Incomplete Dominance
Co-dominance
Multiple Alleles
Sex-linked Traits
Incomplete Dominance
O Shows a blend of both traits if both are
present
O Neither trait is dominant
O Creates a “blend” of the two traits
O Like mixing paint
Incomplete Dominance
Example
O In Four o’clock flowers the alleles for flower color are
both equal therefore neither dominates over the
other. Instead, they blend to create pink flowers if
both genes are present. Show a cross between a red
four o’clock flower (RR) and a white flower (WW).
Let’s try one:
O In humans straight hair (SS) and curly hair
(CC) are traits that have incomplete
dominance, that result in hybrids who have
wavy hair (SC). Cross a curly hair female with
a wavy haired male.
O In andalusian chickens, gray is a blend of
pure black with pure white. Cross a gray
rooster with a gray hen. Give the genotypes
and phenotypes of their offspring.
Co-dominance
O Shows both traits if they are present
O Like co-authors (both names appear on the
cover)
O Like co-captains (both people are captains)
O Problems are done the same way as
incomplete dominance problems
O Only difference would be how you would see
it in person
Co0dominance Example
O In cattle, neither red (RR) nor white (WW) hair
dominates. Instead, both show up if present to
make a “color” called roan (RW). Show the
cross between a roan bull and a roan cow.
Multiple Alleles
O In humans, there are four types of blood;
type A, type B, type AB, and type O. The
alleles A and B are co-dominant to each
other and the O allele is recessive to both A
and B alleles.
O Blood Type (Phenotype)
Type A
O Type B
O Type AB
O Type O
O
Genotype
IAIA or IAi
IBIB or IBi
IAIB (example of Co-Dominance)
ii
Blood Type Example
O A man with blood type AB marries a woman with type
O blood. Show the possible genotypes and
phenotypes for their children.
Let’s try one:
O A woman with Type A blood and a man with
Type AB blood have a baby. The woman is
suing the man for child support, but he is
refusing to pay because he says the baby
(who has Type O blood) is not his. Is he right
or should he be paying child support? Use a
Punnett Square to prove your answer.
O An extremely rich elderly couple died in a car
accident. They had no known living
relatives. A man shows up claiming to be
their long lost son. The elderly couple both
had Type AB blood. The man claiming to be
their son has Type O blood. Does this man
have a claim to their fortune? Use a Punnett
Square to support your answer.
Sex-linked Traits
O In humans sex is determined by the twenty third pair of
O
O
O
O
chromosomes known as “sex chromosomes”.
If you have two x-shaped (XX) chromosomes you are
destined to be a female.
If you have an x and a Y-shaped (XY) chromosomes you
are destined to be a male.
Since the X and Y chromosomes carry different
information, any genes found on the X chromosomes are
referred to as sex-linked genes.
Therefore, women will have two alleles for these genes
because they have two (XX) chromosomes. On the other
hand, men have only one allele for each of these genes
because they have only one X chromosome (XY).
Sex-linked Traits Example
O In fruit flies, the gene for eye color is carried on
the X chromosome, which is a sex chromosome
(sex-linked). The allele for red eyes is dominant
over the allele for white eyes. If a white-eyed
female fruit fly is mated with a red-eyed male,
predict the possible offspring.
Let’s try one:
O Hemophilia is a sex-linked trait. A person with
hemophilia is lacking certain proteins that are
necessary for normal blood clotting. Hemophilia is
caused by a recessive allele so use “N” for normal
and “n” for hemophilia. Since hemophilia is sexlinked, remember a woman will have two alleles (NN
or Nn or nn) but a man will have only one allele (N or
n). A woman who is heterozygous (a carrier) for
hemophilia marries a normal man. Give the genotype
and phenotype ratios along with your Punnett Square.
O In canaries, the green variety with black
eyes is dependent upon a dominant sexlinked factor (B), the cinnamon variety with
brown eyes is due to its recessive allele (b).
A cinnamon male is mated to a homozygous
green female. Show all work and ratios.