The Classic Example of Codominance in Humans is BLOOD TYPE

Blood Type and Sex Linked
Inheritance
Multiple Alleles
Multiple alleles for a particular gene means
that more than two alleles exist in the population.
This does not mean that the gene in a
particular individual possesses more than two
alleles.
An individual can only have a maximum of
two of alleles, one maternal and one paternal, no
matter how many alleles exist in the population.
Example: a population of dogs can have 5
alleles for coat color at one gene.
The Classic Example of Codominance
and Multiple Alleles in Humans is
BLOOD TYPE
Gene that controls ABO blood
type codes for an enzyme that
makes an antigen on blood cells.
Two alleles (IA and IB) are called
“A” and “B” which are
codominant. A and B can both
be expressed on the same cell.
Third allele (i) is called “O”) and it
is recessive to both A and B
Possible Genotypes for
Blood Types
If the Phenotype: A
Genotype(s): AA,
Ao
If the Phenotype:
AB
Genotype(s): AB
If the Phenotype: B
Genotype(s):
BB,Bo
If the Phenotype: O
Genotype(s): oo
Possible combinations of alleles
A Real Problem:
Mom is Type B, Dad is Type O … Baby is Type AB. Is
this possible?
Bo X ____
oo
•Cross: ____
MOM
B
o
o
Bo
oo
o
Bo
oo
Remember, when
in doubt,
heterozygous is
more likely
Check your understanding
Could an AB father have a type O
child?
Why?
Are the father’s
alleles codominant
or recessive?
O is homozygous
recessive. A or B
would hide the o
allele.
Sex Chromosomes:
the 23rd pair in humans
All others (122) are
called
autosomes
Boy Girl

The Sex Chromosomes
The “X” Chromosome
Has essential genes
Can’t live without it!
Females have a 23rd pair = XX (one
usually deactivated)
The “Y” Chromosome
Has few genes
Male 23rd pair = XY
What’s the probability of having a boy or girl?
X
X
X
XX
XX
50%
Y
XY
XY
50%
Some X-Linked Traits:
Hemophilia (missing clotting enzymes)
Bleeding disorders in which it takes a long
time for the blood to clot.
Colorblindness
Defect in color vision
“Male-pattern” baldness
Most are recessive.

A man with normal vision marries a colorblind
woman. What is the probability that they will have a
colorblind SON?
Xc Xc
XY
•Cross: ____ X ____
MOM
Xc
Xc
Remember, Mom
only shows if she is
homozygous!
X
XcX Focus only
XconXSONS
Y
Xc Y
(not daughters)!
Xc Y
100%
(2/2) SONS
A hemophiliac man marries a healthy woman. What is the
probability that they will have any hemophiliac children?
0%
All sons healthy
All daughters carriers
Colorblindness
Red-green colorblindnessRed and green appear to be the
same. X-linked
Red-green affects:
1 in 10 males in the US
1 in 100 females in the US
Remember: Males have just one
X chromosome. All X-linked
alleles are expressed in males.
Other forms:
Total Colorblindness
Blue-Yellow colorblindness
Blue-yellow appears to be the
same. It is autosomal Chromosomes
1 to 22
What do you see?
http://www.ansci.cornell.edu/usdagen/mult_alleles.html
Medline Plus, 2014, US National Library of Medicine, National
Institutes of Health
http://www.nlm.nih.gov/medlineplus/ency/article/002051.htm
Virtual Fruit Fly Lab, 2014,
http://www.mhhe.com/biosci/genbio/virtual_labs/BL_15/BL_1
5.html