INCOMPLETE DOMINANCE, CODOMINANCE, MULTIPLE
ALLELES, POLYGENIC TRAITS
DOMINANT? RECESSIVE?
NEITHER?

NOT ALL GENES SHOW SIMPLE PATTERNS
OF DOMINANT AND RECESSIVE ALLELES.
IN MOST ORGANISMS, GENETICS IS
MORE COMPLICATED, BECAUSE THE
MAJORITY OF GENES HAVE MORE THAN
TWO ALLELES. MANY IMPORTANT TRAITS
ARE CONTROLLED BY MORE THAN ONE
GENE.
INCOMPLETE DOMINANCE
PAIRS OF ALLELES PRODUCE A
HETEROZYGOUS PHENOTYPE THAT
RESULTS IN AN APPEARANCE IN
BETWEEN THE PHENOTYPES OF THE TWO
PARENTAL VARIETIES.
 EXAMPLE: SNAPDRAGON COLOR

INCOMPLETE DOMINANCE
 With
incomplete dominance, a
cross between organisms with
two different phenotypes
produces offspring with a third
phenotype that is a blending of
the parental traits.
INCOMPLETE DOMINANCE SNAPDRAGONS
INCOMPLETE DOMINANCE –
Humans
HYPERCHOLESTEROLEMIA –
DANGEROUSLY HIGH LEVELS OF
CHOLESTEROL IN THE BLOOD
 NORMAL INDIVIDUALS ARE HH
 HETEROZYGOTES ARE Hh (1 IN 500)
 HOMOZYGOUS INDIVIDUALS WITH hh (1
IN 1,000,000) 5X NORMAL CHOLESTEROL
LEVEL

INCOMPLETE DOMINANCE
PROBLEMS

YELLOW GUINEA PIGS CROSSED WITH
WHITE GUINEA PIGS ALWAYS PRODUCE
CREAM COLORED GUINEA PIGS.
 A. COMPLETE THE PUNNETT SQUARE FOR THE
CROSS BETWEEN A WHITE GUINEA PIG AND A
YELLOW GUINEA PIG.
 B. COMPLETE THE PUNNETT SQUARE FOR THE
CROSS BETWEEN TWO CREAM-COLORED GUINEA
PIGS.
 C. GIVE THE PHENOTYPIC AND GENOTYPIC RATIO
FOR THE OFFSPRING IN A AND B.
CODOMINANCE
BOTH ALLELES CONTRIBUTE TO THE
PHENOTYPE OF THE ORGANISM
 EXAMPLE: IN HORSES, THE ALLELE FOR
RED HAIR IS CODOMINANT WITH THE
ALLELE FOR WHITE HAIR. HORSES WITH
BOTH ALLELES ARE ROAN BECAUSE
THEIR COATS ARE A MIXTURE OF BOTH
RED AND WHITE HAIRS

CODOMINANCE
 With
codominance, a cross
between organisms with two
different phenotypes produces
offspring with a third phenotype in
which both of the parental traits
appear together.
CODOMINANCE PROBLEMS

1. WHEN A RED BULL IS BRED TO A
WHITE COW, THE COLOR OF THE CALF IS
ROAN.
 A. COMPLETE THE PUNNETT SQUARE FOR THE
CROSS BETWEEN A RED BULL AND A WHITE COW
 B. GIVE THE GENOTYPIC RATIO AND PHENOTYPIC
RATIO OF THE OFFSPRING.
 C. A ROAN BULL AND A ROAN COW ARE BRED.
GIVE THE GENOTYPIC RATIO AND PHENOTYPIC
RATIO OF THIS CROSS.
MULTIPLE ALLELES
MORE THAN TWO POSSIBLE ALLELES
EXIST IN A POPULATION
 EX. COAT COLOR IN RABBITS –
DETERMINED BY A SINGLE GENE WITH AT
LEAST 4 DIFFERENT ALLELES THAT CAN
PRODUCE 4 POSSIBLE COAT COLORS
 EX. HUMAN GENE FOR EYE COLOR

Multiple Alleles
Human Eye Color
3 known genes that
Explain typical patterns of green, brown,
blue eye color inheritance
 Hazel, grey and multiple shades of green,
brown, blue are not explained – molecular
basis unknown


Human Eye Color Genes







Human Eye Color Genes:
EYCL1 (gey) green dominant over blue
Green/blue eye color,
EYCL2 (bey 1)
EYCL3 (bey 2) brown dominant over blue
Central Brown eye color gene
http://www.athro.com/evo/gen/eyecols.html
MULTIPLE ALLELES
HUMAN BLOOD TYPES – CHROMOSOME 9
ONE GENE, THREE ALLELES, PRODUCE 4
PHENOTYPES
 A PERSON’S BLOOD GROUP MAY BE O, A,
B OR AB
 THESE LETTERS REFER TO 2
CARBOHYDRATES THAT MAY BE FOUND
ON THE SURFACE OF RED BLOOD CELLS
(RBC)


