Genetics
Beyond
Complete
Dominance
Mrs. Dignan’s Biology Class
Complete Dominance
One allele is
completely dominant
over another allele.
 This the most basic
type of genetics
problem.
 Example:Purple
flowers are dominant
over white. Pp is
purple.

Problem#1
In pea plants, yellow
seeds is dominant
over green seeds.
 Two heterozygotes
mate, what are the
phenotypes and
genotypes of their
offspring?

Genotypes: 25%YY,
50% Yy, and 25% yy
 Phenotypes: 75%
yellow, 25% green

Incomplete Dominance
Neither allele is completely dominant.
 Both alleles BLEND.
 Example:There are white and red snapdragon
flowers. RR is red. WW is white. RW is pink.

Problem#2
Flower color (red and
white) in snapdragons
is controlled by
incomplete
dominance
 Two pink
snapdragons mate,
what are the
phenotypes and
genotypes of their
offspring?

Genotypes: 25%RR,
50% RW, and 25%
WW
 Phenotypes: 25%
red, 50% pink, 25%
white

Codominance
Both alleles are
dominant
 The alleles don’t
blend, but they are
both represented in
the phenotype.
 Example:There are
white and red cattle
colorations. RR is
red. WW is white.
RW is white and red
patches (roan).

Problem#3
Fur coloration (red
and white) in cattle is
controlled by
codominance
 A white cow and a
roan (red and white
patches) bull mate.
What are the
phenotypes and
genotypes of their
offspring?

Genotypes: 50%WW,
50% RW
 Phenotypes: 50%
White, 50% roan

Multiple alleles
Some traits are controlled by more than 2 alleles.
 Example: Human blood type. Type A and B are
codominant. Both alleles are completely dominant
over type O.

Problem#4
A type AB woman
mates with a type O
man.
 What are the
phenotypes and
genotypes of their
offspring?

Genotypes: 50%AO,
50% BO
 Phenotypes: 50%
Type A, 50% Type B

Problem#5
A type AB woman
mates with a type AB
man.
 What are the
phenotypes and
genotypes of their
offspring?
 What are the chances
they will have a Type
B daughter?

Genotypes: 25%AA,
50% AB, 25% BB
 Phenotypes: 25%
Type A, 50% Type
AB, 25% type B
 50% female, 50%
male
 ¼*1/2=1/8 (12.5%)

Sex-Linked Inheritance
Some traits are only
found on the X
chromosome
 The y chromosome
does not have a
copy of those genes
because it is so
small.
 Therefore females
have 2 copies of
those genes and
males only have 1.

Sex-Linked Inheritance
These alleles tend to be
recessive.
 If a boy has an allele for
the trait, it shows in his
phenotype.
 A girl would need 2
copies of the recessive
allele to show the trait,
otherwise she is a carrier
(has the allele but it
doesn’t show up in her
phenotype)

Problem#6
A man with normal
color vision
(XCY)mates with a
woman who is a
carrier for color
blindness(XCXc).
 What are the
phenotypes and
genotypes of their
offspring? (include
genders)

Genotypes: 25%
XCXC, 25% XCY,
25%XCXc, 25%XcY.
 Phenotypes: 25%
normal female, 25%
carrier female, 25%
normal male, 25%
color blind male.

Polygenic Inheritance
Many traits are
controlled by two or
more genes.
 These genes may be
on the same
chromosome or on
different
chromosomes.
 Ex) Skin color, hair
color, eye color, and
height

Problem #7


Being able to roll your
tongue is a trait that
exhibits complete
dominance over not being
able to roll your tongue.
Predict the genotypes
and phenotypes of a
 50% Rr, 50%rr
cross between someone
 50% can roll tongue,
who is heterozygous for
50% can’t roll tongue
tongue rolling and
someone who can’t roll
their tongue.
Problem #8
Brown eyes are
completely dominant over
blue eyes (there’s a
separate gene for green
and hazel eyes).
 Predict the genotypes
and phenotypes of a
cross between someone
with blue eyes and
someone who is
heterozygous for brown
eyes.

Genotype: 50% Bb,
50% bb
 Phenotype: 50%
brown eyes, 50%
blue eyes

Problem #9

Harry has blue eyes and can roll his (he is
homozygous dominant). Hermione is
heterozygous for brown eyes, and can’t roll her
tongue. If Harry and Hermione have a baby,
what is the probability their baby will have
brown eyes and will be able to roll it’s tongue?
Dihybrid Cross
To find out the
probability of
inheriting two traits at
once, we need to
create a larger
Punnett square, called
a dihybrid cross
 On each axis, list all
possible combinations
that each parent
could pass on.

Dihybrid Crosses measure Compound Probability

What are the chances a
dice will roll a six?


1/6
What are the chances of
rolling two sixes in a row?

1/6 * 1/6= 1/36

You can apply this same
premise to calculate the
probability of inheriting
two traits:






What is the probability
Harry and Hermione’s child
will have brown eyes?
100% or 1
What is the probability
Harry and Hermione’s child
will be able to roll it’s
tongue?
50% or ½
What is 1 times ½?
½ or 50%
Pedigrees
A pedigree is a
diagram the is drawn
to show the
inheritance of a
particular trait.
 What gender in the
pedigree is most
affected by the
disease?
 What type of
inheritance is this?

Pedigree Key
Problem#10


Draw a pedigree for the following.
Alice and Bob have a two year old son, Charles, who is showing mental
retardation, short stature, and cryptorchidism. Alice has two living,
unaffected, brothers but her eldest brother died at age 9 and a second
brother died aged 10 months. Both had similar problems to Charles. Alice's
father, David, who was symptomless, has a sister, Ethel, who has an
unaffected boy and girl, and a brother, Fred, who also has two unaffected
children. Alice's mother, Gertrude, has two living sisters and had a brother
who had died in childhood and who, she remembers, had been mentally
retarded. Bob has two brothers, Henry and Ignatius, who are still
unmarried. His parents, John and Kate, had tragic lives, both were adopted
and never knew their biological parents and both died as the result of a
road accident.