Download Genetics PART II ppt

SPECIAL TYPES
OF CROSSES
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1. Incomplete Dominance:
When the phenotype of
offspring result in a
BLENDING of the trait.
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The RED (RR) color of a four
o’clock flower is incompletely
dominant to the WHITE (rr)
colored flower…hybrid offspring
will be PINK (Rr).
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Incomplete Dominance
Example: four o’clock flowers
red (RR) x white (WW)
RR = red flower
WW = white flower
W
W
R
RW
RW
R
RW
RW
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Incomplete Dominance
W
W
R RW
RW
R RW
RW
produces the
F1 generation
All RW = pink
(heterozygous pink)
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2. Codominance
When BOTH alleles for a trait are
represented in the heterozygous
offspring.
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The RED (RR) color of hair in horses is
COdominant to white hair (WW)…hybrid
offspring will have a ‘roan’ coat (RW) color…
BOTH red and white hairs (“ROAN”)!
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Red x White: RR x WW
W
R
R
W RW
RW
RW
RW
Roan x Roan: RW x RW
R
W
W RR
RW
R
RW
WW
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Genotype: RW
Genotype: RR, RW, WW
Genotypic Ratio: 4 RW : 0
Genotypic Ratio: 1 RR : 2 RW : 1 WW
Phenotype: Roan
Phenotype: Red, Roan, White
Phenotypic Ratio: 4 Roan : 0 Phenotypic Ratio: 1 Red: 2 Roan:
1 White
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3. Multiple Alleles –
genes for which more
than two different
alleles exist.
Ex: skin color, eye color, hair color,
blood type
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Example: blood type
A (dominant), B (dominant)
AB (codominant), O (recessive)
IA - blood type
B
I – blood type
IAIB – blood type
ii – blood type
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GENOTYPES
PHENOTYPES
AA
Type A
AO
Type A
AB
Type AB
BB
Type B
BO
Type B
OO
Type O
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Codominance Problem
Example: A woman heterozygous for type B blood marries a man who
is heterozygous type A. What are the possible genotypes and
phenotypes of their children?
heterozygous female Type B (IBi)
x
heterozygous male Type A (IAi)
IB
i
Geno – 1 IA IB :
IA
i
IAIB
IBi
IAi
ii
1 IA i :
1 IB i :
1 ii
Pheno – 1 Type A : 1 Type
B: 1 Type AB: 1 Type O
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
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Codominance
Question:
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
girl - type
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Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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4. Sex-linked Traits
Genes carried only on the sex
chromosomes.
Ex: color blindness or hemophilia
XX genotype for females
XY genotype for males
1. Most sex-linked traits are
carried on X chromosome
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1. Most sex-linked traits are
carried on X chromosome
2. The Y chromosome in males is
small and cannot carry/mask any
traits.
3. Therefore, males express sexlinked traits more often than
females.
** Males that are affected don’t usually live past a certain age to pass on their
traits to offspring.
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Genotypes
XX
XX*
X*X*
XY
X0Y
Phenotypes
Normal Female
Carrier Female
Affected Female
Normal Male
Affected Male
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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Sex-linked Trait Problem
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
R
X
rr = white eyed
XY = male
Y
XX = female
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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A woman who is a carrier for colorblindness marries a man who is
colorblind. What are the possible genotypes and phenotypes of their
children?
Genotypes:
Phenotypes:
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Female Carriers
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