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CHAPTER 9
GENETICS
MENDEL’S LAWS
Copyright © 2009 Pearson Education, Inc.
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
Mendel’s law of segregation describes the inheritance
of a single character
 Example of a monohybrid (HETEROZYGOUS) cross
• Parental generation (HOMOZYGOUS): purple flowers 
white flowers
• F1 generation: all plants with purple flowers
• F2 generation: ¾ of plants with purple flowers
¼ of plants with white flowers
 Mendel needed to explain
• Why one trait seemed to disappear in the F1 generation
• Why that trait reappeared in one quarter of the F2
offspring
Copyright © 2009 Pearson Education, Inc.
P generation
(true-breeding
parents)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 ´ F1)
F2 generation
3
–
4
of plants
have purple flowers
1
– of
4
plants
have white flowers
Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
1. Genes are found in alternative versions called ALLELES; a
GENOTYPE is the listing of alleles an individual carries for
a specific gene
2. For each characteristic, an organism inherits two alleles,
one from each parent; the alleles can be the same or
different
– A homozygous genotype has identical
alleles
– A heterozygous genotype has two
different alleles
Copyright © 2009 Pearson Education, Inc.
Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
3. If the alleles differ, the DOMINANT allele determines the
organism’s appearance, and the RECESSIVE allele has no
noticeable effect
– The PHENOTYPE is the appearance or expression of
a trait
– The same phenotype may be determined by more
than one GENOTYPE (the alleles carried by the
organism)
4. Law of segregation: Allele pairs separate (segregate) from
each other during the production of gametes (MEIOSIS)
so that a sperm or egg carries only one allele for each
gene
Copyright © 2009 Pearson Education, Inc.
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1
–
2
1
–
2
P
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
–
2
1
–
2
1
–
2
B
B
B
b
B
B
1
–
4
1
–
4
1
–
2
b
b
B
1
–
4
F2 genotypes
b
b
b
1
–
4
COIN TOSS DEMONSTRATION
• 4 students toss coin once…RESULTS?
• 4 students toss coin 3 times…RESULTS?
• All students toss coin once…RESULTS?
• All students toss coin 3 times…RESULTS?
Homologous chromosomes bear the alleles for
each character
 For a pair of homologous chromosomes,
alleles of a gene reside at the same locus
• Homozygous individuals have the
same allele on both homologous
chromosome
• Heterozygous individuals have a
different allele on each homologous
chromosome
Copyright © 2009 Pearson Education, Inc.
Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Bb
PP
aa
Homozygous
Heterozygous
Homozygous
for the
for the
dominant allele recessive allele
HUMAN EXAMPLES
of
Dominant and Recessive Genes
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
CLASSROOM EXAMPLE
• How many students have free earlobes?
• How many students have attached earlobes?
• Which trait (phenotype) is caused by the
DOMINANT gene?
CLASSROOM EXAMPLE (cont.)
• Free earlobe gene = F
• Attached earlobe gene = f
• What are the possible GENOTYPES for earlobe
shape in the human population?
• What PHENOTYPES are produced by the
different genotypes?
CLASSROOM EXAMPLE (cont.)
• A genetics problem:
If Mary has attached earlobes and John has free
earlobes (John’s mother has attached
earlobes) what are the chances that their
children will have attached earlobes? Free
earlobes?
Use a PUNNETT SQUARE to prove your answer!
CLASSROOM EXAMPLE
1.) Determine the possible genotypes for Mary
and John.
2.) Create a PUNNETT SQUARE.
3.) Determine the PHENOTYPES of the children.
CONNECTION: Genetic traits in humans can be
tracked through family pedigrees
 A pedigree
• Shows the inheritance of a trait in a
family through multiple generations
• Demonstrates dominant or recessive
inheritance
• Can also be used to deduce genotypes
of family members
Copyright © 2009 Pearson Education, Inc.
First generation
(grandparents)
Ff
Ff
Second generation
(parents, aunts,
and uncles)
FF
or
ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Third generation
(two sisters)
Female
Male
Ff
Attached
Free
Ff
ff
MORE GENETICS PROBLEMS
• See LAB MANUAL…
The law of independent assortment is revealed by
tracking two characters at once
 Example of a dihybrid (HETEROZYGOUS) cross
• Parental generation: round yellow seeds  wrinkled green
seeds
• F1 generation: all plants with round yellow seeds
• F2 generation:
9/16 of plants with round yellow seeds
3/16 of plants with round green seeds
3/16 of plants with wrinkled yellow seeds
1/16 of plants with wrinkled green seeds
 Mendel needed to explain
• Why nonparental combinations were observed
• Why a 9:3:3:1 ratio was observed among the F2 offspring
Copyright © 2009 Pearson Education, Inc.
