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Pea Plant Characteristics
Why Pea Plants?
• Easy, cheap, and quick to grow• 7 easily distinguishable contrasting traits• Normally self-pollinate due to tight enclosure
of the parts of the flower• Can be cross-pollinated by cutting out the
stamens from one flower and using them to pollinate another flower with a contrasting trait
Fig. 14-2a
StamensCarpel
Parentalgeneration(P)
TECHNIQUE
1
2
3
4
Mendel’s Crosses• Mendel crossed plants that had bred true for
a certain trait for multiple generations with plants that bred true for the contrasting trait for multiple generations.
• He called these the P generation.• The offspring from these crosses were called
the F1 generation.
• The offspring from crosses between the F1
plants were the called F2 generation.
Fig. 14-2b
Firstfilialgener-ationoffspring(F1)
RESULTS
5
• Mendel observed:• the F1 generation always had a single phenotype• the phenotype of the other parent remained hidden.
Where does the recessive trait go during the F1 generation?
He hypothesized that it is still present in all the offspring, but is not expressed.
• The Law of Dominance:• When two plants are crossed that are pure for
contrasting traits, then one of the traits, the dominant one, will be expressed in the offspring. The other trait, the recessive one, will not be expressed.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mendel’s Laws
• The Law of Segregation:• When an individual produces gametes, the
two “factors” responsible for a trait separate so that a gamete will receive only one factor or the other.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Without knowing about meiosis and genes and how gametes are formed, Mendel determined:
How will the recessive trait ever be expressed if it is dominated by the other trait?
• It can only be expressed when there is no dominant trait present.• He called this genotype homozygous recessive.
Mendel asked:
When writing genotypes, a capital letter represents the dominant trait and a lower-case letter represents the recessive trait.
Tall – TShort - t
Yellow seed – YGreen seed - y
The same letter is normally used.
Gamete Formation
• Due to meiosis, only one chromosome from each homologous pair is in each gamete.
• Thus, there will be one allele for each trait in each gamete.
• Mendel tracked one or two alleles at a time.
Pure Tall Pea Plant X Pure Short Pea Plant
TT X tt
T T t t
Hybrid Tall Pea Plant X Hybrid Tall Pea Plant
Tt X Tt
T t T t
Punnett Squares
• A Punnett square makes it easy to track the alleles which are passed on to offspring by the fertilization of gametes.
A a
A
a
Using a Punnett square:Male gametes
Femalegametes
A a
A AA Aa
a Aa aa
Possible Genotypes
Tracking Traits
• genotype• the alleles of a given gene locus
• phenotype• the physical expression of a gene
What are the possible genotypes for the offspring of each of the following crosses?
1) AA x AA 2) Aa x aa 3) AA x aa 4) aa x Aa5) AA x Aa6) Aa x Aa
1) AA x AA? 100% AA2) Aa x aa? 50% Aa; 50% aa3) AA x aa? 100% Aa4) aa x Aa? 50% Aa, 50% aa5) AA x Aa? 50% Aa, 50% AA6) Aa x Aa? 25% AA, 50% Aa, 25% aa
With only two alleles:• there are a limited number of possible combinations for any one gene• these examples represent the simplest circumstances
Sometimes there are genes with three or more alleles
• Ex. blood types
Sometimes one allele does not have complete dominance over the other.
If I flip a coin one time, what is the probability that it will land on heads?
Probability
1 out of 2 chances, or 50%
Since I got heads (or tails) the first time, what is the probability that I will get heads on my next flip?
It is still 50%.
If I keep flipping the coin and getting heads, does that affect the probability of the outcome of my next flip?
No, the chances of getting heads or tails is the same on each flip.
What is the probability that I will get heads on two coin tosses in a row?
You are now asking for the probability of flipping heads and flipping heads again.
• The probability of two events combined occurring will be much lower than one event alone
• the chance of two coin tosses both being of a particular outcome is less likely than one coin toss of a particular outcome.
• Multiply the probabilities of a combination of possible outcomes.
Rule of Multiplication
• Multiply whenever there is a statement of one probability “and” another probability.
What is the probability that a couple would have three boys?
The separate probabilities are 1/2 for each boy, so the combined probability would be
1/2 x 1/2 x 1/2 = 1/8
What is the probability that parents with the genotypes Aa and Aa would have a child with the genotype AA?
If the probability of getting dealt an ace from a deck of cards is 4 out of 52 (4/52), what is the probability of getting dealt an ace or a king?
The probability of getting an ace is 4/52 and the probability of getting a king is 4/52; to learn what chance there is of getting dealt either card, you must add the probabilities together to get the total probability:
4/52 + 4/52 = 8/52
What is the probability of getting any face card?
Calculate this two ways:• first, count up the total number of face cards (12/52) • verify this, using the rule of addition: (kings) 4/52 + (queens) 4/52 + (jacks) 4/52 = 12/52
Adding the probabilities of a combination ofindependent outcomes is called the rule of addition.
Rule of Addition
Use this whenever there is a statement of one probability “or” another probability.
What is the probability of my being dealt an ace or a face card from one deck of cards?
