecular Basis of Different Inheritance Patterns
Genetics and Probability
Key Concepts:
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Mendel’s Laws of Inheritance
Gregor Mendel, 1822-1884
Entered monastery and became a priest
Historic experiments with pea plants
His paper was ignored at the time, but his findings were independently rediscovered years later
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Garden Pea, Pisum sativum
Many different variable traits
Normally self-fertilizing Female gamete fertilized by male gamete from same plant
Easy to breed true-breeding lines (exhibit the same trait)
Large flowers make crosses easy when desired Cross-fertilization or hybridization
Several advantageous properties:
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Character
Flower color
Purple White
Variants (Traits)
Flower position
Axial Terminal
Seed color
Yellow Green
Seed shape
Round Wrinkled
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Character Variants (Traits)
Pod color
Green Yellow
Pod shape
Smooth Constricted
Height
Tall Dwarf
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Stigma
Stamen
Ovary
Ovule
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© Nigel Cattlin/Photo Researchers, Inc.
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Transfer pollen from stamens of white flower to the stigma of a purple flower.
Remove stamens from purple flower.
1 2
Stamens
Stigma
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P generation True-breeding parents
F1 generation Offspring of P cross
Monohybrids (if parents differ in one trait)
F2 generation F1 self-fertilizes
Recessive trait reappears
Single-factor cross Where the experimenter follows only a single trait
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×
All tall offspring (monohybrids)
Experimental approach
P generation
Tall Dwarf
Cross-fertilization
F1 generation
F2 generation
Tall offspring Dwarf offspring
(a) Mendel’s protocol for making monohybrid crosses
3 1 :
Inheritance pattern
TT × tt
All Tt (tall)
TT tt Tt
(Tall) (Dwarf)
1 : 2 : 1
Self-fertilization
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(b) Mendel’s observed data for all 7 traits
Round wrinkled seeds
Yellow green seeds
Purple white flowers
Axial terminal flowers
Tall dwarf stem
F2 generation
5,474 round, 1,850 wrinkled
6,022 yellow, 2,001 green
705 purple, 224 white
651 axial, 207 terminal
787 tall, 277 dwarf
14,949 dominant, 5,010 recessive
Smooth constricted pods
882 smooth 299 constricted
Green yellow pods
428 green, 152 yellow
THEDATA
P cross
Total
F1 generation
All purple
All axial
All yellow
All round
All green
All smooth
All tall
All dominant
Ratio
3.15:1
3.14:1
3.01:1
2.96:1
2.82:1
2.95:1
2.84:1
2.98:1
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Mendel’s three important ideas
1. Traits are dominant and recessive Dominant variant is displayed in hybrids Recessive variant is masked by dominant
2. Genes and alleles Particulate mechanism of inheritance His “unit factors” are genes Every individual has two genes for a character A gene has two variant forms, or alleles
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3. Segregation of alleles Two copies of a gene carried by an F1 plant
segregate (separate) from each other, so that each sperm or egg carries only one allele
F2 traits follow approximately 3:1 ratio
Mendel’s Law of Segregation
Two copies of a gene segregate from each other during the transmission from parent to offspring.
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Tt
TT Tt Tt tt
T × t
Tt
T × t
Gametes
F1 generation
3 tall offspring
1 dwarf offspring
F2 generation
Segregation: Alleles separate into different haploid cells that eventually give rise to gametes.
Fertilization: During fertilization, male and female gametes randomly combine with each other.
1
2
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Genotype and phenotype
Genotype – The genetic composition of an individual
TT – homozygous dominant
tt – homozygous recessive
Tt – heterozygous
Phenotype – Physical or behavioral characteristics that are the result of gene expression
TT and Tt are tall
tt is dwarf
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Punnett square
Step 1. Write down genotypes of parents Male parent: Tt
Female parent: Tt
Step 2. Write down the possible gametes that each parent can make
Male gametes: T or t
Female gametes: T or t
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Step 3. Create an empty Punnett square.
T t
♀
♂ Male gametes
T
t
Fem
ale
gam
etes
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T
T
t
t
♀
♂ Male gametes
Fem
ale
gam
etes
TT Tt
Tt tt
Step 4. Fill in the possible genotypes.
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Step 5. Determine relative proportions of genotypes and phenotypes.