ABO BLOOD GROUPS
Blood Types:
What do they look like?
ABO BLOOD GROUPS
Blood Type
Genotypes
ABO
Enzymes
Present
RBC
Antigens
Present
Serum
Antibodies
"A"
AA, Ai
"H", "A"
A, H
anti-B
"B"
BB, Bi
"H", "B"
B, H
anti-A
"AB"
AB
"H", "A",
"B"
A, B, H
none
"O"
ii
"H"
H
anti-A,
anti-B
ABO BLOOD GROUP
PROBLEMS

COMPLETE A PUNNETT SQUARE FOR:
1.
2.
3.
4.
5.
6.
HOMOZYGOUS TYPE A x TYPE O
HETEROZYGOUS TYPE A x TYPE O
HOMOZYGOUS TYPE A x HOMOZYGOUS TYPE B
HETEROZYGOUS TYPE B x HOMOZYGOUS TYPE A
HETEROZYGOUS TYPE A x HETEROZYGOUS TYPE B
HETROZYGOUS TYPE B x TYPE O
Practice problems

A TYPE AB MALE MARRIES A TYPE O
FEMALE.
 WHAT ARE THEIR GENOTYPES?
 COMPLETE A PUNNETT SQUARE TO SHOW THE
POSSIBLE GENOTYPES OF THEIR CHILDREN.
Practice problems

A TYPE A MALE MARRIES A TYPE B
FEMALE. THEY HAVE TWO CHILDREN.
ONE CHILD IS TYPE AB AND THE OTHER
CHILD IS TYPE O.
 WHAT ARE THE GENOTYPES OF THE PARENTS?
 COMPLETE A PUNNETT SQUARE TO SHOW THE
GENOTYPES OF THESE TWO CHILDREN AND
FOR ANY FUTURE CHILDREN.
Practice problems

A TYPE A MALE IS MARIED TO A TYPE O
FEMALE. USING PUNNETT SQUARES,
EXPLAIN HOW IT WOULD BE POSSIBLE
FOR THEM TO HAVE A:
 TYPE A CHILD.
 TYPE B CHILD.
 TYPE O CHILD.
Practice problems

A TYPE AB MALE MARRIES A TYPE B
FEMALE. USING PUNNETT SQUARES,
EXPLAIN HOW IT WOULD BE POSSIBLE
FOR THEM TO HAVE A:
 TYPE AB CHILD
 TYPE A CHILD
 TYPE O CHILD
 TYPE B CHILD
Practice problems

A TYPE AB MALE MARRIES A TYPE AB
FEMALE. USING PUNNETT SQUARES,
EXPLAIN HOW IT WOULD BE POSSIBLE
FOR THEM TO HAVE A:
 TYPE AB CHILD
 TYPE A CHILD
 TYPE B CHILD
 TYPE O CHILD
Practice problems

A TYPE O MALE MARRIES A TYPE O
FEMALE.
 WHAT ARE THE POSSIBLE GENOTYPES OF
THEIR CHILDREN?
Practice problems

A TYPE A MALE, WHOSE FATHER IS TYPE
O, MARRIES A TYPE O FEMALE. WHAT ARE
THE POSSIBLE GENOTYPES OF THEIR
CHILDREN?

THE MALE FROM THE PREVIOUS PROBLEM
HAS A BROTHER WHO IS TYPE B. WHAT
IS THEIR MOTHER’S BLOOD TYPE?

THE BROTHERS IN THE ABOVE PROBLEMS
HAVE A SISTER WHOSE BLOOD TYPE IS
O. IS THIS POSSIBLE?
POLYGENIC TRAITS
TRAITS CONTROLLED BY TWO OR MORE
GENES
 EX. 3 GENES INVOLVED MAKING THE
REDDISH-BROWN PIGMENT IN THE EYES
OF FRUIT FLIES
 EX. WIDE RANGE OF HUMAN SKIN
COLOR DUE TO MORE THAN 4 DIFFERENT
GENES CONTROLLING THE TRAIT

POLYGENIC TRAIT – COAT
COLOR IN MICE
SEX-LINKED TRAITS
 IN
HUMANS, SEX IS DETERMINED
BY THE 23RD PAIR OF
CHROMOSOMES – THE SEX
CHROMOSOMES!
 THE SEX CHROMOSOMES ARE THE X
AND THE Y CHROMOSOMES
 XX – FEMALE
 XY – MALE
SEX-LINKED TRAITS
MOST SEX-LINKED GENES ARE
X-LINKED GENES.
 WHY? THE X CHROMOSOMES ARE
LONGER AND CONTAINS THOUSANDS
MORE GENES THAN THE Y CHROMOSOME.
 X CHROMOSOME CONTAINS 1098 GENES.
 Y CHROMOSOME CONTAINS 26 GENES.

