dihybrid crosses incomplete dominance codominance multiple alleles in a nutshell!
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Dihybrid CrossesIncomplete Dominance
CodominanceMultiple Alleles
In a nutshell!
Review: Dominant/Recessive
• One allele is dominant over the other (capable of masking the recessive allele)– Example: Purple flowers are dominant over white
PP = purple pp = white Pp = purple
Review Problem: Dominant/Recessive
• In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.
P
p
P p
pp
Pp
Pp
PP- PP (1); Pp (2); pp (1)- ratio 1:2:1
- purple (3); white (1)- ratio 3:1
GENOTYPES:
PHENOTYPES:
Review of Monohybrid Crosses
• Remember, monohybrid crosses involve only ONE trait
• Practice… In fruit flies, red eyes are dominant over white eyes.– Cross a white-eyed fly with a homozygous
dominant red-eyed fly.– Cross two heterozygous red-eyed flies.
Draw a Punnett square for each cross,
and determine the genotypic and phenotypic ratios.
• Can you determine the genotype of a white-eyed fly just by looking at it? Why?
– Yes- it’s rr– There is only one genotype for white eyes
• Can you determine the genotype of a red-eyed fly just by looking at it? Why?
– No- it could be RR or Rr– There are two genotypes for red eyes
• How could you determine the genotype of the red-eyed fly?– Perform a test cross
r r
R
R
Rr Rr
Rr Rr
Rr
Rr rr
RR
R
R
r
r
Review of Monohybrid Crosses
• What two organisms would you cross in the test cross?– The unknown red-eyed fly with a white eyed fly
• If the red-eyed fly was RR, what would the offspring of the test cross look like?– All of the offspring would have red eyes
• If the red-eyed fly was Rr, what would the offspring of the test cross look like?– Half of the offspring would have red eyes, half would
have white eyes
Dihybrid Cross:a cross that shows the possible offspring
for two traits
Fur Color:B: Black b: White
Coat Texture:R: Roughr: Smooth
In this example, we will cross a heterozygous individual with
another heterozygous individual.
Step 1: Determine the parents’ genotypes
Step 2: You must find ALL possible gametes that can be made from each parent.
Remember, each gamete must have one B and one R.
Parent 1: BbRr Parent 2: BbRr
Parent 1: BbRr Parent 2: BbRr
Parent 1
Possible gametes:
BR
Br
bR
br
Parent 2
Possible gametes:
BR
Br
bR
br
Step 3: arrange all possible gametes for one parent along the top of your Punnett Square, and all possible gametes for the other parent down the side of your Punnett Square…
Fur Color:B: Black b: White
Coat Texture:R: Roughr:
Smooth
BbRr x BbRr
BR
bR
br
bR
Br
BR
br
Br
Step 4: Find the possible genotypes of the offspring
Fur Color:B: Black b: White
Coat Texture:R: Roughr:
Smooth
BbRr x BbRr
BR
bR
br
bR
Br
BR
br
Br
BBRR
BbRR BbRr
BBRr BBrr BbRr Bbrr
BbRR BbRr bbRR bbRr
BbRr Bbrr bbRr bbrr
BBRr
BR bR
br
bR
Br
BR
br
Br
BBRR
BbRR
BbRr
BBRr BBrr BbRr Bbrr
BbRR
BbRr bbRR
bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the offspring would have a black, rough coat?
How many of the offspring would have a black, smooth coat?
How many of the offspring would have a white, rough coat?
How many of the offspring would have a white, smooth coat?
Fur Color:B: Black b: White
Coat Texture:R: Roughr:
Smooth
BR bR
br
bR
Br
BR
br
Br
BBRR
BbRR
BbRr
BBRr BBrr BbRr Bbrr
BbRR
BbRr bbRR
bbRr
BbRr Bbrr bbRr bbrr
BBRr
How many of the offspring would have black, rough coat?
How many of the offspring would have a black, smooth coat?
How many of the offspring would have a white, rough coat?
How many of the offspring would have a white, smooth coat?
Fur Color:B: Black b: White
Coat Texture:R: Roughr:
Smooth
Phenotypic Ratio
9:3:3:1
• On a blank sheet of paper…In pea plants, yellow seeds (Y) are dominant over
green seeds (y), and rounded peas (R) are dominant over wrinkled peas (r).
Cross a plant that is heterozygous for both traits with a plant that is homozygous recessive for both traits. Draw a Punnett square to show all possible offspring, and determine the phenotypic ratio.
YyRr X yyrr
YR yR
yr
yr
yr
yr
yrYr
YyRr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
YyRr Yyrr yyRr yyrr
Yyrr
Incomplete Dominance• A third (new) phenotype appears in the
heterozygous condition.
• Flower Color in 4 O’clocks
RR = red rr = white Rr = pink
Problem: Incomplete Dominance
• Show the cross between a pink and a white flower.
- Rr (2); rr (2); RR (0)- ratio 2:2:0
- pink (2); white (2) Red (0)- ratio 2:2:0
R r
r
r
rrRr
rrRr
GENOTYPES:
PHENOTYPES:
Codominance
• The heterozygous condition, both alleles are expressed equally
• Sickle Cell Anemia in Humans
NN = normal cells
SS = sickle cells NS = some of each
Problem: Codominance• Show the cross between an individual with
sickle-cell anemia and another who is a carrier but not sick.
N S
S
S
NS
NS
SS
SS
- NS (2) SS (2)- ratio 2:2:0
- carrier (2); sick (2)- ratio 2:2:0
GENOTYPES:
PHENOTYPES:
Multiple Alleles
• There are more than two alleles for a trait
• Blood type in humans
• Blood Types?– Type A, Type B, Type AB, Type O
• Blood Alleles?– A, B, O (in book – IA, IB, I)
Rules for Blood Type
• A and B are codominant– AA = Type A– BB = Type B– AB = Type AB
• A and B are dominant over O– AO = type A– BO = type B– OO = type O
Problem: Multiple Alleles• Show the cross between a mother who has type O
blood and a father who has type AB blood.
- AO (2) BO (2)
- type A (2); type B (2)- ratio 2:2:0:0
GENOTYPES:
PHENOTYPES:
O O
A
B
AO
BO
AO
BO
Problem: Multiple Alleles• Show the cross between a mother who is
heterozygous for type B blood and a father who is heterozygous for type A blood.
-AB (1); BO (1); AO (1); OO (1)- ratio 1:1:1:1
-type AB (1); type B (1) type A (1); type O (1)- ratio 1:1:1:1
GENOTYPES:
PHENOTYPES:
A O
B
O
AB
OO
BO
AO
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