T t
♀
♂ Male gametes
TT Tt
Tt tt
Genotype ratio
TT:Tt:tt
1:2:1
Phenotype ratio
tall:dwarf
3:1
T
t
Fem
ale
gam
etes
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Testcross
A dwarf pea plant must be tt
A tall pea plant could be either TT or Tt, so genotype must be determined by a testcross
Cross the unknown individual (TT or Tt) to a homozygous recessive individual (tt) If some offspring are dwarf, unknown individual must
have been Tt If all offspring are tall, unknown individual was TT
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Dominant phenotype; genotype could be TT or Tt
Recessive phenotype; genotype must be tt
If plant with dominant phenotype is TT, all offspring will be tall.
Alternatively, if plant with dominant phenotype is Tt, half of the offspring will be tall and half will be dwarf.
T T
t
t
Tt Tt
Tt Tt
T t
t
t
×
tt
tt Tt
Tt
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Two-factor cross
Follows inheritance of two different traits
Can determine linkage
Possible patterns: Two genes are linked – variants found together
in parents are always inherited as a unit
Two genes are independent – variants are randomly distributed
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YYRR yyrr
×
YYRR yyrr
×
YyRr YyRr
P generation
Gametes
F1 generation
Sperm Sperm
Eg
g
Eg
g
YYRR
YYRr
YyRR
YyRr Yyrr yyRr yyrr
yyRr yyRR
YyRr yyrr
YyRr YYRR
YyRr
YYrr
YYRr YyRR YyRr
Yyrr YyRr
F2 generation
(a) Hypothesis: linked assortment
(b) Hypothesis: independent assortment
yr YR
YR yr
YR
yr
YR yr
YR yR yr Yr
YR
yr
Yr
yR
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Dihybrid offspring – offspring are hybrids with respect to both traits
Data for F2 hybrids is consistent with independent assortment
P cross 315 yellow, round seeds 101 yellow, wrinkled seeds 108 green, round seeds
32 green, wrinkled seeds
(c) The data observed by Mendel
×
F1 generation
F2 generation
Yellow, round seeds
Yellow, round seeds
Green, wrinkled seeds
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Sperm Sperm
Eg
g
Eg
g
YYRR
YYRr
YyRR
YyRr Yyrr yyRr yyrr
yyRr yyRR
YyRr yyrr
YyRr YYRR
YyRr
YYrr
YYRr YyRR YyRr
Yyrr YyRr
F2 generation YR yr
YR
yr
YR yR yr Yr
YR
yr
Yr
yR
Round
s
S
S s
y
Y
Y y
SS S
S s s s
s YY Y
Y y y y
y
Yellow
Dihybrid Cross
Probabilities Round=SS, Ss is ¾ Wrinkled=ss is ¼
Probabilities Yellow=YY, Yy is ¾ Green=yy is ¼
Yellow and round Proportion of peas that are yellow = ¾
Proportion of peas that are round = ¾
To determine the proportion of both yellow and round you have to multiply the proportion of each individual phenotype, thus,
¾ x ¾ = 9/16 are yellow and round
Calculating Genetic Probabilities with Mendelian Inheritance
Yellow and wrinkled Proportion of peas that are yellow = ¾
Proportion of peas that are wrinkled = ¼
To determine the proportion of both yellow and wrinkled you have to multiply the proportion of each individual phenotype, thus,
¾ x ¼ = 3/16 are yellow and wrinkled
Calculating Genetic Probabilities with Mendelian Inheritance
Calculating Genetic Probabilities with Mendelian Inheritance
Green and round Proportion of peas that are green = ¼
Proportion of peas that are round = ¾
To determine the proportion of both green and round you have to multiply the proportion of each individual phenotype, thus,
¼ x ¾ = 3/16 are green and round
Calculating Genetic Probabilities with Mendelian Inheritance
Green and wrinkled Proportion of peas that are green = ¼
Proportion of peas that are wrinkled = ¼
To determine the proportion of both green and wrinkled you have to multiply the proportion of each individual phenotype, thus,
¼ x ¼ = 1/16 are green and wrinkled
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Law of Independent Assortment
Alleles of different genes assort independently of each other during gamete formation.
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Chromosome Theory of Inheritance
1. Chromosomes contain the genetic material (DNA). Genes are found in the chromosomes.
2. Chromosomes are replicated and passed from parent to offspring. They are also passed from cell to cell during the development of a multicellular organism.
