SBI3U – Unit 2: Genetics
Date: ____________________
6.1 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
______________________________________________________________________________
Incomplete Dominance
 When neither allele for a gene completely conceals the presence of the other;
 Both alleles are ___________________________________, producing a
___________________________ (intermediate expression of a trait, looks like a ____________)
 E.g. flower colour in snapdragon plant
 E.g. familial hypercholesterolemia in humans p. 243 (top of textbook page)
Example 1:
Cross between a true-breeding red-flower plant and a true-breeding white-flower plant
Red snapdragon
x
CRCR
white snapdragon
CWCW
Cross between F1 offspring CRCW
pink snapdragon
CRCW
x
pink snapdragon
CRCW
SBI3U – Unit 2: Genetics
Date: ____________________
Codominance
 Both dominant alleles are ______________________________ in a heterozygote at the
______________________
 E.g. Hair colour in cattle
 E.g. Sickle cell anemia in humans (Heterozygote advantage, p. 244)
Example 2:
Cross between a true-breeding red bull and a true-breeding white cow
Red Bull
HrHr
x
White Cow  Roan Calf
HwHw
Example of Codominance – Colours in Cows and Bulls
Section 6.1 (Part 1) - Learning Check p. 244 #1-6
HrHw
SBI3U – Unit 2: Genetics
Date: ____________________
RECALL:
 Mendel experimented with pea plants; only 2 possible alleles
 tall vs. short; round vs. wrinkled; yellow vs. green; etc.
What if there were multiple alleles?????
 Many traits in humans and other species are the result of interactions of ___________________
____________________________________________________
 A gene with more than two alleles is said to have __________________________
 E.g.
______________________________________________________________________________
______________________________________________________________________________
Q: What happens when genes have multiple alleles?
A: When there are multiple alleles for a given characteristic, the alleles have a
_____________________________________________
 Hierarchy represented by capital letters with superscript numbers or letters
Example 3: Rabbit Coat Colour
DOMINANCE HIERARCHY
Dominance Order
Genotype
C C
AGOUTI
C C , CCCch, CCCh, CCCc
CHINCHILLA
CchCch, CchCh, CchCc
HIMALAYAN
ChCh, ChCc
ALBINO
CcCc
 Agouti dominant over other colours
Q: Cross between agouti CCCh & albino CcCc?
F1 phenotypes:
SBI3U – Unit 2: Genetics
Date: ____________________
Blood Types
 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
__________________________________
 What is an antigen protein?
 _____________________________________________________________________
 Gene is designated, I and has 3 common alleles:
 IA , IB, i
4 different phenotypes (blood types), 6 genotypes
 Type A  ___________________________________________________________________
 Type B  ___________________________________________________________________
 Type AB  _______________________________________
 Type O  ____________________________
Of the three alleles that determine blood type, one (_____) is recessive to the other two, and other two
(____________________) are codominant




Presence of allele IA, produces an ‘________________________’
Presence of allele IB, produces a ‘________________________
Presence of allele IA and IB, produces both ‘___________________ and ‘___________________’
Presence of allele i, produces ____________antigen
SBI3U – Unit 2: Genetics
Date: ____________________
Example 4: Blood Types
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) IAIB x i i (Parent #2)
Learning Check p. 247 #1-10
SBI3U – Unit 2: Genetics
Date: ____________________
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.
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.
_____________________________________________________________________________________
_____________________________________________________________________________________
Continuous Variation
Continuous variation 
_____________________________________________________________________________________
_____________________________________________________________________________________
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 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.
Polygenetic Inheritance
_____________________________________________________________________________________
_____________________________________________________________________________________
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, ____________________________________________.
Review Questions p. 250 #1-10