Mendelian Genetics
2/12/2017
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16 round seeds & green
pods
3/16 round seeds & yellow
pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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Incomplete Dominance
and
Codominance
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Incomplete Dominance
Incomplete Dominance
F1 hybrids have an appearance somewhat
in between the phenotypes of the two
parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
r
r
RR = red flower
rr = white flower
2
r
r
produces the
F1 generation
R
R
R
R
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3
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Incomplete Dominance
4
Codominance
Two alleles are expressed (multiple
alleles) in heterozygous individuals.
Example: blood type
1.
2.
3.
4.
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G. Podgorski, Biol 1010
5
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Mendelian Genetics
2/12/2017
Another Codominance Problem
Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
IA
IB
i
i
IB
i
IB
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7
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Codominance
Codominance
Question:
Answer:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents?
boy - type O (ii)
AB (IAIB)
X
IA
i
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Example: Eye color in fruit flies
Traits (genes) located on the sex
chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on
X chromosome
G. Podgorski, Biol 1010
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Sex-linked Traits
Sex-linked Traits
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i
IB
girl - type
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8
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
11
Xy chromosome - male
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12
2
Mendelian Genetics
2/12/2017
Sex-linked Trait Problem
Sex-linked Trait Solution:
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
Xr
Xr
RR = red eyed
Rr = red eyed
XR
rr = white eyed
XY = male
XX = female
Xr
Xr
XR
Y
Y
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14
Female Carriers
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G. Podgorski, Biol 1010
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