The probability of getting an ace is 4/52 andthe probability of getting a face card is 12/52, so the probability of either outcome occurring is 4/52 + 12/52 = 16/52
If a child with a dominant gene A is affected by a particular disease, what is the probability of a couple having a child with this disease if they have the genotypes of Aa (father) and aa (mother)?
The probability of the father giving the child an “A” allele is 1/2 and the probability of the mother giving the child an “A” allele is 0, so the total probability is 1/2 + 0 = ½
Verify this using a Punnett square.
The Testcross
• How can we tell the genotype of an individual with the dominant phenotype?
• Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
• The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
• If any offspring display the recessive phenotype, the mystery parent must be heterozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-7
TECHNIQUE
RESULTS
Dominant phenotype, unknown genotype:
PP or Pp?
Predictions
Recessive phenotype, known genotype: pp
If PP If Ppor
Sperm Spermp p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
or
All offspring purple 1/2 offspring purple and1/2 offspring white
Fig. 14-7a
Dominant phenotype, unknown genotype:
PP or Pp?
Predictions
Recessive phenotype, known genotype: pp
If PP If Ppor
Sperm Spermp p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
TECHNIQUE
Fig. 14-7b
RESULTS
All offspring purple
or
1/2 offspring purple and1/2 offspring white
The Law of Independent Assortment
• Mendel derived the law of segregation by following a single trait
• The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one trait
• A cross between such heterozygotes is called a monohybrid cross
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Mendel identified his third law of inheritance by following two characters at the same time
• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-8
EXPERIMENT
RESULTS
P Generation
F1 Generation
Predictions
Gametes
Hypothesis ofdependentassortment
YYRR yyrr
YR yr
YyRr
Hypothesis ofindependentassortment
orPredictedoffspring ofF2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YRYYRR
YYRR YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr Yyrr
Yyrr
yyRR yyRr
yyRr yyrr
yyrr
Phenotypic ratio 3:1
EggsEggs
Phenotypic ratio 9:3:3:1
1/21/2
1/2
1/2
1/4
yr
1/41/4
1/41/4
1/4
1/4
1/4
1/43/4
9/163/16
3/161/16
Phenotypic ratio approximately 9:3:3:1315 108 101 32
Fig. 14-8a
EXPERIMENT
P Generation
F1 Generation
Predictions
Gametes
Hypothesis ofdependentassortment
YYRR yyrr
YR yr
YyRr
Hypothesis ofindependentassortment
orPredictedoffspring ofF2 generation
Sperm
Sperm
YR
YR
yr
yr
Yr
YR
yR
Yr
yR
yr
YRYYRR
YYRR YyRr
YyRr
YyRr
YyRr
YyRr
YyRr
YYRr
YYRr
YyRR
YyRR
YYrr Yyrr
Yyrr
yyRR yyRr
yyRr yyrr
yyrr
Phenotypic ratio 3:1
EggsEggs
Phenotypic ratio 9:3:3:1
1/21/2
1/2
1/2
1/4
yr
1/4 1/41/4
1/4
1/4
1/4
1/4
1/43/4
9/163/16
3/161/16
Fig. 14-8b
RESULTS
Phenotypic ratio approximately 9:3:3:1315 108 101 32
• Using a dihybrid cross, Mendel developed the
• Law of independent assortment
• Each pair of alleles segregates independently of each other pair of alleles during gamete formation
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
We now know • Mendel’s Law of Independent Assortment
• Applies only to genes on different, nonhomologous chromosomes
• The traits he studied were, in fact, on nonhomologous chromosomes
• Genes located near each other on the same chromosome tend to be inherited together
Solving Complex Genetics Problems with the Rules of Probability
• The multiplication and addition rules can predict the outcome of crosses involving multiple characters– A dihybrid cross is equivalent to two or more
independent monohybrid crosses occurring simultaneously
– In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
• Many heritable characters are not determined by only one gene with two alleles.
• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may deviate from Mendelian patterns:– When alleles are not completely dominant or
recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Degrees of Dominance
• In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
• In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-10-1
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
Fig. 14-10-2
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
Fig. 14-10-3
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
Incomplete dominance in snapdragon color
• A dominant allele does not subdue a recessive allele; alleles don’t interact
• Alleles are simply variations in a gene’s nucleotide sequence
• For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Relation Between Dominance and Phenotype
• Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
– At the organismal level, the allele is recessive
– At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
– At the molecular level, the alleles are codominant
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more common in populations than recessive alleles
• For example, one baby out of 400 in the United States is born with extra fingers or toes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
• In this example, the recessive allele is far more prevalent than the population’s dominant allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Multiple Alleles• Most genes exist in populations in more than two
allelic forms
• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-11
IA
IB
i
A
B
none(a) The three alleles for the ABO blood groups and their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cellappearance
Phenotype(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
Pleiotropy
• Most genes have multiple phenotypic effects, a property called pleiotropy
• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Extending Mendelian Genetics for Two or More Genes
• Some traits may be determined by two or more genes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Epistasis• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second locus
– For example, in mice and many other mammals, coat color depends on two genes
• One gene determines the pigment color (with alleles B for black and b for brown)
• The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-12
BbCc BbCc
Sperm
EggsBC bC Bc bc
BC
bC
Bc
bc
BBCC
1/41/4
1/41/4
1/4
1/4
1/4
1/4
BbCC BBCc BbCc
BbCC bbCC BbCc bbCc
BBCc BbCc
BbCc bbCc
BBcc Bbcc
Bbcc bbcc
9 : 3 : 4
Epistasis- One gene affects the expression of another gene
Polygenic Inheritance
• Quantitative characters vary in the population along a continuum
• usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
• Skin color in humans is an example of polygenic inheritance
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-13
Eggs
Sperm
Phenotypes:
Number ofdark-skin alleles: 0 1 2 3 4 5 6
1/646/64
15/6420/64
15/646/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/81/8
1/81/8
1/81/8
1/81/8
AaBbCc AaBbCc
PolygenicInheritance
Height in Human Beings
Frequency Distributions in Polygenic Inheritance
Nature and Nurture: The Environmental Impact on Phenotype
• Phenotype for a character depends on environment as well as genotype
– The norm of reaction is the phenotypic range of a genotype influenced by the environment
– For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-14
Himalayan Rabbit
When an ice pack was kept on a shaved portion of the rabbit’s back, the new fur grew back black instead of white.