The law of independent assortment is revealed by
tracking two characters at once
 Law of independent assortment
• Each pair of alleles segregates
independently of the other pairs of alleles
during gamete formation
• R = round; r = wrinkled
Y = yellow; y = green
• For genotype RrYy, four gamete types are
possible: RY, Ry, rY, and ry
Copyright © 2009 Pearson Education, Inc.
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
1
–
4
ry
1
–
4
RY
Eggs
1
–
2
RY
1
–
4
ry
Hypothesized
(not actually seen)
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RY
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
rY
Eggs
1
–
4
1
–
4
9
––
16
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
R
Y
r
y
Metaphase I
of meiosis
(alternative
arrangements)
r
R
Y
y
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
r
R
y
Y
R
Y
Metaphase I
of meiosis
(alternative
arrangements)
r
R
Y
y
r
Anaphase I
of meiosis
y
R
r
Y
y
Metaphase II
of meiosis
r
R
Y
y
r
R
Y
y
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
r
R
y
Y
R
Y
y
Y
y
R
R
Y
y
Anaphase I
of meiosis
r
Y
R
r
R
Y
Metaphase I
of meiosis
(alternative
arrangements)
r
Metaphase II
of meiosis
r
Y
y
r
R
Y
y
y
Y
Y
r
r
r
1
– ry
4
1
– rY
4
Fertilization among the F1 plants
F2 generation
R
Gametes
y
1
– RY
4
r
9
:3
:3
:1
y
y
R
R
1
–
4
Ry
QUESTION…
• Why would genes independently assort even
if they are on the same chromosome???
HINT: Think Prophase I (meiosis)…
R
Y
R
y
r
Y
r
y
R Y
r
Tetrad
y
Crossing over
Gametes
CONNECTION: Many inherited disorders in humans are controlled by a single gene
 Inherited human disorders show
• Recessive inheritance
– Two recessive alleles are needed to show disease
– Heterozygous parents are carriers of the diseasecausing allele
– Probability of inheritance increases with inbreeding,
mating between close relatives
• Dominant inheritance
– One dominant allele is needed to show disease
– Dominant lethal alleles are usually eliminated from
the population
Copyright © 2009 Pearson Education, Inc.
EXAMPLES of HUMAN DISORDERS
The next slide lists a number of Recessive and
Dominant AUTOSOMAL disorders…
AUTOSOMAL means that the defective gene is
found on one of the autosomes.
AUTOSOMES = Chromosomes that are NOT one
of the sex chromosomes
SEX Chromosomes = The X or Y chromosomes
that determine the sex of the human
XX = Female
XY = Male
Genetics Problem
• Cystic Fibrosis (Autosomal Recessive):
• If Mary and John are both heterozygous
(carriers) for the cystic fibrosis gene, what are
the chances that their children will have cystic
fibrosis???
Parents
Normal
Carrier
Cc
Normal
Carrier
Cc
´
Sperm
Offspring
C
c
C
CC
Normal
Cc
Normal
(carrier)
c
Cc
Normal
(carrier)
Eggs
cc
Cystic
Fibrosis
Genetics Problem
• Huntington’s (Autosomal Dominant):
• Only Genotypes = Hh (Huntington’s) or hh
(normal)
• If Mary is normal and John will eventually
develop Huntington’s disease, what are the
chances that their children will develop
Huntington’s disease???
Parents
Mary
hh
John
Hh
´
Sperm
H
h
Hh
Huntington’s
hh
Normal
Hh
Huntington’s
hh
Normal
h
Offspring
Eggs
h
VARIATIONS ON MENDEL’S
LAWS
Copyright © 2009 Pearson Education, Inc.
Incomplete dominance results in intermediate
phenotypes
 Incomplete dominance
• Neither allele is dominant over the
other
• Expression of both alleles is observed
as an intermediate phenotype in the
heterozygous individual
Copyright © 2009 Pearson Education, Inc.
