Observable Patterns of
Inheritance
Chapter 12
Key Concepts
• What is a gene and how does it influence
appearance or inheritance of traits?
• What is an allele? How do we locate a gene on a
chromosome?
• What evidence is there of assortment and
reassortment of genes in Mendelian genetics?
• What is a dominant versus a recessive trait/gene?
• Determine whether a gene is dominant or recessive
in a biological monohybrid cross.
• Is there truly clear lines between dominant and
recessive traits?
• How doe the environment affect all of the biological
traits? Nature versus Nuture
• Complete a monohybrid and a dihybrid cross.
Determine genotypic ratios and phenotypic ratios.
Earlobe Variation
• Whether a person has attached or
detached earlobes depends on a single
gene
• Attached earlobes: two copies of the
recessive allele for this gene
• Detached earlobes: either one or two
copies of the dominant allele
Early Ideas about Heredity
• People knew that sperm and eggs
transmitted information about traits
• Blending theory
• Problem:
– Would expect variation to disappear
– Variation in traits persists
Gregor Mendel
• Strong background
in plant breeding
and mathematics
• Using pea plants,
found indirect but
observable
evidence of how
parents transmit
genes to offspring
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a
chromosome
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele that is paired with it
Allele Combinations
• Homozygous
– having two identical alleles at a locus
– AA or aa
• Heterozygous
– having two different alleles at a locus
– Aa
Genetic Terms
A pair of homologous
chromosomes
Figure 11.4
Page 179
A gene locus
A pair of alleles
Three pairs of genes
Figure 11.4
Page 179
Genotype & Phenotype
• Genotype refers to particular genes an
individual carries
• Phenotype refers to an individual’s
observable traits
• Cannot always determine genotype by
observing phenotype
Tracking Generations
• Parental generation
mates to produce
P
• First-generation offspring
mate to produce
F1
• Second-generation offspring
F2
Monohybrid Crosses
Experimental intercross between
two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa
?
Mendel’s
Monohybrid
Cross Results
F2 plants showed
dominant-torecessive ratio that
averaged 3:1
Figure 11.5
Page 180
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf
Probability
The chance that each outcome of a
given event will occur is proportional to
the number of ways that event can be
reached
Monohybrid
Cross
Illustrated
True-breeding
homozygous recessive
parent plant
F1
PHENOTYPES
aa
True-breeding
homozygous dominant
a
parent plant
Aa
Aa
Aa
Aa
a
A
Aa
Aa
A
Aa
Aa
AA
An F1 plant
self-fertilizes
and produces
gametes:
F2
PHENOTYPES
Aa
A
AA
Aa
Aa
aa
a
A AA Aa
Figure 11.7
Page 181
a
Aa
aa
Mendel’s Theory
of Segregation
• An individual inherits a unit of
information (allele) about a trait from
each parent
• During gamete formation, the alleles
segregate from each other
Test Cross
• Individual that shows dominant
phenotype is crossed with individual
with recessive phenotype
• Examining offspring allows you to
determine the genotype of the dominant
individual
Punnett Squares of
Test Crosses
Homozygous
recessive
a
a
Homozygous
recessive
a
a
A
Aa
Aa
A
Aa
Aa
a
aa
aa
A
Aa
Aa
Two phenotypes
All dominant phenotype
Dihybrid Cross
Experimental cross between individuals
that are homozygous for different
versions of two traits
Dihybrid Cross: F1 Results
purple
flowers,
tall
TRUEBREEDING
PARENTS:
AABB
GAMETES:
AB
x
white
flowers,
dwarf
aabb
AB
ab
ab
AaBb
F1 HYBRID
OFFSPRING:
Figure 11.9 (1)
Page 183
All purple-flowered, tall
Dihybrid Cross: F2 Results
AaBb X AaBb
1/4 AB 1/4 Ab 1/4 aB
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
1/4 ab
1/16
AABB
1/16
AABb
1/16
AaBB
1/16
AaBb
1/16
AABb
1/16
AAbb
1/16
AaBb
1/16
Aabb
1/16
AaBB
1/16
AaBb
1/16
aaBB
1/16
aaBb
1/16
AaBb
1/16
Aabb
1/16
aaBb
1/16
aabb
9/16 purple-flowered, tall
3/16 purple-flowered, dwarf
3/16 white-flowered, tall
1/16 white-flowered, dwarf
Figure 11.9(2)
Page 183
Independent Assortment
• Mendel concluded that the two “units”
for the first trait were to be assorted into
gametes independently of the two
“units” for the other trait
• Members of each pair of homologous
chromosomes are sorted into gametes
at random during meiosis
Independent Assortment
Metaphase I:
A
A a
a
B
B b
b
OR
A
A a
a
b
b B
B
Metaphase II:
Gametes:
A
A
a
a
A
A
a
a
B
B
b
b
b
b
B
B
B
A
B
A
1/4 AB
b
a
b
a
1/4 ab
b
A
b
A
1/4 Ab
B
a
B
a
1/4 aB
Impact of Mendel’s Work
• Mendel presented his results in 1865
• Paper received little notice
• Mendel discontinued his experiments in
1871
• Paper rediscovered in 1900
Dominance Relations
Complete dominance
Incomplete dominance
Codominance
Incomplete
Dominance
X
Incomplete
Homozygous
Homozygous
parent
parent
Dominance
All F1 are
heterozygous
X
Figure 11.10
Page 184
F2 shows three phenotypes in 1:2:1 ratio
Codominance: ABO Blood Types
• Gene that controls ABO type codes for
enzyme that dictates structure of a
glycolipid on blood cells
• Two alleles (IA and IB) are codominant
when paired
• Third allele (i) is recessive to others
ABO Blood Type:
Allele Combinations
Range of genotypes:
Blood
types:
IA IA
IB IB
or
or
IA i
IA IB
IB i
ii
A
AB
B
O
Figure 11.11
Page 184
ABO and Transfusions
• Recipient’s immune system will attack
blood cells that have an unfamiliar
glycolipid on surface
• Type O is universal donor because it
has neither type A nor type B glycolipid
Epistasis
• Interaction between the products of
gene pairs
• Common among genes for hair color in
mammals
Coat Color
in
Retrievers
BBEE
X
bbee
F1 puppies
are all BbEe
F2 puppies
Figure 11.13
Page 186
BE
Be
bE
be
BE
BBEE
BBEe
BbEE
BbEe
Be
BBEe
BBee
BbEe
Bbee
bE
BbEE
BbEe
bbEE
bbEe
be
BbEe
Bbee
bbEe
bbee
black
brown
yellow
Continuous Variation
• A more or less continuous range of
small differences in a given trait among
individuals
• The greater the number of genes and
environmental factors that affect a trait,
the more continuous the variation in
versions of that trait
Human Variation
• Some human traits occur as a few
discrete types
– Attached or detached earlobes
– Many genetic disorders
• Other traits show continuous variation
– Height
– Weight
– Eye color
(line of bell-shaped curve indicates
continuous variation in population)
Range of values for the trait
Number of individuals with
some value of the trait
Number of individuals with
some value of the trait
Describing Continuous
Variation
Range of values for the trait
Temperature Effects
on Phenotype
• Rabbit is homozygous for
an allele that specifies a
heat-sensitive version of an
enzyme in melaninproducing pathway
• Melanin is produced in
cooler areas of body
Figure 11.18
Page 190
Environmental Effects on
Plant Phenotype
• Hydrangea macrophylla
• Action of gene responsible for floral
color is influenced by soil acidity
• Flower color ranges from pink to blue