Temperature affected the expression of the gene for fur color.
Many human traits follow Mendelian patterns of inheritance
• Humans are not good subjects for genetic research
– Generation time is too long– Parents produce relatively few offspring– Breeding experiments are unacceptable
• However, basic Mendelian genetics endures as the foundation of human genetics
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Pedigree Analysis
• A pedigree is a family tree that describes the interrelationships of parents and children across generations
• Inheritance patterns of particular traits can be traced
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-15a
KeyMale
Female
AffectedmaleAffectedfemale
Mating
Offspring, inbirth order(first-born on left)
Fig. 14-15b
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Widow’s peak No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Wwor
Fig. 14-15c
Attached earlobe
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Ff Ff
Ff Ff Ff
ff Ff
ff ff ff
ff
FF or
orFF
Ff
• Pedigrees can also be used to make predictions about future offspring
• We can use the multiplication and addition rules to predict the probability of specific phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Recessively Inherited Disorders
• Many genetic disorders are inherited in a recessive manner
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The Behavior of Recessive Alleles
•Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)
– Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-16
Parents
Normal Normal
Sperm
Eggs
NormalNormal(carrier)
Normal(carrier) Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
• If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
• Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
• Most societies and cultures have laws or taboos against marriages between close relatives
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Cystic Fibrosis
• Cystic fibrosis– the most common lethal genetic disease in the
United States, striking one out of every 2,500 people of European descent
– results in defective or absent chloride transport channels in plasma membranes
– Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Sickle-Cell Disease
• Sickle-cell disease affects one out of 400 African-Americans
– caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
– Symptoms include physical weakness, pain, organ damage, and even paralysis
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Dominantly Inherited Disorders
• Some human disorders are caused by dominant alleles
– Dominant alleles that cause a lethal disease are rare and arise by mutation
– Achondroplasia is a form of dwarfism caused by a rare dominant allele
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-17
Eggs
Parents
Dwarf Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd dd
dD
Dd dd
ddDd
d
d
• Huntington’s disease is a degenerative disease of the nervous system
• The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
Huntington’s Disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Multifactorial Disorders
• Many diseases, such as heart disease and cancer, have both genetic and environmental components
• Little is understood about the genetic contribution to most multifactorial diseases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Testing and Counseling
• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Tests for Identifying Carriers
• For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fetal Testing
• In amniocentesis, the liquid that bathes the fetus is removed and tested
• In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
• Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-18a
Fetus
Amniotic fluidwithdrawn
Placenta
Uterus Cervix
Centrifugation
Fluid
Fetalcells
Severalhours
Severalweeks
Severalweeks
Bio-chemical
tests
Karyotyping
(a) Amniocentesis
Fig. 14-18b
(b) Chorionic villus sampling (CVS)
Bio-chemical
tests
Placenta Chorionicvilli
Fetus
Suction tubeinsertedthroughcervix
FetalcellsSeveral
hours
SeveralhoursKaryotyping
Newborn Screening
• Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States
– Phenylketonuria – inability to metabolize the amino acid phenylalanine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-UN2
Degree of dominance
Complete dominanceof one allele
Incomplete dominanceof either allele
Codominance
Description
Heterozygous phenotypesame as that of homo-zygous dominant
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygotes: Bothphenotypes expressed
Multiple alleles
Pleiotropy
In the whole population,some genes have morethan two alleles
One gene is able toaffect multiplephenotypic characters
CRCR CRCW CWCW
IAIB
IA , IB , i
ABO blood group alleles
Sickle-cell disease
PP Pp
Example
Fig. 14-UN3
DescriptionRelationship amonggenes
Epistasis One gene affectsthe expression ofanother
Example
Polygenicinheritance
A single phenotypiccharacter isaffected bytwo or more genes
BbCc BbCc
BCBC
bC
bC
Bc
Bc
bc
bc
9 : 3 : 4
AaBbCc AaBbCc
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