Flower Color in Plants…
P generation
Red
RR
White
rr
r
R
Gametes
F1 generation
Pink
Rr
Gametes
1
–
2
R
1
–
2
r
Sperm
1
–
2
F2 generation
R
1
–
2
r
1
–
2
R
RR
rR
1
–
2
r
Rr
rr
Eggs
Hypercholesterolemia in Humans…
Genotypes:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
Sickle Cell Anemia in Humans
• HH = Normal Hemoglobin
• Hh = Sickle Cell Trait
• hh = Sickle Cell Anemia
The sickle cell gene also affects many phenotypic
characters
 Pleiotropy
• One gene influencing many characteristics
• The gene for sickle cell disease
– Affects the type of hemoglobin produced
– Affects the shape of red blood cells
– Causes anemia
– Causes organ damage
– Is related to susceptibility to malaria
Copyright © 2009 Pearson Education, Inc.
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Genetics Problem: Sickle Cell
Anemia
• If Mary has normal hemoglobin and John has
sickle cell anemia, what are the chances that
their children will have sickle cell anemia? Will
have sickle cell trait? Will have normal
hemoglobin?
Sickle Cell Anemia Problem (cont.)
• What is Mary’s genotype?
• What is John’s genotype?
• Create the Punnett Square…
Many genes have more than two alleles in the
population: MULTIPLE ALLELES
 Multiple alleles
• More than two alleles are found in the
population
• A diploid individual can carry any two of
these alleles
• The ABO blood group has three alleles,
leading to four phenotypes: type A, type B,
type AB, and type O blood
Copyright © 2009 Pearson Education, Inc.
CODOMINANCE
 Codominance
• Neither allele is dominant over the
other
• Expression of both alleles is observed
as a distinct phenotype in the
heterozygous individual
• Observed for type AB blood
Copyright © 2009 Pearson Education, Inc.
Blood
Group
(Phenotype) Genotypes
Red Blood Cells
Antibodies
Present in
Blood
Anti-A
Anti-B
O
ii
A
I AI A
or
I Ai
Carbohydrate A
Anti-B
B
IBIB
or
IBi
Carbohydrate B
Anti-A
AB
IAIB
—
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
GENETICS PROBLEMS
• ABO BLOOD TYPE…See Lab Manual
A single character may be influenced by many
genes: POLYGENIC INHERITANCE
 Polygenic inheritance
• Many genes influence one trait
• Skin color is affected by at least
three genes
Copyright © 2009 Pearson Education, Inc.
P generation
aabbcc
AABBCC
(very light) (very dark)
F1 generation
AaBbCc
AaBbCc
Sperm
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
F2 generation
1
–
8
1
–
8
1
–
8
1
–
8
Fraction of population
Eggs
20
––
64
1
–
8
1
–
8
1
–
8
1
–
8
15
––
64
6
––
64
1
––
64
1
––
64
6
––
64
15
––
64
20
––
64
15
––
64
6
––
64
1
––
64
Skin color
The environment affects many characters
 Phenotypic variations are influenced by the
environment
• Skin color is affected by exposure to
sunlight
• Susceptibility to diseases, such as cancer,
has hereditary and environmental
components
Copyright © 2009 Pearson Education, Inc.
SEX CHROMOSOMES
AND
SEX-LINKED GENES
Copyright © 2009 Pearson Education, Inc.
Chromosomes determine sex in many species
 X-Y system in mammals (HUMANS), fruit flies
• XX = female; XY = male
 X-O system in grasshoppers and roaches
• XX = female; XO = male
 Z-W in system in birds, butterflies, and some fishes
• ZW = female, ZZ = male
 Chromosome number in ants and bees
• Diploid = female; haploid = male
Copyright © 2009 Pearson Education, Inc.
X
Y
GENETICS PROBLEM
• What are the chances that Mary and John’s
children will be Boys? Girls?
• Mary = XX
• John = XY
• Create a PUNNETT SQUARE…
CONNECTION: Sex-linked disorders affect mostly
males
 Males express X-linked disorders such as
the following when recessive alleles are
present in one copy
• Hemophilia
• Colorblindness
• Duchenne muscular dystrophy
Copyright © 2009 Pearson Education, Inc.
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
GENETICS PROBLEMS
• SEX LINKED TRAITS…See Lab Manual