Codominance and Sex-linked
traits
CODOMINANCE
• 2 alleles are dominant and both are
expressed
Most common examples are blood type and flower
color.
Blood Type
• 3 possible alleles: A, B, O
• 4 possible PHENOTYPES:
A
AB
B
O
Both alleles A and B are dominant. When
both A and B alleles are present, they are
both expressed.
Type O is recessive.
Blood Type
There are multiple genotypes:
Phenotype A = Genotype AA, Ao
Phenotype B = Genotype BB. Bo
Phenotype AB = Genotype AB
Phenotype O = Genotype oo
Blood Type
• Punnett Squares are worked the same
way as before:
A
A
• Cross AA with Bo
What are the
possible
Genotypes?
Phenotypes?
B
o
AB
AB
Ao
Ao
Sex-linked traits
• Sex is determined by the X and Y
chromosomes.
XX is female
XY is male
Sex-linked traits
Genotype uses Xs and Ys.
Female = XX
Male = XY
Sex-linked traits
Cross a male and a female.
What are the frequencies of:
female offspring? Male offspring?
X
X
X
XX
XX
Y
XY
XY
Sex-linked traits
Some genes are only found on the
X or the Y chromosome. These traits are
SEX-LINKED.
Common examples include Hemophilia and
colr-blindness.
Sex-linked traits
The alleles you are looking at are written as
superscripts on the X and Y
chromosomes.
XBXB
XBY
Sex-linked traits
Example: The gene for colorblindess is only
found on the X chromosome
B= normal vision; b= colorblind
Female with normal vision: XBXb or XBXB
Female with colorblindness: XbXb
Male with normal vision: XBY
Male with colorblindness: XbY
Sex-linked traits
Punnett squares work the same way, use X
and Y in the genotypes.
Cross a heterozygous
normal vision female
with a normal vision
male.
Genotypes?
Phenotypes?
XB
Xb
XB
XBXB
XBXb
Y
XBY
XbY