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Chapter 14
Mendel and the Gene Idea
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
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• Gregor Mendel
– experiments with garden peas
Figure 14.1
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Mendel’s Experimental Approach
• Why peas?
= available in many varieties
= could strictly control mating
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Mendel’s Experimental Approach
Stamens (Male)
Carpel (Female)
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Mendel’s Experimental Approach
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Mendel’s Experimental Approach
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Genetics Vocabulary
Alternative versions of genes = Alleles
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Genetics Vocabulary
Organism inherits 2 alleles:
• 1 from mom, 1 from dad
– A genetic locus is represented twice
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Genetics Vocabulary
• If the two alleles at a locus differ…
• Dominant allele = determines appearance
• Recessive allele = no noticeable effect on
appearance
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Genetic Vocabulary: Homozygous vs. Heterozygous
• Homozygous for a particular gene
– Identical pair of alleles for that gene
• Ex: PP (2 purple flower alleles)
• True-breeding
- Homozygous dominant (PP)
- Homozygous recessive (pp)
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Homozygous vs. Heterozygous
• Homozygous for a particular gene
– Identical pair of alleles for that gene
• Ex: PP (2 purple flower alleles)
• True-breeding
• Heterozygous for a particular gene
– Has a pair of alleles that are different for that
gene
• Ex: Pp (1 purple allele, 1 white allele)
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Homozygous or Heterozygous?
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Genetics Vocabulary
• An organism’s genotype (EX: Pp, PP, pp)
– genetic makeup
• An organism’s phenotype (Ex: Purple or white)
– physical appearance
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Phenotype versus genotype
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Phenotype versus genotype
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• Mendel used :
• Characters that varied in an “either-or” manner
• Varieties that were “true-breeding”
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Typical Mendelian Experiment
Parental Generation
Hybridization
F1 Generation
F1 self-pollinate
F2 generation
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All Purple
Hybrids
3:1
Purple : White
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• Does Mendel’s segregation model account for
the 3:1 ratio observed in the F2 generation?
– We can answer this question using a Punnett
square
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Other pea plant characters
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• Construct a Punnett Square for the following
crosses:
• Seed color: Y = Yellow, y = green
– YY X Yy
– Expected ratio observed in offspring?
• Seed shape: R = Round, r = wrinkled
– Rr X rr
– Expected ratio observed in offspring?
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The Testcross
• In pea plants with purple flowers
– Genotype is not obvious (Pp or PP)?
= Perform testcross
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The testcross
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The testcross
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Monohybrid Cross
Mendel Followed a single trait (ex: flower color)
• The P = true-breeding (PP or pp)
• The F1 offspring = monohybrids (heterozygous
for one character) (Pp)
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Dihybrid Cross
• Mendel followed 2 characters at the same time
• P generation = Cross two, true-breeding
parents differing in two characters
– YYRR X yyrr
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Character 1
Y =YELLOW
y =green
Character 2
R=ROUND
r = wrinkled
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• Mendel followed 2 characters at the same time
• P generation = Cross two, true-breeding
parents differing in two characters
– YYRR X yyrr
• F1 generation = Produces dihybrids
(heterozygous for both characters)
– YyRr
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2. Independent Assortment of Chromosomes
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2. Independent Assortment of Chromosomes
Homologous orient randomly at metaphase I of meiosis
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• How are two characters transmitted from
parents to offspring?
1. As a package? (Ex: yellow and round YR)
• =Dependent Assortment
2. Independently?
• =Independent Assortment
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A dihybrid cross
Only YR and yr as
inherited from P
generation?
YR Yr yR yr ?
