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Jay Phelan
What Is Life? A Guide To Biology
First Edition
CHAPTER 7
Mendelian Inheritance
© 2010 W. H. Freeman and Company
INHERITANCE OF GENES
Maternal
chromosome pair
Paternal
chromosome pair
Gene
Maternal gamete: egg
Paternal gamete: sperm
Humans have
23 pairs of
chromosomes
(46 individual
chromosomes)
and, thus, two
copies of each
gene.
Each human
gamete has just
one copy of each
chromosome and,
thus, one copy of
each gene.
Gametes unite during fertilization.
Child inherits one set of
chromosomes from each
parent and, thus, two
copies of each gene.
FISH ODOR SYNDROME
Egg
Sperm
Normal
version
of FMO3
gene
Defective version of
the FMO3 gene,
responsible for fish
odor syndrome
Gametes unite during fertilization.
Genes for
FMO3
Child inherits one copy of
the defective version of the
FMO3 gene. The child is a
“silent carrier” and will not
have fish odor syndrome.
If two copies of the defective
version of FMO3 were
present, the child would
develop fish odor syndrome.

Once breeders recognized the
existence of heredity, selective
breeding—such as for increased
body size in cattle—became possible.
EXAMPLES OF SINGLE-GENE TRAITS
Cleft chin
Non-cleft chin
EXAMPLES OF SINGLE-GENE TRAITS
Unattached earlobes
Attached earlobes
EXAMPLES OF SINGLE-GENE TRAITS
Widow’s peak
Straight hairline
The mistaken idea that a tiny, pre-made human existed in
every sperm cell was introduced in the 1600s. This theory
remained popular through the 1800s.
MENDEL’S RESEARCH APPROACH
Gregor Mendel
(1822–1884)
Three features of Mendel’s
methodical research were
critical to its success.
DOMINANT AND RECESSIVE TRAITS
1
Mendel crossed
true-breeding
purple-flower
plants with
true-breeding
white-flower
plants.
True-breeding
purple-flower plant
True-breeding
white-flower plant
DOMINANT AND RECESSIVE TRAITS
1
Mendel crossed
true-breeding
purple-flower
plants with truebreeding whiteflower plants.
True-breeding
purple-flower plant
The purple-colored
flower is the
dominant trait,
while the whitecolored flower is a
recessive trait.
2
Then, Mendel
crossed two of
the purple-flower
offspring.
True-breeding
white-flower plant
All offspring have purple flowers.
DOMINANT AND RECESSIVE TRAITS
The purple-colored
flower is the dominant
trait, while the whitecolored flower is a
recessive trait.
All offspring have purple flowers.
2 Then, Mendel
crossed two of
the purpleflower offspring.
Most offspring have purple flowers,
but some have white flowers.

