Mendel’s Peas were ideal for
learning about inheritance, but
they do not represent the norm…
• Traits in pea plants are determined by just
two alleles
• In peas, one allele is clearly dominant &
the other is clearly recessive
• However, things aren’t always this clearcut and simple in the world of genetics.
What if Mendel looked at mice?
 If a female black mouse and a male
white mouse were crossed, what will
the offspring look like?
- 100% are GREY
 If the F1 offspring were crossed, what
will there offspring look like?
- 25% black
- 50% grey
- 25% white
Incomplete Dominance
• A cross between two organisms with
different traits results in an offspring with
a third phenotype that is a blending of the
parental traits.
• It’s like mixing paints:
– Red + White = Pink
– Red does not totally block (dominate) white,
we end up with something in-between.
Inheritance in Snapdragons
Let’s look at cattle….
This cow resulted from a cross between a cow
with red fur and a cow with white fur.
This is called ‘roan’ fur; red & white fur together.
Codominance
• Similar to incomplete dominance in that
there is a 3rd phenotype
• In COdominance, the “recessive” and
“dominant” alleles appear together in the
phenotype of hybrid organisms.
• Red x White = red & white
Polygenic (Multifactorial) Traits
• Phenotype is determine by more than one
gene
• Often results in gradations, where each
gene has an additive effect
Ex) If 10 gene loci are turned on plant will be
20cm tall, if only 5 loci are turned on plant
will be 10cm tall
• Results in a bell-shaped curve
– Skin color & Height are examples in humans
Phenotype Distribution:Polygenic Traits
Multiple Alleles
• More than 2 alleles for a particular trait
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
chhc
ch,cCc
h
ch
AIbino:
Chinchilla:
Himalayan:
cc CC,
cc
c,hCc
, or
cch
c,hhor
cch
c
Full color:
, or
Cc
c = albino; no color;
recessive to all other
alleles
ABO Blood Types
• In addition to having multiple alleles, ABO
blood type also exhibits codominance
• ‘IA’ & ‘IB’ are codominant
• ‘i’ is recessive
ABO Blood Typing
Genotype
IA IA or IA i
IB IB or IB i
IAIB
ii
Blood Type
What does your ABO blood type mean?
 Remember the ‘flags’
on our cell membranes?
They help cells to
recognize each other.
 Some of those flags
‘announce’ your blood
type. We call these
flags antigens
Blood
Type
A
B
AB
O
Antigen
ABO Blood Transfusions
Blood
Type
A
B
AB*
O*
Can receive:
Summary of ABO Blood Types
Rh Factor
• Blood can also be categorized as + or –
• This refers to the presence (dominant) or
absence (recessive) of the Rh antigen
Phenotype Genotype(s)
Rh+
Rh-
Antigen
Chromosomal Theory of Inheritance
• This theory states that genes occupy specific loci on
chromosomes and it is the chromosomes that undergo
segregation and independent assortment during meiosis.
Gene Linkage & Mapping Chromosomes
• According to the chromosomal theory of inheritance, genes on the
same chromosome are more likely to be inherited together
• Crossing over helps to increased variation, but the closer two
genes are on a chromosome the more likely they are to be “linked”
• The frequency of crossing over between two genes can be used to
estimate the relative positions of genes on chromosomes
Sex Chromosomes & Autosomes
• Recall that two of the 46 human chromosomes
are known as sex chromosomes, because they
determine the individual’s sex.
– Females have two copies of an X chromosome.
– Males have one X chromosome and one Y
chromosome.
• The remaining 44 chromosomes are known as
autosomal chromosomes or autosomes.
Sex-Linked Genes
• Located on one of the
sex chromosomes
(X or Y)
• Since the X
chromosome is
longer, it has many
genes not found on
the Y chromosome.
• Most sex-linked
genes are X-linked
genes.
Sex-Linked Genes
X-Linked Inheritance
• Examples:
– Hemophilia
• The protein necessary for normal blood clotting is missing
– Colorblindness
• Defective version of one or all of the 3 genes responsible for
color vision
– Male Pattern Baldness
• Hair loss
– Duchenne Muscular Dystrophy
• Weakening and loss of skeletal muscle
• These traits are recessive & more common in males.
Why?
Possible Inheritance of Colorblindness Allele
Complete the following sex-linked crosses:
Eva and Paul just had a son,
Michael. Paul is has normal
color vision, but Eva’s father
was colorblind. What is the
likelihood that Michael is
colorblind?
Laura and Steve are expecting
their first child. They are
concerned about the chances
their child might be
hemophiliac because both
Steve and Laura’s father are
hemophiliac. What is the
probability of Laura and Steve
having a hemophiliac child?
Complete the following pedigrees.
Which is for a sex-linked trait? How do you know?