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Materials: none
• Catalyst: Beyonce and Jay Z are having
another baby. Both parents are heterozygous
for Hitchhiker’s thumb. Hitchhikers thumb is
dominant to no Hitchhiker’s thumb. What are
the possible genotypes and phenotypes of
their offspring and in what percentages will
they be seen?
Materials: none
• Catalyst: Compare and contrast Codominance
with Incomplete Dominance. You may use a
table or Venn Diagram to help you out.
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Announcements
• Unit 3 Test this week (Thurs/Fri)
– If you do well you get to go to Six Flags in May!!
• Unit 3 Study guide due on Wednesday
– Can be found online at msnay.weebly.com
• LAST WEEK TO TURN IN LATE WORK FOR UNIT
3! No work/Mutant Makers projects will be
accepted past Friday!
• Grades post tomorrow!
• Tutorial Today 3:15 – 4:30
Star Quiz 14
Objectives
• I can explain the concepts of codominance
and incomplete dominance.
• I can determine the genotypes and
phenotypes of codominant and incompletely
dominant traits in offspring.
• I can explain the concept of sex-linked traits in
mammals.
Codominance:
(NOTES)
• Both alleles are expressed on different parts
of the organism.
• Heterozygous = see both phenotypes
• CO means BOTH SHOW!
BB’
RR’
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Codominance Example: (NOTES)
• There are 3 alleles to classify blood type
in humans (A, B, O).
• Blood type AB = codominant
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Incomplete dominance: (NOTES)
• No dominant allele
• Blending of traits
• Heterozygous = blended phenotype
Example: snapdragon flower color:
RR = red
Rr = pink
rr = white
How is Incomplete Dominance Shown
on a Punnett Square?
• When the heterozygous genotype is shown,
label it as having the “blended” phenotype.
• Ex:
RR Rr
Rr rr
heterozygous cross
Red= RR= 25%
Pink=Rr=50%
White=rr=25%
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Silent Practice
• The color of pythons is a codominant trait. A
dark brown python (BB) mates with a light
brown python (B’B’). What are the
genotypes and phenotype probabilities of
the offspring?
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Genetic Disorder Inheritance
– A heterozygote for a recessive disorder is a carrier.
– Disorders caused by dominant alleles are
uncommon.
(dominant)
Sex-linked traits
• Genes on sex chromosomes are called sex-linked genes.
– Y chromosome genes = male characteristics.
– X chromosome genes = many traits.
Male mammals have an XY genotype
– All of a male’s
sex-linked
genes are
expressed.
– If a disorder is
carried on the X
chromosome,
the male will
express it.
Female mammals have an XX genotype
– Females must have 2 copies of a recessive sex
linked trait to express the disorder.
Examples of Sex Linked Disorders
• Red – Green Colorblindness
– Recessive trait
– Found on the x chromosome
• Male Pattern Baldness
– Recessive trait
– Found on the x chromosome
• Hemophilia
– Recessive trait
– Found on the x chromosome
Sex Linked Disorders Summary
(LISTEN)
• Males express sex linked traits more
frequently than females. (XY vs. XX)
• If the gene is found on the X chromosome, it
will be expressed in males no matter if it is
dominant or recessive.
• For females, they must have 2 copies of the
gene to express that disorder.
Classwork 3.11
• Directions: complete the worksheet using your
notes to correctly answer the questions.
REMEMBER:
• Co means both show!
• Incomplete = blend
When you are done, turn it in to the blue tray
and work on your Unit 3 Study Guide!
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