Dominance: Co-Dominance, Incomplete Dominance and Blood Types
Inheritance Patterns Incomplete dominance Co dominance.
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Transcript of Inheritance Patterns Incomplete dominance Co dominance.
Incomplete Dominance What happens when a
true breeding red snapdragon (RR)
crosses with a
True breeding white snapdragon (R'R')?
Snapdragon plants
Result?
p. 143
The F1 generation are all heterozygous (RR') Pink
What happened to these traits? Did they blend?
F2 generation yielded…
F1 F2
1 red: 2 pink: 1 white (different than that of Mendel’s F2 monohybrid cross ratio of 3:1)
Conclusion: traits do not blend Explanation: Genes code for specific
proteins Most are enzymes with
different degrees of catalytic activity
In this case, One allele (R) codes for
an enzyme that produces red pigment
Conclusion: traits do not blend The other allele (R') codes
for an enzyme that does not produce pigment
So, heterozygous plants (RR'), with only one allele for the functional enzyme produces just enough red pigment to display pink. (not completely dominant!)
2 alleles with functional enzymes (RR) produce enough pigment to show red
Incomplete dominance
A condition in which the heterozygous phenotype is intermediate between the 2 homozygous phenotypes
We use RR' (not Rr) to indicate incomplete dominance
Sample problem In four O’clock plants,
red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of
1. The F1 generation?
Solution What is required?
The flower colour of F1 generation?
What is given? Red flowers (RR) incompletely dominant White flowers (R'R') Heterozygous flowers (RR') are pink
Your turn… In four O’clock plants,
red flowers are incompletely dominant over white flowers. The heterozygous flowers are pink. If a red-flowered four o’clock plant is crossed with a white-flowered four o’clock plant, what will be the flower colour of
The F1 generation crossed with its red parent?
The F1 generation crossed with its white parent?
Solution What is required?
F1 (RR') X red parent (RR) F1 (RR') X white parent (R'R')
What is given? Red flowers (RR) incompletely dominant
White flowers (R'R')
Heterozygous flowers (RR') are pink
Co dominance Sometimes, both alleles of a given
characteristic are dominant Therefore, in heterozygotes, both alleles
are expressed.
I.e. Black rooster is crossed with a white hen
Result?
Black and white plumage Both colours were
expressed
Multiple alleles In addition to co dominance, Many genes may have multiple alleles:
More than two alleles available to determine the phenotype of an organism
I.e. co dominance & multiple alleles
I.e. ABO blood group There are 3 different
alleles for blood type: IA IB
i Each type indicates a
specific glycoprotein present on blood cells (People with type O have
no glycoproteins on their blood cells)
(People with AB have both types present)
A is dominant to O B is dominant to O AB are both co
dominant
Sample problem If a woman has blood type AB, and a man
has blood type A, what possible blood types will their children have?
Solution:
IAIA IAIB
IAIA IAIB
IAIA IAIB
IAi IBi
IA IB
IA
i
2 type A, 2 type AB 2 type A, 1 type AB, 1 type B
IA IB
IA
IA
Your turn… Suppose a man with
blood type B marries a woman with type AB blood. What blood types would you expect to find among their children?
What is required? Possible blood types
of children
What is given? Man – IBIBor IBi Woman - IAIB