Make a punnett square for each case
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• Concept 14.2: The rules of probability govern
Mendelian inheritance
• Multiplication Rule
• Addition Rule
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The Multiplication and Addition Rules Applied to
Monohybrid Crosses
• The multiplication rule
– Probability that two or more independent
events will occur together
– Ex: coin toss
– Heads ½ X Heads ½ = ¼
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• Ex: Probability in a monohybrid cross
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Rule of Addition
• Probability that any one of two or more
exclusive events will occur
Ex: Heterozygotes:
¼Rr + ¼rR = ½
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• A multi-character cross
= two or more independent monohybrid crosses
occurring simultaneously
• Calculate the chances for various genotypes:
1. Consider each character separately
2. Go back to question being asked
3. Multiply individual probabilities together
4. Use Rule of addition (if necessary)
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• 3 characters = trihybrid cross
– Purple flowers (Pp), Yellow (Yy), Round (Rr)
– Purple flowers (Pp), green (yy), wrinkled (rr)
• PpYyRr X Ppyyrr
Question: What percentage of the offspring
from this cross would be predicted to have
purple flowers and green and wrinkled
seeds?
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• 1. Consider each character separately
(make a punnett square for each character)
PpYyRr X Ppyyrr:
– Pp X Pp =
– Yy X yy =
– Rr X rr =
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• 1. Consider each character separately
(make a punnett square for each character)
– PpYyRr X Ppyyrr
– Pp X Pp = ¼ PP, ½ pP, ¼ pp
– Yy X yy = ½ Yy, ½ yy
– Rr X rr = ½ Rr, ½ rr
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2. Go back to the original Question
PpYyRr X Ppyyrr
– Pp X Pp = ¼ PP, ½ pP, ¼ pp
– Yy X yy = ½ Yy, ½ yy
– Rr X rr = ½ Rr, ½ rr
Question: What percentage of the offspring
from this cross would be predicted to have
purple flowers and green and wrinkled
seeds?
Start by listing all genotypes that fulfill
this condition:
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2. Go back to the original Question
PpYyRr X Ppyyrr
– Pp X Pp = ¼ PP, ½ Pp, ¼ pp
– Yy X yy = ½ Yy, ½ yy
– Rr X rr = ½ Rr, ½ rr
Question: What percentage of the offspring from this
cross would be predicted to have purple flowers
and green and wrinkled seeds?
Start by listing all genotypes that fulfill
this condition:
Ppyyrr, PPyyrr
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3. Calculate probability for each genotype
– Pp X Pp = ¼ PP, ½ pP, ¼ pp
– Yy X yy = ½ Yy, ½ yy
– Rr X rr = ½ Rr, ½ rr
• Ppyyrr ½ X ½ X ½
• Ppyyrr
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= 2/16
3. Calculate probability for each genotype
– Pp X Pp = ¼ PP, ½ pP, ¼ pp
– Yy X yy = ½ Yy, ½ yy
– Rr X rr = ½ Rr, ½ rr
• Ppyyrr ½ X ½ X ½
= 2/16
• PPyyrr ¼ X ½ X ½
=1/16
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4. Rule of addition
2/16 Ppyyrr
+1/16 Ppyyrr
3/16
= chance that the offspring from this cross
would have purple flowers and green and
wrinkled seeds
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• A multi-character cross #2
= two or more independent monohybrid crosses
occurring simultaneously
• Calculate the chances for various genotypes:
1. Consider each character separately
2. Go back to question being asked
3. Multiply individual probabilities together
4. Use Rule of addition
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• 3 characters = trihybrid cross #2
– white flowers (pp), Yellow (Yy), wrinkled (rr)
– Purple flowers (Pp), green (yy), Round (Rr)
• ppYyrr X PpyyRr
Question: What percentage of the offspring
from this cross would be predicted to have
white flowers and green and wrinkled
seeds?
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• 1. Consider each character separately
(make a punnett square for each character)
ppYyRr X Ppyyrr:
– pp X Pp = ½ Pp, ½ pp
– Yy X yy = ½ Yy, ½ yy
– rr X Rr = ½ Rr, ½ rr
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2. Go back to the original Question
ppYyRr X Ppyyrr:
– pp X Pp = ½ Pp, ½ pp
– Yy X yy = ½ Yy, ½ yy
– rr X Rr = ½ Rr, ½ rr
Question: What percentage of the offspring
from this cross would be predicted to have
white flowers and green and wrinkled
seeds?
