PROBABILTY
 P= number of ways that a given outcome can occur
total number of possible outcomes
Gene Interactions
There are three main types of gene
interactions:
1) Multiple Alleles
There are many traits that are controlled by
more than two alleles for example, eye colour
in Drosophila is controlled by four possible
alleles.
The following phenotypes and dominance
hierarchy is possible:
wild type (red) > apricot > honey > white
Multiple Alleles
 Wild type- the most commonly seen trait
 Mutant- non-wild-type traits
When dealing with multiple alleles, it is no longer
necessary to use upper & lower case letters  both
letters & upper case numbers are used.
phenotype
wild type
Apricot
honey
white
genotype
E1
E2
E3
E4
Predict the genotypes and phenotypes of the F1
generation from the mating of wild type (E1E4) with
apricot (E2E3).
Predict the genotypes and phenotypes of the F1 generation
from the mating of wild type (E1E4) with apricot (E2E3).
E1
E4
E2
E1E2
E2E4
E3
E1E3
E3E4
In the F1 generation ½ are wild type; ¼ apricot; ¼ honey
Do Now
phenotype
genotype
E1
wild type
Apricot
E2
honey
E3
white
E4
With the above information, make two heterozygous crosses what phenotypes do you get?
2) Incomplete Dominance
In some heterozygotes, both alleles of a pair are
expressed in the phenotype. These alleles are said
to be equally dominant. This lack of dominance is
known as incomplete dominance.
eg.
P1 Black  White
F1
P2 Grey
F2
Grey
×
Grey
Ex. Snapdragons
An interaction between the alleles in the
heterozygote shows an intermediate
phenotype.
WW
Ww
ww
3) Codominance
A form of incomplete dominance where
two alleles are expressed in such a way
that the effect of each is noticed
separately in the phenotype.
Both parental phenotypes can be
distinguished in the heterozygote
offspring.
The expression of one allele does not
mask the expression of another.
Ex. A red bull crossed with a white cow =
roan calf (calf has intermingled white & red hair)
Roan cattle and horses have both
coloured and white hair.
Blue Roan
Strawberry Roan
The ABO blood group system is another
example.
Inheritance of blood groups is determined by
the gene “I” which has three different alleles,
only two of which can occur at the locus at
once. The alleles are responsible for producing
antigens on the surface of the red blood cells,
which determines the blood group.
Alleles A and B are co-dominant so that when
they are both present, both A and B antigens are
produced. Both A and B are dominant to O.
Blood Types (A, B, O, AB)
Allele IA – formation of blood factor A (antigen A)
Allele IB – formation of blood factor B (antigen B)
Allele I – no factors result
Genotype Blood Type
IAIA or IAi
A
IBIB or IBi
B
ii
O
IAIB
AB
Type A
Type B
Type AB
Type O
IA an IB are codominant and i is
recessive.
Rh antigens are straight dominant
vs. recessive.
Blood types of
North America
Blood types of
North America
with RH.
Example:
A mother with blood type A has a child with Blood type
O. The father is blood type B. Indicate the genotypes
of the parents.
Father can be IBIB or IBi and mother can be IAIA or IAi.
To have a child that is type O (ii) both mother and father
must contribute the i allele, therefore the father must be
IBi and the mother is IAi
Case Study
 Page 611: A Mystery of Blood Types
Pedigrees
• A chart or register showing a line of ancestors
• Circles represent females, squares represent males,
solid circles & squares represent those who have the
trait being studied.
•Horizontal lines between circles and squares represent
mating between male and female.
• A vertical line joins parents and children.
• Pedigrees are often used to study sex-linked traits
such as color-blindness and hemophilia
PEDIGREE SUMMARY
This will help you determine a trait in a pedigree as
with autosomal or X-linked:
Autosomal Dominant
 must be in each generation
 affected individuals transmit to minimum ½ of their
offspring
 males and females are equally affected
 cannot have carriers, they will all be affected!
Autosomal Recessive
 may skip a generation
 affected offspring generally have normal
(but heterozygous) parents
 male and female are equally affected
X-linked Dominant
 very likely to be observed in each generation
 females pass on to half of either sex
 no transmission from father to son (only
daughters)
X-linked Recessive
 affect males more than females
 no transmission from father to son
 daughters of males are carriers
 females pass onto ½ sons
 affected females have affected fathers and carrier
mothers
Pedigree analysis
Probability
• Probability is the likelihood of an event
happening.
• Probability can be expressed by the
following formula:
Probability = # of chances for an event
# of possible combinations
Therefore, when a coin is tossed, there are
two possibilities – heads or tails.
What are the chances of getting heads?
1 head/2 head/tail =
50 % chance
What is the probability of two coins being
tossed and getting heads?
½ = 50 % chance
Coin 1 –
Coin 2 –
½ = 50 % chance
The Rule of Independent Events –
Chance has no memory. This means that previous
events will not affect future events. Ex. If you
tossed two heads in a row, the probability of
tossing heads again will still be ½.
