Advanced Genetics:
Multiple Alleles & Sex-Linked Traits
Biology Honors
8.5, 16.7 (blood types)
8.6 (sex-linked traits)
Multiple Alleles
Multiple Alleles
• Traits are the result of more
than 3 or more types of
alleles
• Example: blood typing
• There are 3 different alleles
for blood type (A, B, & O)
• A is dominant to O
• B is dominant to O
• A and B are codominant
Multiple Alleles
Blood Transfusions
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Type A = A antigens (proteins)
Type B = B antigens
Type AB = A & B antigens
Type O = No antigens
Blood can only be transferred to a
body of a person who's immune
system will "recognize" it.
• O is the universal donor
• O blood has no antigens are that are
foreign to the body
• AB is the universal recipient
• Person with AB blood has both the A and B
antigens already in the body
• Rh factor counts too!
• (+) can receive (+) or (-)
• (-) can only receive (-)
• Ever see ER shows where doctors call for O
neg. blood?
Rh Factor
• Another gene for proteins
on the membrane of
blood cells
• You have them (+) or you
don’t (-)
• (+) is dominant to (-)
Sample Crosses
1. Cross a man with type AB blood with a woman who is heterozygous
for type A blood
• AB x AO
• Geno: ¼ AA; ¼ AB; ¼ AO; 1/4 BO
• Pheno: ½ type A; ¼ type B; ¼ type AB
Sample Crosses
2. Cross a man who is heterozygous for type A blood
with a woman who is heterozygous for type B blood.
• AO x BO
• Geno: ¼ AB; ¼ BO; ¼ AO; ¼ OO
• Pheno: ¼ type AB; ¼ type B; ¼ type A; ¼ type O
Sex-Linked Traits
Sex-linked Traits
• A gene that is found
only on the X
chromosome and not
the Y chromosome
• Examples:
• Colorblindness
• Hemophilia
• Duchenne Muscular
Dystrophy
• Male Pattern Baldness
Female
Male
Sex-linked Traits
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More common in men because they only have one X chromosome
Females need to have the gene on both X’s to have the trait
Females that are heterozygous for the trait are carriers
Carriers appear normal but can pass the trait to their offspring
Colorblindness
• Recessive gene (b) on X
chromosome
• Condition in which certain
colors cannot be distinguished
• Red/Green color blindness is
most common (about 99%)
• Causes problems in
distinguishing reds and greens
• No treatment
• Life is normal
Hemophilia
• Recessive gene (h) on the X
chromosome
• Bleeding disorder in which the
blood does not clot normally
• Persons with hemophilia may bleed
for a longer time than others after
an injury or accident
• They also may bleed internally,
especially in the joints
• Live life cautiously, receive
treatments to help blood clot
normally
Duchenne Muscular Dystrophy
• Recessive gene (d) on the X chromosome
• Rapidly-worsening muscle weakness that starts in the legs and
pelvis, and later affects the whole body
• No cure
• Treatment is aimed at control of symptoms to maximize the
quality of life
• Death usually occurs by age 25
Sample Crosses
1. Cross a normal man with a
woman who is a carrier for
hemophilia
• XHY x XHXh
• Geno: ¼ XHXH; ¼ XHXh; ¼ XHY;
¼ XhY
• Pheno: ½ females will be
normal; ½ females will be
carriers; ½ males will be
normal; ½ males will be
hemophiliacs
Sample Crosses
2. Cross a colorblind male with
a female who is a carrier for
colorblindness.
• XbY x XBXb
• Geno: ¼ XBXb; ¼ XbXb; ¼ XBY;
¼ XbY
• Pheno: ½ females will be
colorblind; ½ females will be
carriers; ½ males will be
normal; ½ males will be
colorblind