The recessive trait for the white-colored
flower must have been lurking in the previous
generation, even though it is not visible.
MENDEL’S LAW OF SEGREGATION
According to Mendel’s law of segregation, only one of the two
alleles for a gene is put into a gamete. At fertilization, offspring
receive from each parent one allele for each gene.
Heterozygous pea plant
Heterozygous pea plant
Two different alleles
(white, purple)
for the same gene
(flower color)
MEIOSIS Each gamete gets one copy of each gene.
MENDEL’S LAW OF SEGREGATION
FERTILIZATION Each fertilized egg gets two copies of each gene.
Homozygous
recessive
Heterozygous
Heterozygous
Homozygous
dominant
PHENOTYPE: Little or no pigment in the eyes, hair, and skin
GENOTYPE: Homozygous for the recessive allele for albinism
PUNNETT SQUARE: ALBINISM
A Punnett square is a useful tool for determining the possible
outcomes of a cross between two individuals.
Cross 1
MOTHER
albino
homozygous
aa
a
FATHER
pigmented
homozygous
AA
GAMETES
a
GAMETES
A
Aa
Aa
Aa
Aa
A
OFFSPRING Genotype
All heterozygous Aa
Phenotype
All pigmented
MOTHER
pigmented
heterozygous
Aa
Cross 2
A
FATHER
pigmented
heterozygous
Aa
GAMETES
a
GAMETES
A
AA
Aa
Aa
aa
a
OFFSPRING
Genotype
Phenotype
1/4 homozygous dominant AA
2/4 heterozygous Aa
1/4 homozygous recessive aa
3/4 pigmented
1/4 albino
GENETICS AND PROBABILITY
IF…
The mother is albino, and
the father is heterozygous
aa
Aa
100%
a
50%
a
1.0
A
×
THEN…
There is a 100% chance that
the mother’s egg will carry
the recessive a allele and a
50% chance that a sperm will
carry the recessive a allele
a
0.5
Multiply the two components together
to determine the overall probability.
AND…
= 0.5 or 50%
chance the
offspring will
be albino.
aa
TAY-SACHS AND PROBABILITY
IF…
Parents are heterozygous
for Tay-Sachs
Tt
Tt
50%
T
t
50%
T
THEN…
There is a 50% chance
that a gamete will carry
the recessive t allele
t
AND…
×
0.5
0.5
= 0.25 or 25%
chance the
child will be
homozygous
Multiply the two components together
for Tay-Sachs.
to determine the overall probability.
tt
TEST-CROSS: WHITE ALLIGATORS
TEST-CROSS: WHITE ALLIGATORS
MOTHER
white
homozygous
mm
Test-cross
FATHER
pigmented
unknown genotype
M_
(MM or Mm)
m
GAMETES
m
GAMETES
M
M
Unknown allele
could be M or m.
m
Mm
Mm
Mm
Mm
Mm
Mm
OFFSPRING (if unknown genotype is MM)
Genotype
Phenotype
All heterozygous Mm
All pigmented
OFFSPRING (if unknown genotype is Mm)
Genotype
Phenotype
2/4 heterozygous Mm
2/4 pigmented
2/4 homozygous recessive mm
2/4 white
PEDIGREE
A pedigree is a useful tool to document a trait of interest across
multiple generations of family members.
Grandfather
Aunt
First
cousin
Grandmother
Uncle
First
cousin
Grandfather
Mother
Father
Sister
Me
Grandmother
Uncle
Aunt
Sister
Female exhibiting trait of interest
Female not exhibiting trait
Male exhibiting trait of interest
Male not exhibiting trait
ANURY
Anury is a condition in dogs and some other animals in which
they are born without a tail. The condition is inherited as a
recessive trait.
ANURY
?
Can you figure out the genotype of the individual labeled “1”?
IF…
A dog has no
tail, but his
father
was normal
THEN…
The normallooking mother
(1) of the
tailless dog
must be
heterozygous.
PEDIGREE
Female with anury
1
Female with tail
Male with anury
Male with tail
?
?
Can you now determine
the probability of this
puppy having anury?
INCOMPLETE DOMINANCE: SNAPDRAGONS
Incomplete dominance occurs when a heterozygote exhibits an
intermediate phenotype between the two homozygotes.
Cross 1
The superscript W represents the allele
that produces no pigment (white
flower). The superscript R represents the
allele that produces pigment
(red flower).
MOTHER
white-flower
homozygous
CW CW
CW
FATHER
red-flower
homozygous
CRCR
GAMETES
CW
GAMETES
CR
CWCR
CWCR
CWCR
CWCR
CR
OFFSPRING Genotype
All heterozygous CWCR
Phenotype
All pink flowers
INCOMPLETE DOMINANCE: SNAPDRAGONS
Cross 2
CW
GAMETES
CR
CW
GAMETES
FATHER
pink-flower
heterozygous
CW CR
MOTHER
pink-flower
heterozygous
CW CR
CWCW
CWCR
CWCR
CRCR
CR
OFFSPRING Genotype
Phenotype
1/4 homozygous CRCR
1/4 red flowers
2/4 heterozygous CWCR
2/4 red flowers
1/4 homozygous CWCW
1/4 white flowers
CODOMINANCE: SICKLE-CELL DISEASE
Codominance occurs when a heterozygote displays characteristics of both
homozygous parents.
CODOMINANCE: SICKLE-CELL DISEASE
HbAHbA
Homozygote
HbSHbA
Heterozygote
HbSHbS
Homozygote
Individual does
not have sicklecell disease.
Individual carries a
defective allele and
has an intermediate
condition. Some
cells become sickled
under extreme
conditions.
Individual has two
copies of the
defective
hemoglobin allele
and has sickle-cell
disease.
MULTIPLE ALLELISM: BLOOD TYPE
Multiple allelism occurs when there are three or more alleles for a
gene within a population. An individual still inherits only two alleles—
one from each parent.
Three alleles possible:
A (dominant to O and codominant with B)
B (dominant to O and codominant with A)
O (recessive to A and B)
Gene that
determines
blood type
Type A
A
A
Type B
or
A
B
B
Type A
Type AB
A
B
O
O
or
O
B
O
Red blood cells have 6 different genotypes (AA, AO, BB,
BO, AB, and OO). These genotypes result in 4 different
phenotypes (type A, type B, type AB, and type O).
BLOOD TYPE, ANTIGENS, AND ANTIBODIES
Antigens are
chemicals on the
surface of some cells.
They act as signposts
that tell the immune
system whether the
cell belongs in the
body. Antibodies are
immune system
molecules that attack
cells with foreign
antigens.
Type A
A antigens
B antibodies
Type B
B antigens
A antibodies
Type AB
A and B
antigens
Neither A nor B
antibodies
Type O
Neither A
nor B
antigens
A and B
antibodies
THE SCIENCE BEHIND BLOOD DONATION
BLOOD TYPE
CAN DONATE TO
CAN RECEIVE FROM
Type A
Type A
Type AB
Type A
Type O
Type B
Type B
Type AB
Type B
Type O
• Has A antigens
• Produces antibodies that attack
B antigens
• Has B antigens
• Produces antibodies that attack
A antigens
Type AB
• Has A and B antigens
• Produces no antibodies
• Universal recipient
Type O
• Has no antigens
• Produces antibodies that attack
A and B antigens
• Universal donor
Type AB
Type A Type B