Product of the probabilities of two separate
events:
½ × ½ = ¼ therefore a 25% chance
The Product Rule –
The probability of two or more independent
events occurring together is the product of the
individual probabilities if each individual event
occurs separately.
Dihybrid Crosses
Mendel also studied
two separate traits
with a single cross by
using the same
procedure he had
used for studying
single traits.
Mendel crossed a purebred yellow round pea
with a purebred green wrinkled pea.
Pure breeding round = RR
Pure breeding wrinkled = rr
Pure breeding yellow = YY
Pure breeding green = yy
Genotype for the yellow, round parent is RRYY
Genotype for the green, wrinkled parent is rryy
P1
YYRR x yyrr
Purebred yellow round × Purebred green wrinkled
The entire F1
Male gametes
generation has the
YR
YR
YR
YR
genotype: YyRr
and is
yr
Y yR r Y yR r Y yR r Y yR r
phenotypically
yellow
round!
Female
gametes
yr
Y yR r Y yR r Y yR r Y yR r
yr
Y yR r Y yR r Y yR r Y yR r
yr
Y yR r Y yR r Y yR r Y yR r
F1
Now let’s cross the F1 generations with one
another and see what we get…
9/16 Yellow Round
3/16 Yellow wrinkled
3/16 green Round
1/16 green wrinkled
The
phenotypic
ratio 9:3:3:1 is
the ratio you
will find in all
heterozygous
dihybrid
crosses!
Example:
In summer squash, white fruit color is dominant
“W” and yellow fruit color is recessive “w”.
Another allele produces disc shaped fruit “S” while
its recessive allele “s” yields sphere-shaped fruit. If
a pure breeding white disc variety is crossed with a
homozygous yellow sphere variety, the F1 are all
white disc hybrids. If the F1 generation is allowed
to mate, what would be the expected phenotypic
ratio in the F2 generation?
Try this!
P1
WWSS x wwss
F1 All offspring will be WwSs
P2
WwSs x WwSs
WS
Ws
wS
ws
WS WWSS WWSs WwSS WwSs
Ws WWSs WWss WwSs Wwss
wS
WwSS
WwSs wwSS wwSs
ws
WwSs
Wwss wwSs
wwss
F2
Rhesus Factor & Birth
P1 : Female Rh- × Male Rh+
Baby is Rh+ because father is. Mother’s
blood produces antibodies upon birth, (since
blood mixes at birth). First baby is okay.
Second pregnancy- mom’s antibodies can
now move across the placenta and cause
baby’s RBC’s to clump (agglutinate) if second
baby is also Rh+. This decreases oxygen
delivery in the baby – “blue baby.”
What can be done?
 Mom can be given an injection of a drug that
inhibits antibody production immediately after
delivery.
What happens if this is undetected?
 Baby could be given a blood transfusion
while in the womb. Fairly uncommon.
Blood Types & Rhesus Factor Question
R – dominant allele (Rh+)
r – recessive allele (Rh-)
Example: A woman homozygous for blood
type A and heterozygous for the rhesus
allele, Rh+, has a child with a man with
type O blood who is Rh-. What is the
probability that their child will have blood
There
will
be
a
type A, Rh+?
50% chance.
Do Now
 Biology creatures have many strange traits. Blinking eyes “B” is dominant and burning eyes are recessive
“b”. Another allele produces pleasant smelling pheromones “P” while its recessive allele “p” yields rotting
fruit pheromones. If a pure breed blinking pleasant smelling biology creature is crossed with a heterozygous
blinking pleasant smelling biology creature, what would be the expected phenotypic ratio?
Techniques used in order to produce the
genotype or phenotype that you want:
Selective Breeding: the crossing of desired
traits from plants or animals to produce
offspring with both characteristics
Inbreeding: the process by which
breeding stock is drawn from a limited
number of individuals possessing
desirable phenotypes.
Hybridization: Blending of desirable
but different traits.
Gene Interaction
1) Many traits studied by Mendel were controlled
by one gene.
2) Some traits are regulated by more than one
gene; many of your characteristics are determined
by several pairs of independent genes –
polygenic.
eg.) skin color, eye color and height, feather colour in parakeets
3) One trait controlled by more than one allele
(multiple alleles).
Eg. Blood types, Drosophilia eye colour
4) Genes that interfere with the
expression of other genes are called
epistatic.
Example:
-allele B produces a black coat color in dog
- b produces a brown coat color
-A second gene, W prevents the formation
of pigment
- w does not prevent color
- genotype wwBb would be black
- genotype WwBb would appear white
-W allele masks the effect of the B color gene
- If wwBb is crossed with a WwBb, state the
phenotypes produced.
8/16 = white
6/16 = black
2/16 = brown
5) Complementary Interaction occurs when two
different genotypes interact to produce a phenotype
that neither is capable of producing by itself.
Example:
-Allele R produces a rose comb in chickens
-Allele P (on a different chromosome) produces a
pea comb.
- R and P alleles both present = walnut comb
- The absence of rose and pea alleles results in an
individual with a single comb