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BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
Chapter 14
Mendel and the Gene Idea
From PowerPoint® Lectures for Biology: Concepts & Connections
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Gregor Mendel
• Gregor Mendel is the father of modern genetics.
• He was an Austrian monk from the 1860s.
Stamen
Carpel
Figure 9.2A, B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel bred pea
plants and studied
how their traits
changed
depending on the
parent plants.
White
1
Stamens
Carpel
PARENTS
(P)
• A character is a
heritable feature
like flower color
• A trait is a
variation of a
character (purple
or white)
Removed
stamens
from purple
flower
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
OFFSPRING
(F1)
Figure 9.2C
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Planted
seeds
from pod
• Mendel studied
seven pea
characteristics
FLOWER
COLOR
Purple
White
Axial
Terminal
SEED
COLOR
Yellow
Green
SEED
SHAPE
Round
Wrinkled
POD
SHAPE
Inflated
Constricted
POD
COLOR
Green
Yellow
STEM
LENGTH
Tall
Dwarf
FLOWER
POSITION
• True-breeding
plants make
offspring that are
the same as the
parents every time
Figure 9.2D
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.3 Mendel’s principle of segregation describes the
inheritance of a single characteristic
• From his
experimental data,
Mendel deduced
that an organism
has two genes
(alleles) for each
inherited
characteristic
– One characteristic
comes from each
parent!!!
Figure 9.3A
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P GENERATION
(true-breeding
parents)
Purple flowers
White flowers
All plants have
purple flowers
F1
generation
Fertilization
among F1
plants
(F1 x F1)
F2
generation
3/
of plants
have purple flowers
4
1/
4 of plants
have white flowers
GENETIC MAKEUP (ALLELES)
• A sperm or egg
carries only one
allele of each pair
P PLANTS
Gametes
– The 2 alleles for a
gene separate
during gamete
formation, and each
gamete gets a
different one
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
All Pp
1/
2
1/
P
P
2
p
P
Eggs
Sperm
PP
– This is the law of
segregation
F2 PLANTS
Phenotypic ratio
3 purple : 1 white
p
p
Pp
Pp
pp
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Law of Independent Assortment
• Another law Mendel discovered is the Law of
Independent Assortment which says that each
allele segregates independently from another
(traits aren’t linked unless they are on the same
chromosome)
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Dominant and Recessive
• A dominant allele is represented by a CAPITAL
letter. It is always expressed when present.
(BB or Bb)
• A recessive allele is represented by lower case
letter. It is only expressed when an individual
has 2, one from mom and one from dad. (bb
only)
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Genotype and Phenotype
• A genotype is the genetic make-up of an
individual, expressed in letters. (BB, Bb, bb)
• A phenotype is the physical appearance of an
individual, determined by his or her genotype.
(black, brown, short, tall, etc)
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Homozygous and Heterozygous
• Homozygous: when both alleles of a genotype
are the same (either both dominant, BB, or
both recessive, bb)
• Heterozygous: when one allele is dominant and
one is recessive (Bb only)
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Punnett Squares
• Punnett Squares are used to show the mating
of two parents and the possible offspring they
can produce.
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Monohybrid crosses
• When we are looking at one trait (such as eye
color or height), we do a monohybrid cross.
• Parents are called P
• Children are called F1
• Grandchildren are called F2
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers (P) are dominant to white flowers, p
• How would we represent a homozygous purple flower?
• PP
• A heterozygous flower?
• Pp
• What is the phenotype of a flower that is Pp?
• Purple
• What is the genotype of a white flower?
• pp
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Purple flowers, P, are dominant to white, p.
• Show a Punnett Square crossing a homozygous
purple flower with a heterozygous purple
flower.
• PP x Pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers, P, are dominant to white, p.
• Show a Punnett Square crossing two
heterozygous flowers.
• Pp x Pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers, P, are dominant to white, p.
• Show a Punnett Square crossing a homozygous
purple and homozygous white flower.
• PP x pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers, P, are dominant to white, p.
• Cross a heterozygous plant with a white plant.
• Pp x pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Purple flowers, P, are dominant to white, p.
• Cross two white plants.
• pp x pp
• What are the genotypic and phenotypic ratios?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Practice
• What color is PP? Pp? pp?
• What is the phenotype of Pp?
• What is the genotype of a white flower?
• What is the genotype of a heterozygous flower?
• What is the phenotype of a heterozygous flower?
• What is the phenotype of a homozygous recessive flower?
• What is the genotype of a homozygous dominant flower?