KARYOTYPE
SEX-LINKED TRAITS
FOR EACH GENE EXCLUSIVELY ON THE X
CHROMOSOME, THERE ARE TWO ALLELES
OF EACH GENE
 MALES, XY, HAVE ONLY ONE ALLELE
 A MALE, XY, WITH A RECESSIVE ALLELE
ON THE X CHROMOSOME, WILL ALWAYS
EXHIBIT THAT RECESSIVE TRAIT
BECAUSE THERE IS NO CORRESPONDING
ALLELE ON THE Y CHROMOSOME.

EXAMPLES OF SEX-LINKED
TRAITS
 IN HUMANS:
 Red-Green Color Blindness
 Hemophilia
 Duchenne Muscular Dystrophy
 IN CATS
 Calico coat color
 IN FRUIT FLIES:
 Eye Color
WORKING PUNNETT
SQUARES

IDENTIFY EACH INDIVIDUAL AS MALE OR
FEMALE ACCORDING TO THEIR SEX
CHROMOSOMES
 EX. XX or XY

THE LETTERS REPRESENTING THE TRAIT
ARE PLACED AS SUPERSCRIPTS ABOVE
THE X CHROMOSOME
 EXAMPLE: XR or Xr
EYE COLOR IN FRUIT FLIES
THE GENE FOR EYE COLOR IS ON THE X
CHROMOSOME.
 THE ALLELE FOR RED EYES IS DOMINANT
OVER WHITE EYES
 PROBLEM:

 IF A WHITE-EYED FEMALE FRUIT FLY IS MATED
WITH A RED-EYED MALE, PREDICT THE POSSIBLE
OFFSPRING
COAT COLOR IN CATS

Coat color in cats is an X-linked gene, with
alleles for black and orange-brown
 XBXB and XBY cats will have a black coat
 XOXO and XOY will have an orange-brown coat
 female cats with XBXO are Calico!!
Pedigrees
 a diagram of a family history that shows relationships
between family members and their status with respect
to a particular hereditary condition.
 The affected person through which the pedigree is
discovered is indicated by an arrow on pedigrees.
 Males are denoted by squares and females by circles.
 A filled symbol indicates that an individual is affected by
a specific condition.
 Generations are represented on horizontal levels;
 parents on one level and the children of those parents
together on a level below them.
Pedigrees Autosomal Dominant
Autosomal Dominant

Dominant conditions are expressed in individuals
who have just one copy of the mutant allele. The
pedigree on the right illustrates the transmission
of an autosomal dominant trait. Affected males
and females have an equal probability of passing
on the trait to offspring. Affected individual's
have one normal copy of the gene and one
mutant copy of the gene, thus each offspring has
a 50% chance on inheriting the mutant allele. As
shown in this pedigree, approximately half of the
children of affected parents inherit the condition
and half do not.
Pedigrees–Autosomal Recessive
Autosomal Recessive

Recessive conditions are clinically manifest
only when an individual has two copies of the
mutant allele. When just one copy of the
mutant allele is present, an individual is a
carrier of the mutation, but does not develop
the condition. Females and males are
affected equally by traits transmitted by
autosomal recessive inheritance. When two
carriers mate, each child has a 25% chance
of being homozygous wild-type (unaffected);
a 25% chance of being homozygous mutant
(affected); or a 50% chance of being
heterozygous (unaffected carrier).
Pedigrees-X-Linked Recessive
X-Linked Recessive

X-linked recessive traits are not clinically manifest
when there is a normal copy of the gene. All X-linked
recessive traits are fully evident in males because
they only have one copy of the X chromosome, thus
do not have a normal copy of the gene to compensate
for the mutant copy. For that same reason, women
are rarely affected by X-linked recessive diseases,
however they are affected when they have two copies
of the mutant allele. Because the gene is on the X
chromosome there is no father to son transmission,
but there is father to daughter and mother to
daughter and son transmission. If a man is affected
with an X-linked recessive condition, all his daughter
will inherit one copy of the mutant allele from him.
Pedigrees-X-Linked Dominant
X-Linked Dominant

Because the gene is located on the X
chromosome, there is no transmission from
father to son, but there can be transmission
from father to daughter (all daughters of an
affected male will be affected since the father
has only one X chromosome to transmit).
Children of an affected woman have a 50%
chance of inheriting the X chromosome with
the mutant allele. X-linked dominant
disorders are clinically manifest when only
one copy of the mutant allele is present.
Famous Pedigree


The PEDIGREE of
Nicholas (Nikolia) II ROMANOV (last
CZAR) of RUSSIA

(Nicholas Alexandrovitch); Knight of
the Garter; (since he possessed Russia,
he may be considered the wealthiest man
ever)