Transcript Document

Revisiting Dihybrid Punnett Squares
• What exactly is a dihybrid cross?
– Predicting the outcomes from crossing 2
traits.
• Why do we use them?
– Instead of doing to monohybrid crosses, you
can combine them to see all the possible
combinations from that particular mother and
father.
Dihybrid Crosses Cont’d
• What do the tops and sides of a Punnett Square
represent?
– The gametes from the mother and father that are going to
mix (fertilize) to make the possible offspring in the middle.
Each gamete is haploid. The mother’s
eggs can have either W or w; father’s
sperm can have either W or W.
W
W
W
WW WW
w
Ww
Ww
Webbed feet (W) in ducks
are dominant to un-webbed
(w) feet. If a mother duck is
heterozygous for webbed
feet, and the father is
homozygous for webbed
feet, will any of their
ducklings have un-webbed
feet?
Mother = Ww, Father = WW
Dihybrid Crosses Cont’d
• Why do we have a Punnett Square with 4 boxes on
each side instead of 2?
– There will be 2 possible alleles that can be passed on for
each trait. You will have 4 total options to include all the
possibilities of combining the 2 traits.
Mother: WB,Wb, wB, wb
Short beaks (b) in ducks
are recessive to long
beaks (B) feet. If a father
duck has a short beak,
and the mother is a
carrier for short beaks,
will any of their ducklings
have the same type of
beak as their mother??
Father: Wb, Wb, Wb, Wb
Mother = bb, Father = Bb
The mother can pass on a B or a b; the
father can pass on a b or a b towards
the beak trait.
Remember, the mother can pass on a
W or a w allele for webbed feet and the
father can pass on a W or a W…
SO, all the possible gamete
combinations are:
Putting it all together…
Mother’s gamete choices: WB, Wb, wB, wb
Father’s gamete choices: Wb, Wb, Wb, Wb
By taking all the daughter cell possibilities, we’re covered no matter
what gametes end up getting fertilized to restore diploidy.
Wb Wb Wb Wb
WB
WWBb
WWBb
WWBb
Wb
WWbb WWbb
WWbb
WWbb
wB
WwBb
WwBb
WwBb
WwBb
Wwbb
Wwbb
Wwbb
Wwbb
wb
WWBb
Continuing our exploration of
patterns of inheritance with…
Codominance,
Polygenic Inheritance
and Blood Types
Codominance
• Heterozygote expresses both alleles’
conditions
– Ex. A black rooster bred with a white hen
produces a black and white checkered
chicken.
• Usually uses both capital letters (black= B,
white=W , checkered = BW)
Codominance practice: cross a
black rooster with a white hen
Key:
W
W
B = black
W = white
B
BW BW
B
BW BW
BW = checkered
Results:
100% checkered
(BW)
Now let’s cross a checkered hen
with a checkered rooster.
B
W
B
BB
BW
W
BW
WW
Results:
25% Black
50% Checkered
25% White
(1:2:1)
Polygenic Inheritance
• Trait controlled by 2 or more genes
• May be on the same or different
chromosomes
• Shows a range, intermediate is
most common phenotype
• Upper and lower case letters used
Skin color: A polygenic trait
Determining # of Genes Involved in Skin Color
Expected
distribution- 4
genes
Number of individuals
Observed
distribution
of skin color
Expected
distribution1 gene
Light
Expected
distribution- 3
genes
Right
Range of skin color
And the answer is…
• Skin color is actually due to 5 genes
• Genotypes darkest to lightest:
–AABBCCDDEE would be darkest
skinned
–AaBbCcDdEe would be medium
skinned
–aabbccddee would be lightest skinned
Influence of External Environment
• In arctic foxes temperature has an effect on
the expression of coat color. In winter, fur is
white; in summer, fur is brown
Influence of External Environment
• Leaves can have different sizes, thicknesses,
and shapes depending on the amount of light
they receive.
Influence of Internal Environment
• The internal
environments of males
and females are
different because of
hormones and
structural differences.
• An organism’s age can
also affect gene function.
– Ex. Adult male lion’s
manes
Coloration may also be caused by differences in
hormones between sexes
Peacock (male) Peahen (female)
Blood type quick facts
• Red blood cells are called erythrocytes
• Proteins on their surfaces are called
antigens, controlled by genes
• Antigens make antibodies to foreign
substances, which includes RBCs with
different antigens on their surface
• 4 phenotypes: A, B, AB, O
• 3 alleles: IA, IB, i
Phenotype A
Surface molecule A
• The lA allele is
dominant to i, so
inheriting either the
lAi alleles or the lA lA
alleles from both
parents will give
you type A blood.
• Surface molecule
A is produced.
Phenotype B
• The lB allele is also
dominant to i.
• To have type B
blood, you must
inherit the lB allele
from one parent and
either another lB
allele or the i allele
from the other.
• Surface molecule B is
produced.
Surface molecule B
Phenotype AB
• The lA and lB alleles
are codominant.
• If you inherit the lA
allele from one parent
and the lB allele from
the other, your red
blood cells will produce
both surface molecules
and you will have type
AB blood.
Surface molecule B
Surface molecule A
Phenotype O
•No antigens produced
Check your chart!
Blood
Group
Antigens
Antibodies
Can
receive
from
Can give to
A
A
B
A or O
A or AB
B
B
A
B or O
B or AB
AB
A and B
Both
AB
O
None
None
A, B, AB,
O
O
A, B, AB,
O
Possible genotypes for each
phenotype:
•A=
or
• B = IBIB or IBi
A
B
• AB = I I
• O = ii
A
A
I I
A
I i
Now let’s put your skills to the test…
• Can you save these patients?