Chapter 6 pp. 241 – 277
SBI 3U1
Section 6.1 (Part 1)
SBI 3U1
pp. 242 - 244
Beyond Mendel

With advanced technology, scientists
realized that patterns of inheritance are
more complicated than Mendel proposed

Patterns of inheritance observed by
scientists that were between dominant and
recessive…..
Incomplete Dominance

When neither allele for a gene completely
conceals the presence of the other;

Both alleles are EQUALLY DOMINANT,
producing a NEW PHENOTYPE (intermediate
expression of a trait, looks like a BLEND)
E.g. flower colour in snapdragon plant
 E.g. familial hypercholesterolemia in humans
p. 243 (top of textbook page)

Incomplete Dominance Example
 Cross
between a true-breeding red-flower
plant and a true-breeding white-flower plant
Red snapdragon
x
white snapdragon
CR CR
CWCW
CR
CR
CW CRCW CRCW
CW CRCW CRCW
-All pink (100%)
- A new
phenotype (blend)
-CRCW in F1
Incomplete Dominance Example
Incomplete Dominance Example
 Cross
between F1 offspring CRCW
pink snapdragon
CR C W
CR
CR CRCR
x
pink snapdragon
C R CW
CW
CRCW
CRCR : CRCW : CWCW
1 red : 2 pink : 1 white
CW
CWCR
CWCW
in F2
Incomplete Dominance Example
Codominance
 Both
dominant alleles are EQUALLY
EXPRESSED in a heterozygote at the
SAME TIME
 E.g.
Hair colour in cattle
 E.g. Sickle cell anemia in humans
(Heterozygote advantage, p. 244)
Codominance Example
 Cross
between a true-breeding red bull and
a true-breeding white cow
x White Cow  Roan Calf
Hw Hw
HrHw
Red Bull
HrHr
Hr
Hr
Hw
HrHw
HrHw
Hw
HrHw
HrHw
- All roan (100%)
- Some red
strands, some
white strands of
hair
- HrHw in F1
Codominance Example
HwHw
HrHw
HrHw
HrHr
HrHw
HrHw
SUMMARY – 3 Types of Dominance

Complete – ONLY ONE allele in the
genotype is seen in the phenotype

Incomplete – a MIXTURE of the alleles
in the genotype is seen in the phenotype

Co-Dominant – BOTH alleles in the
genotype are seen in the phenotype
To Do List
Section 6.1 (Part 1)

Learning Check p. 244 #1-6
Section 6.1 (Part 2)
SBI 3U1
pp. 245 - 250
RECALL:
 Mendel experimented with pea plants; only
2 possible alleles
 tall vs. short; round vs. wrinkled; yellow vs.
green; etc.
Multiple Alleles
 Many
traits in humans and other species
are the result of interactions of more than 2
alleles for one gene
 A gene
with more than two alleles is said to
have multiple alleles
 E.g.
Human Blood Group, Rabbit Coat
Colour, Eye Colour in Fruit flies, Human
Eye Colour
Multiple Alleles
Q: What happens when genes have multiple
alleles?
A: When there are multiple alleles for a given
characteristic, the alleles have a
DOMINANCE HIERARCHY
 Hierarchy
represented by capital letters
with superscript numbers or letters
Example 2: Rabbit Coat Colour
DOMINANCE HIERARCHY
Dominance Order
Genotype
AGOUTI
CCCC, CCCch, CCCh, CCCc
CHINCHILLA CchCch, CchCh, CchCc
HIMALAYAN ChCh, ChCc
ALBINO
CcCc
 Agouti
dominant over
other colours
Example 2: Rabbit Coat Colour
Dominance Order
AGOUTI
CHINCHILLA
HIMALAYAN
ALBINO
Genotype
CCCC, CCCch, CCCh, CCCc
CchCch, CchCh, CchCc
ChCh, ChCc
CcCc
Q:Cross between agouti CCCh & albino CcCc?
CC
Ch
F1 phenotypes
Cc
CCCc
ChCc
- 2/4 are agouti (CCCc)
- 2/4 are himalayan (ChCc)
Cc
CCCc
ChCc
Blood Type

A single gene determines a person’s blood type

This gene determines what type of antigen protein,
if any is attached to the cell membrane of RBC’s

What is an antigen protein?
 A molecule that stimulates the body’s immune system

Gene is designated, I and has 3 common alleles:
•IA , IB, i
Blood Type

4 different phenotypes (blood types), 6 genotypes

Type A  IAIA homozygotes, IAi heterzygotes

Type B  IBIB homozygotes, IBi heterzygotes

Type AB  IAIB heterzygotes

Type O  ii
Of the three alleles that determine blood type,
one (i) is recessive to the other two, and other
two (IA and IB) are codominant

Blood Type
 Presence
of allele IA, produces an ‘A
 Presence
of allele IB, produces a ‘B
antigen’
antigen’
 Presence
of allele IA and IB, produces
both ‘A antigen’ and ‘B antigen’
 Presence
antigen
of allele i, produces NO
Blood Type
Blood Type Example
Determine the possible phenotypes and genotypes of
the F1 generation offspring of a parent with type AB
blood and parent with type O blood.
(Parent #1)
IA
i
i
IA
i
IA i
IAIB x
ii (Parent #2)
IB
IBi
IBi
- 2/4 are Type A (IA i)
- 2/4 are Type B (IBi)
-Both are heterozygote
To Do List
Section 6.1 (Part 2)

Learning Check p. 247 #1-10
Section 6.1 (Part 3)
SBI 3U1
pp. 245 - 250
Environmental Effect on Inheritance
Environmental conditions often affect the
expression of traits.
E.g. Some genes are influenced by temperature.
Dark colour in Himalayan rabbits, is on the
cooler parts of their bodies: the face, ears, tails,
and feet.
The dark colouring is the result of a gene that is
only active below a certain temperature.
Environmental Effect on Inheritance
One way to study the effect of the environment on
expression of traits is to study genetically identical
organisms placed in different surroundings.
E.g. Identical twins are genetically identical.
Differences in the activity of their genes can be
due to environmental effects.
Continuous Variation
Continuous variation  a range of variation in
one trait resulting from the activity of many genes
These are traits for which the phenotypes vary
gradually from one extreme to another.
E.g. height and skin colour in humans, ear length
in corn, and kernel colour in wheat.
Continuous Variation
Continuous traits cannot be placed into discrete
categories because they vary over a continuum.
E.g. Height in humans varies over a wide range of
values. People cannot be categorized as only
short or tall.
Traits that exhibit continuous variation are usually
controlled by more than one gene and in some
cases, involve several genes.
Continuous Variation
Polygenetic Inheritance
Traits that are controlled by many genes are called
polygenic traits.
A group of genes that all contribute to the same
trait is called a polygene.
Each dominant allele contributes to the trait. Recessive
alleles do not contribute to the trait. For skin colour, the
more dominant alleles a person has, the darker their
skin.
Polygenetic Inheritance
To Do List
Section 6.1 (Part 3)

Review Questions p. 250 #1-10