Start by listing all genotypes that fulfill
this condition:
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2. Go back to the original Question
ppYyRr X Ppyyrr:
– pp X Pp = ½ Pp, ½ pp
– Yy X yy = ½ Yy, ½ yy
– rr X Rr = ½ Rr, ½ rr
Question: What percentage of the offspring from this
cross would be predicted to have white flowers and
green and wrinkled seeds?
Start by listing all genotypes that fulfill
this condition:
ppyyrr
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3. Calculate probability for each genotype
ppYyRr X Ppyyrr:
– pp X Pp = ½ Pp, ½ pp
– Yy X yy = ½ Yy, ½ yy
– rr X Rr = ½ Rr, ½ rr
• ppyyrr ½ pp X ½ yy X ½ rr
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= 1/8
Concept 14.3
• Inheritance patterns are often more complex
than predicted by simple Mendelian genetics
• The relationship between genotype (Ex: Pp)
and phenotype (Ex: purple) is rarely simple
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The Spectrum of Dominance
• Complete dominance
– Phenotypes of the heterozygote and dominant
homozygote are identical
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• Codominance
– Two dominant alleles affect the phenotype in
separate, distinguishable ways
• Ex: human blood group MN
• MM = RBC with M molecules
• NN = RBC with N molecules
• MN = ?
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Incomplete dominance
F1 hybrid phenotype is between the phenotypes of the
two parental varieties
Figure 14.10
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Dominance and Phenotype
• Dominant and recessive alleles
– Do not “interact”
– Different alleles = synthesis of different
proteins that produce a phenotype
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Dominance and Phenotype
• Dominant and recessive alleles
– Do not “interact”
– Different alleles = synthesis of different
proteins that produce a phenotype
• Ex: flower color
– White (W) vs. Red (R)
– W= protein that produces white pigment
– R = protein that produces red pigment
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Multiple Alleles
• Most genes exist in populations
– In more than two allelic forms
1
2
1
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2
3
• The ABO blood group in humans
– Is determined by multiple alleles:
• 3 different alleles for enzyme I
– IA = attaches the A carbohydrate
– IB = attaches the B carbohydrate
– i = attaches neither A nor B
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Table 14.2
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• Complex inheritance patterns
• Codominance
• Incomplete dominance
• Multiple alleles
– Mendel’s fundamental laws still apply!
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Concept 14.4:
• Human traits follow Mendelian patterns of
inheritance
• Humans = not convenient subjects for genetic
research
How can we study Human Genetics?
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Concept 14.4:
• Human traits follow Mendelian patterns of
inheritance
• Humans = not convenient subjects for genetic
research
How can we study Human Genetics?
= Pedigree analysis
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Male =
Female =
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Follow Attached earlobe = ff
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• Carriers?
• Disease condition = aa
• No disease symptoms = Aa or AA
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Mating of Close Relatives
• Mating between relatives
– Can increase the probability of the appearance
of a genetic disease
cc
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Cc
CC
Cc
Cc
• Albinism- recessive phenotype
• Only aa
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Albinism- recessive phenotype
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• Human achondroplasia phenotype
• The phenotype is determined by a dominant
allele = AA or Aa
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Human achondroplasia: Dominant allele disease
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PHENYLKETONURIA - [PKU]
pp
• Autosomal recessive disorder
• Gene for phenylalanine hydroxylase (PAH),
found on chromosome 12 mutated
• PAH converts the amino acid phenylalanine to
tyrosine
• No PAH = concentration of phenylalanine in
the body can build up to toxic levels
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PKU: Recessive disease (pp)
Pp
pp
Pp/PP
Pp
Pp
Pp
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Pp
Pp
Review!
• Mendel’s Pea experiments:
• Experimental method
• Typical Mendelian experiment:
– P, F1, F2
• Monohybrid cross vs. Dihybrid cross
• Law of Segregation and Law of Independent
assortment
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Review!
• Solving Multi-hybrid crosses with probability
• More complex inheritance patterns:
– Co-dominance, Incomplete dominance,
Multiple alleles
• Pedigree Analysis
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