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Incomplete Dominance
• In incomplete dominance, the heterozygous
genotype produces a phenotype that is in
between the dominant and recessive ones.
• For example, if RR makes red flowers, and rr
makes white flowers, then Rr makes PINK
flowers (instead of red).
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Practice: Red flowers show incomplete dominance.
RR is red, Rr is pink, and rr is white.
• Cross two pink flowers. Show a Punnett square
and genotypic and phenotypic ratios of the
offspring.
• Rr x Rr
• Cross a red flower with a white flower.
• RR x rr
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Blood types
• There are 4 possible blood types:
A, B, AB, and O.
• A = iAiA (homozygous A) or iAiO (heterozygous A)
• B= iBiB (homozygous B) or iBiO (heterozygous B)
• AB = iAiB (always heterozygous)
• O = iOiO (always homozygous)
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Practice.
• 1. Cross a woman with AB blood with a man
with O blood.
• 2. Cross a woman with homozygous A blood
with a man with AB blood.
• 3. Cross a woman with heterozygous B blood
with a man with heterozygous A blood.
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Dihybrid Crosses
• Instead of crossing just one trait, dihyrbrid
crosses show the crossing of two separate
traits.
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Dihybrid examples: Dark (D) is dominant to light
(d) and Brown (B) is dominant to blond (b).
• 1. Cross two heterozygous individuals.
• 2. Cross a heterozygous individual with one
who is light and heterozygous for brown.
• 3. Cross a homozygous dark, heterozygous
brown with a heterozygous dark, blond
individual.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Rule of Multiplication
• If you flip two coins at once, the probability
that both end up heads is ½ x ½ = ¼
• Cross PpYyRr x Ppyyrr and calculate the
probability of getting at least 2 recessive traits
•
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Sex-linked diseases
• Any gene located on a sex chromosome is
called a sex-linked gene.
• Examples include color blindness, baldness,
hemophilia, and muscular dystrophy.
• These recessive diseases usually affect men
more than women.
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– This is because the bad genes are carried on the
X. Men only have one X, so one bad gene means
they have the condition.
– Women have 2 Xs, so if only one has it they are
only carriers.
– XRXR is a normal female
– XRXr is a female who is a carrier
– XrXr is an affected female
– XRY is a normal male
– XrY is an affected male
R = red-eye allele
r = white-eye allele
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.22B-D
Practice: color blindness is a recessive sex linked
disease. Use the letters B and b for these
problems:
• 1. Cross a woman who is a carrier with a
normal man.
• 2. Cross a normal woman with a colorblind
man.
• 3. Cross a woman who is a carrier with a color
blind man.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-linked recessive inherited disorders include:
• Color blindness
• Hemophilia (excessive bleeding)
• Muscular dystrophy (weak muscles)
• Male patterned baldness
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Pedigrees
• Pedigrees are used to trace traits through a
family tree.
• Circles are girls, squares are boys.
• Filled in circles and squares represent
individuals affected by a disease.
• A horizontal line connecting the symbols
represents marriage, vertical lines represent
offspring.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Turn to the pedigree on the back of your handout!
• The filled in circles and squares have attached
earlobes.
• Let’s list the genotypes and phenotypes of all
the individuals that we can.
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Everything that’s left over…
• A testcross is done to figure out an unknown
genotype. The mystery genotype is crossed
with a homozygous recessive individual.
• A carrier is heterozygous for a disease but does
not show symptoms. They CAN pass it on to
offspring.
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Other types of inheritance
• Pleiotropy: Genes can affect more than one
phenotype (sickle-cell and malaria)
• Epistasis: One gene affects how a second gene
is expressed
• Polygenic Inheritance: Many genes affect one
phenotype (skin color)
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Recessive inherited diseases include…
• Deafness
• Albinism
• Cystic fibrosis (excess mucus in lungs)
• PKU (mental retardation)
• Sickle Cell Disease (red blood cell damage)
• Tay Sachs (lipid accumulation in brain)
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Dominant inherited diseases include…
• Dwarfism
• Alzheimer’s
• Huntington’s Disease (mental deterioration
and tremors starting in middle age)
• High Cholesterol
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Fetal Screening for genetic diseases
• Least invasive is ultrasound, which uses sound
waves to produce a picture of a fetus.
• In chorionic villus sampling (CVS) a narrow
tube is inserted into the uterus to suction off
fetal tissue, which is used for karyotyping.
• The most invasive is amniocentesis, in which a
needle is used to withdraw amniotic fluid to
test for genetic disorders.
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings