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Incomplete DominanceCodominanceMultiple Alleles
In a nutshell!
HW: Complete the HW
Packet (by Monday)
Resubmission of genetics Quiz 2 by Monday
Review: Dominant/Recessive
• One allele is dominant over the other (capable of masking the recessive allele)
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:
Incomplete Dominance
• A third (new) phenotype appears in the heterozygous condition.
RR = red rr = white Rr = pink
Problem: Incomplete Dominance
• Show the cross between a red and a white flower.
- RR (1); Rr (2); rr (1)- ratio 1:2:1
-Red (1); Pink (2); white (1)- ratio 1:2:1
R r
R
r
Rr
rrRr
GENOTYPES:
PHENOTYPES:
RR
Problem: Incomplete Dominance
• Show the cross between a pink and a white flower.
- Rr (2); rr (2)- ratio 1:1
- pink (2); white (2)- ratio 1:1
R r
r
r
rrRr
rrRr
GENOTYPES:
PHENOTYPES:
Codominance• The heterozygous condition,
both alleles are expressed equally
NN = normal cells
SS = sickle cells NS = some of each
Sickle Cell Anemia in Humans
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 1:1
- carrier (2); sick (2)- ratio 1:1
GENOTYPES:
PHENOTYPES:
What do you notice about the letters N and S in the Genotypes of F1?
How Do We Determine If a Trait IS
• Dominant/Recessive• Incomplete
Dominance• Co-Dominance
• Rr • Rr
• RW
Multiple Alleles
• Sometimes there are more than two alleles for a trait – can you think of any?
• Blood type in humans
• Blood Types?– Type A, Type B, Type AB, Type O
• Blood Alleles?– A, B, O (often expressed – 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.
-IA Io (2) IB Io (2)- ratio 1:1
- type A (2); type B (2)- ratio 1:1
GENOTYPES:
PHENOTYPES:
Io Io
IA
IB
IA Io
IB Io
IA Io
IB Io
What do you notice about the letters N and S in the Genotypes of F1?
Blood Type – Two Ways to Write It
In Cases of CO-Dominance
• Both alleles are expressed in the heterozygote and both take a CAPITAL CASE letter
• An index letter may identify the allele
Therefore (Blood):
• Normal haemoglobin allele is also HbN
• Sickle cell allele is also HbS
Comprehension Questions
• ROAN CATTLE• Red Hair (RR)• White (WW)• HETEROZYGOUS = • Is this
– A: Codominance– B: Incomplete Dominance
What color is the Heterozygous Offspring?
BIRD WINGS
• White Wings (WW)
• Blue Wings (BB)
• Heterozygous =
• What gives it away that it is CO-Dominance?
Rose Color
• Red Rose (RR)• White Rose (ww)• Heterozygous =• Is this
– A: Codominance– B: Incomplete Dominance
• What gives it away that it is Incomplete-Dominance?
Human Skin Color
• Incomplete dominance
• Controlled by 4 to 6 genes!!!
• (Take Away – ONE CHARACTERISTIC MAY BE CONTROLLED BY MULTIPLE GENES!)