3. The nucleus of a diploid cell contains two sets of chromosomes, found in homologous pairs. Maternal and paternal sets of homologous chromosomes are functionally equivalent; each set carries a full complement of genes.
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Chromosome Theory of Inheritance
4. At meiosis, one member of each chromosome pair segregates into each daughter nucleus. During the formation of haploid cells, the members of different chromosome pairs segregate independently of each other.
5. Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring.
Chromosomes and segregation
Mendel’s Law of Segregation can be explained by the pairing and segregation of homologous chromosomes during meiosis
The physical location of a gene on a chromosome is called its locus
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Gene locus—site on chromosome where a gene is found. A gene can exist as 2 or more different alleles.
T—Tall allele
t—Dwar fallele Genotype: Tt (heterozygous)
Pair of homologous chromosomes
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Homologs segregate into separate cells during meiosis I.
2
Chromosomes replicate, and cell progresses to metaphase of meiosis I.
Sister chromatids separate during meiosis II to produce 4 haploid cells.
1
3
Diploid cell
Four haploid cells
Heterozygous (Tt) cell from a tall plant
Homologues paired with each other
t t T T
t t T T
t t T T
t T
Sister chromatids
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Chromosomes and independent assortment
The Law of Independent Assortment can also be explained by the behavior of chromosomes during meiosis
Random alignment of chromosome pairs during meiosis I leads to the independent assortment of genes found on different chromosomes
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Four haploid cells Four haploid cells
y Y
R r
y Y
R r
y y Y Y Y Y y y
r r R R
y
R R
Y Y Y
Y Y
or
Heterozygous diploid cell (YyRr) to undergo meiosis
Heterozygous diploid cell (YyRr) to undergo meiosis
R R
y Y
r r
y y
r r
R R y
r y
r
r r R R
y R
y r r
Y Y R
Metaphase I (can occur in different ways)
Chromosomes replicate, and cell progresses to metaphase of meiosis I. Alignment of homologs can occur in more than one way.
1
Homologs segregate into separate cells during meiosis I.
2
Sister chromatids separate during meiosis II to produce 4 haploid cells.
3
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Pedigree Analysis of Human Traits
Inherited trait is analyzed over the course of several generations in one family
Cystic fibrosis (CF) example
Approximately 3% of Americans of European descent are heterozygous carriers of the recessive CF allele, and phenotypically normal
Individuals who are homozygous exhibit disease symptoms
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A family pedigree for Cystic Fibrosis, a recessive trait.
Unaffected individual
Affected individual
Presumed heterozygote
Female
Male
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(a) Human pedigree showing cystic fibrosis
I
II
III
I-1 I-2
II-1 II-2 II-3 II-4 II-5
III-1 III-2 III-3 III-4 III-5 III-6 III-7
Recessive Inheritance
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Many of the alleles causing human genetic disease are recessive, like Cystic Fibrosis
But some are dominant, like Huntington disease Huntington disease has an autosomal dominant
inheritance pattern Gene is on one of 22 pairs of autosomes
Disease genes can also be found on the sex chromosomes
Disease genes can be recessive or dominant, autosomal or sex-linked
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A family pedigree for Huntington disease, an autosomal dominant trait.
I
II
III
II-1 II-2 II-3
I-1 I-2
II-4 II-5 II-6 II-7
III-1 III-2 III-3 III-4
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Dominant Inheritance
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Molecular Basis of Different Inheritance Patterns
Simple Mendelian inheritance Alleles are dominant or recessive Phenotype ratios follow Mendel’s laws
More complex forms of inheritance Incomplete dominance Codominance
Understanding gene function at the molecular level explains differences in inheritance patterns
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Recessive allele does not affect phenotype of heterozygote
Single copy of the dominant allele makes enough functional protein to provide a normal phenotype, masking recessive allele
Sometimes heterozygote may even upregulate the lone functional allele to provide high enough expression
Simple Mendelian Inheritance
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Example: Purple pigment, P
One P allele makes enough functional protein to provide a normal phenotype
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Genotype
Amount of functional protein P produced
Phenotype
The relationship of the normal (dominant) and mutant (recessive) alleles displays simple Mendelian inheritance.
