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Complex Genetics
When Mendel’s rules don’t exactly
apply…especially in human traits!
Incomplete Dominance
• Medium Phenotype
• When the
heterozygote is in
between the recessive
and dominant.
Problem
• A red 4 o’clock is crossed a pink 4 o’clock.
What are % of the genotypes and
phenotypes of the offspring.
Codominance
• When both phenotypes
are expressed.
• Use capital letters to
represent both genes.
Problem
• A black cow is mated with a white cow.
Both black and white are dominant. What
will be the phenotype of the offspring.
Show a Punnett Square.
Multiple Alleles
• When there is more
than one allele for a
gene.
–
–
–
–
Blood type
I is dominant
i is recessive
I can be IA or IB
Problem
• A mother is Blood type A. The father is
blood type B. What is their genotypes if
they have a child who is blood type O?
– A:
– B:
– O:
Punnett Squares to show answer:
Getting Blood!
Blood Types
Sex Linked
• When a gene is on the
female X
chromosome.
• Females get two
copies, males only get
one.
• If males have a mutant
recessive, they will
have the disease.
Example: Colorblindness
• Rods--Black and White
vision
• Cones--Color Vision
• Gene for cones on X
chromosome
– XBXB; XBXb-Normal
Female
– XbXb-Colorblind Female
– XBY-Normal Male
– XbY-Colorblind Male
Problem
• A man is normal vision. His wife is normal
vision but her mom was colorblind. What is
the chance of their daughters being
colorblind? Their sons? Show in a Punnett
Square in proper notation.
Polygenic Inheritance
• Many genes affect a
phenotype. Most
visual traits have a
range.
• Ex. Eye color, hair
color, height
Problem
• Skin color is influenced by 3 major genes.
Dominant genes cause greater production of
melanin.
• Which of the following would be the
genotype for a darker skin phenotype?
– AABbcc
– Aabbcc
– AaBbCC
Pleiotropy
• One mutation has many
effects
• Example--Cystic Fibrosis
– What type of mutation?
– What are two organ systems
that it effects?
– Could there be an
evolutionary significance to
the CF allele?
Epistasis
•
•
•
•
•
When one gene effects the outcome of another gene.
B_E_ : Black fur and black skin
B_ee: Yellow fur and black skin
bbE_: Brown fur and brown skin
Bbee: Yellow fur; brown skin
– What does each trait code for?
– How does the skin trait affect fur color?
Review: Dihybrid cross
• How is a dihybrid cross
different than a four box
Punnett Square?
• How does the dihybrid
Cross show Mendel’s
Law of Segregation?
• Law of Indepdent
Assortment?
• Random Fertilization?
Practice Makes Perfect!
• Mendelian’s Law of
Dominance
– Make your own 5
problems and answer
• Non-Mendelian
Genetics
– Worksheets
• Corn Lab
– Lab Notebook!