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Incomplete dominance Co-dominance Multiple alleles IB topic 4.3 (pages 103-104)
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Review: Dominant/Recessive One allele is dominant over the other (capable of masking the recessive allele) PP = purple pp = white Pp = purple
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Review Problem: Dominant/Recessive PpPp P p pp Pp PP - PP (1); Pp (2); pp (1) - ratio 1:2:1 - purple (3); white (1) - ratio 3:1 GENOTYPES: PHENOTYPES: In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants.
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Incomplete Dominance A third (new) phenotype appears in the heterozygous condition. Flower Color in 4 O’clocks RR = red rr = whiteRr = pink
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Problem: Incomplete Dominance Show the cross between a pink and a white flower. GENOTYPES: - Rr (2); rr (2) - ratio 1:1 PHENOTYPES: - pink (2); white (2) - ratio 1:1 rrRr rr Rr rrrr
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Codominance The heterozygous condition, both alleles are expressed equally Sickle Cell Anemia in Humans NN = normal cells SS = sickle cells NS = some of each
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Codominance Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S SSSS SS - NS (2) SS (2) - ratio 1:1 - carrier (2); sick (2) - ratio 1:1 GENOTYPES: PHENOTYPES:
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Multiple Alleles There are more than two alleles for a trait Blood type in humans Blood Types (4 different phenotypes) Type A, Type B, Type AB, Type O Blood Alleles Pg. 104
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Blood types The letters refer to 2 carbohydrates (A substance and B substance) that may be found on the surface of the red blood cells A person’s blood cells may have: One type or the other (A or B) Both (type AB) Neither (type O)
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Blood types continued The four blood groups result from various combinations of the three different alleles of one gene Genotypes: I A (A carbohydrate) I B (B carbohydrate) i (giving rise to neither) Because each person carries 2 alleles, there are 6 possible genotypes
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Blood types continued Both the I A and I B alleles are dominant to the i allele Therefore, I A I A and I A i individuals have type A I B I B and I B i individuals have type B I A I B individuals have type AB (codominant) ii individuals have type O
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Why is this important? Matching compatible blood groups is critical for blood transfusions A person’s blood produces specific proteins called antibodies again foreign blood factors If the donors blood has a factor (A or B) that is foreign to the recipient, antibodies produced by the recipient bind to the foreign molecules and cause the donated blood cells to clump together BAD!!
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Random facts O is the universal donor AB is the universal recipient + and – comes from the Rh test Another type of antigen Key test for pregnant woman If fetus and mother are Rh incompatible, the mother can receive a vaccine
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Problem: multiple alleles Show the cross between a mother who has type O blood and a father who has type AB blood. - I A i (2); I B i (2) - ratio 1:1 - AO (2); BO (2) - ratio 1:1 GENOTYPES: PHENOTYPES: O O ABAB AO BO AO BO
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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. -I A I B, I B i, I A i, ii - 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 BOBO AB OO BO AO
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Mix-up at the hospital One busy night in an understaffed maternity unit, four children were born about the same time. Then the babies were mixed up (by mistake); it was not certain which child belonged to which family. Fortunately, the children had different blood groups: A, B, AB, and O. The parents blood groups were also known: Mr. and Mrs. Jones (A x B) Mr. and Mrs. Gerber (O x O) Mr. and Mrs. Lee (B x O) Mr. and Mrs. Santiago (AB x O) The nurses were able to decide which child belonged to which family. Deduce how this was done.
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Answer Jones: parents of the AB child Lees: parents of the B child Gerbers: parents of the O child Santiagos: parents of the A child
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How? Jones A x B AA x BB (AB), AO x BO (AB or O), AA x BO (AB or A), AO x BB (AB or B) So, they could be the parents of any of the kids Lees B x O BB x OO (B), BO x OO (B or O) Parents of either the B or O children Gerbers O x O OO x OO Must be parents of the O child Therefore, Lees must be parents of the B child Santiagos AB x O AB x OO (A or B) Can’t be parents of the B child b/c of the Gerbers, so they are the parents of the A child Therefore, the Jones are the parents of the AB child
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