Colorless precursor molecule
Protein P Purple pigment
100% 50% 0%
Purple Purple White
PP Pp pp
Recessive Alleles That Cause Diseases May Have Multiple Effects on Phenotype
In many human genetic diseases, a recessive allele fails to produce a specific functional protein
Over 7,000 human disorders are caused by mutations in single genes
Most single-gene diseases are recessive, but some are dominant
Pleiotropy – mutation in a single gene has multiple effects
Example: Cystic Fibrosis (CF)
Normal CF allele codes for transporter protein that regulates chloride ion balance
Mutation diminishes function of transporter, causing multiple pleiotropic effects:
Thick mucus in lungs is due to water imbalance caused by ion balance
Sweat is very salty because salt cannot be recycled back into body without transporter
Some males are infertile because Cl- transporter is needed for proper development of vas deferens
Incomplete dominance
Heterozygote shows intermediate phenotype
Neither allele is dominant
example: Pink four-o’clocks 50% of normal protein
not enough to give red color
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CRCR
CRCw
CR
CR
CR
Cw
Cw
Cw
F2 generation
Eg
g
CRCw CwCw
CRCw CRCR
P generation Red White
Gametes
CwCw
×
Self-fertilization of F1 off spring
F1 generation
Sperm
Pink
Multiple alleles – three or more variants in a population
Phenotype depends on which two alleles are inherited
example: ABO blood types in humans Type AB is codominant – expresses both alleles equally
Codominance
O A B A B
Table 16.3 The ABO Blood Group
Antigen A Antigen B
Galactose RBC
Antigen A Antigen B
RBC
AB
IAIB IBIB or IBi
Against A
A and B
Neither
RBC N-Acetyl- galactosamine
IAIA or IAi
Against B
Neither A nor B
Against A and B
ii
RBC
Blood type:
Genotype:
Surface antigen:
Antibodies:
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Figure 12.13 Codominance: ABO Blood Reactions Are Important in Transfusions
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Role of environment
The environment plays a vital role in phenotype
Genotype provides the plan to create a phenotype; the environment provides nutrients and energy to carry out the plan
Norm of reaction – effects of environmental variation on a phenotype
example: Genetically identical plants grow to different heights in different temperatures
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1
2
0
3
45 55 65 75 85 95
Temperature (°F)
Hei
gh
t (f
eet)
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Role of environment
Example: Phenylketonuria (PKU) disease
Can develop normally if given a diet free of phenylalanine
If diet contains phenylalanine, symptoms include mental retardation, underdeveloped teeth and foul-smelling urine
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Environment influences expression of PKU within a family.The child in the middle was raised on a phenylalanine-free diet; the ones on either side were not.
Figure 12.11 Multiple Alleles Generate Diversity: Rabbit Coat Color
C: Dark gray cch: Chinchilla ch: Light gray c: Albino
Hierarchy of Dominance
Figure 12.14 Genes May Interact Epistatically (Part 1)
Gene 1 B or b: Which pigment (B is dominant to b) Gene 2 E or e: Determines if there is pigment (E is dominant to e)
Epistasis: Phenotype of one gene is affected by another gene
Figure 12.14 Genes May Interact Epistatically (Part 2)
Genetic Linkage and Recombination Recombination frequency (RF):
Two loci are closer, there is less crossing over
Higher RF means two genes are farther away
Low RF means two genes are very close and are less likely to be separated by recombination
Figure 12.21 Steps toward a Genetic Map Arbitrary reference point
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Genetics and Probability
Probability – the chance that an event will have a particular outcome
For a single coin toss, chance of getting heads
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Example: Self-fertilization of a pea plant heterozygous for the height gene (Tt)
Punnett square predicts that 1/4 of the offspring will be dwarf
Tt
Tt TT
tt
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Sample size
Prediction accuracy depends on number of events observed – the sample size
Random sampling error – deviation between observed and expected outcome
Larger samples have smaller sampling errors
Humans have small families and observed data may be very different from expected outcome
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Product rule
Probability that two or more independent events will occur is equal to the product of their individual probabilities
If we toss a coin twice, what is the probability that we will get heads both times?
The product rule says that it is equal to the probability of getting heads on the first toss (1/2) times the probability of getting heads on the second toss (1/2), or one in four
½ x ½ = ¼
Sum rule
Probability that one of two or more mutually exclusive outcomes will occur is the sum of the probabilities of the possible outcomes
In a cross between two heterozygous (Tt) pea plants, we may want to know the probability of a particular offspring being a homozygote (either TT or tt)
¼ + ¼ = ½ Half the offspring will be homozygotes
Tt
Tt TT
tt