Type AB Type O
Type A Type B
Type AB Type O
Type O
Individuals with type O blood are universal donors.
Individuals with type AB are universal recipients.
POLYGENIC TRAITS
PLEIOTROPY: ONE GENE, MANY EFFECTS
Pleiotropy occurs when one gene influences multiple, different traits.
Heterozygote for the sickle-cell trait
An allele that causes cells to sickle has
two effects:
1
It disrupts red blood cells’ oxygen
delivery.
2
It causes red blood cells to be
inhospitable to malarial parasites.
PLEIOTROPY: ONE GENE, MANY EFFECTS
Someone with no sickled cells
does not have sickle-cell anemia
but is susceptible to malaria.
Malarial parasite
infecting a red
blood cell
SEX-LINKED TRAITS: COLOR-BLINDNESS
A sex-linked trait is carried on the X chromosome. Women carry two
copies of the X chromosome, while men carry an X chromosome and
a Y chromosome.
X
Gene with instructions
for light-sensitive
proteins within the eye
Two alleles possible:
R (produces functioning
light-sensitive proteins)
r (produces defective
light-sensitive proteins)
SEX-LINKED TRAITS: COLOR-BLINDNESS
TO BE COLOR-BLIND
X
r
X X
Y
Male must inherit colorblindness allele (r) from
his mother.
r
r
Female must inherit color-blindness
allele (r) from both parents.
SEX-LINKED TRAITS: COLOR-BLINDNESS
TO HAVE NORMAL VISION
X
R
X X
Y
Male must inherit normal
color-vision allele (R)
from his mother.
R
X X
r
R
Female can inherit normal colorvision allele (R) from either her
mother or her father.
R

Siamese cats and some rabbits have genes that
produce light fur at warmer temperatures and
dark fur at colder temperatures (such as on the
tail, nose, ears, and feet).
MENDEL’S LAW OF INDEPENDENT ASSORTMENT
Mendel’s law of independent assortment states that one trait does not
influence the inheritance of another trait.
MOTHER
albino
homozygous aa
dimpled chin
homozygous DD
Cross 1
a
FATHER
pigmented
homozygous AA
non-dimpled chin
homozygous dd
D
GAMETES
a
D
A
d
Aa
Dd
Aa
Dd
Aa
Dd
Aa
Dd
A
d
OFFSPRING
Genotype
All heterozygous Aa
All heterozygous Dd

Phenotype
All pigmented
All dimpled chin
In this example, having a dimpled chin does
not affect which alleles are inherited for skin
pigmentation.
MENDEL’S LAW OF INDEPENDENT ASSORTMENT
Mendel’s law of independent assortment states that one trait does not
influence the inheritance of another trait.
MOTHER
pigmented
heterozygous Aa
dimpled chin
heterozygous Dd
Cross 2
A
FATHER
pigmented
heterozygous Aa
dimpled chin
heterozygous Dd
D
GAMETES
a
D
A
D
AA
DD
Aa
Dd
Aa
Dd
aa
dd
aA
d
OFFSPRING
Genotype
1/4 homozygous dominant AA
1/4 homozygous dominant DD
2/4 heterozygous Aa
2/4 heterozygous Dd
1/4 homozygous recessive aa
1/4 homozygous recessive dd
Phenotype
3/4 pigmented
3/4 dimpled chin
1/4 albino
1/4 non-dimpled chin
LINKED GENES
If this gamete is fertilized, the linked genes will be
passed on to the offspring together as a group.
Homologous chromosomes
Linked genes
Maternal
copy
Paternal
copy
When crossing over occurs,
linked genes usually stay
together.

Homologous chromosomes Each of the four chromatids
after the exchange of genetic gets packaged into a gamete.
information
If genes are far apart on a chromosome or are on different chromosomes,
the inheritance of one does not influence the inheritance of another.