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Non-Mendelian Genetics

This review covers non-Mendelian genetics, including the concepts of dominant and recessive alleles, as well as incomplete dominance, codominance, multiple alleles, polygenic traits, and sex-linked traits. Examples and cross calculations are provided to illustrate these concepts.

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Non-Mendelian Genetics

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  1. Non-Mendelian Genetics

  2. phenotype: purple phenotype: white phenotype: purple Mendelian Genetics: Dominant & Recessive Review • One allele is DOMINANT over the other (because the dominant allele can “mask” the recessive allele) genotype: PP genotype: pp genotype: Pp

  3. PP Pp Pp pp 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 GENOTYPES: - PP (25%) Pp (50%) pp (25%) - ratio 1:2:1 P p PHENOTYPES: - purple (75%) white (25%) - ratio 3:1

  4. Non-Mendelian Genetics • Incomplete Dominance • Codominance • Multiple Alleles • Polygenic Traits • Sex-Linked Traits

  5. RR = red rr = white Rr = pink Incomplete Dominance • a third (new) phenotype appears in the heterozygouscondition as a BLEND of the dominant and recessivephenotypes. Ex - Dominant Red (R) + Recessive White (r) = Hybrid Pink (Rr)

  6. R r r r Rr Rr rr rr Problem: Incomplete Dominance • Show the cross between a pink and a white flower. GENOTYPES: - RR (0%) Rr (50%) rr (50%) - ratio 1:1 PHENOTYPES: - pink (50%); white (50%) - ratio 1:1

  7. NS = some of each SS = sickle cells NN = normal cells Codominance • in the heterozygous condition, both alleles are expressed equally with NO blending! Represented by using two DIFFERENT capital letters. • Example: Dominant Black (B) + Dominant White (W) = Speckled Black and White Phenotype (BW) • Sickle Cell Anemia - sick

  8. Codominance Example: Speckled Chickens • BB = black feathers • WW = white feathers • BW = black & white speckled feathers • Notice – NO GRAY! NO BLEND! Each feather is either black or white

  9. Codominance Example: Rhodedendron • R = allele for red flowers • W = allele for white flowers • Cross a homozygous red flower with a homozygous white flower.

  10. Codominance Example:Roan cattle • cattle can be red (RR – all red hairs) white (WW – all white hairs) roan (RW – red and white hairs together)

  11. Codominance Example:Appaloosa horses • Gray horses (GG) are codominant to white horses (WW). The heterozygous horse (GW) is an Appaloosa (a white horse with gray spots). • Cross a white horse with an appaloosa horse. W W G W GW GW WW WW

  12. NS SS SS NS Problem: Codominance • Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S GENOTYPES: - NS (50%) SS (50%) - ratio 1:1 S S PHENOTYPES: - carrier (50%) sick (50%) - ratio 1:1

  13. Multiple Alleles • there are more than twoalleles for a gene. Ex – blood type consists of two dominant and one recessive allele options. Allele A and B are dominant over Allele O (i)

  14. Multiple Alleles:Lab Mouse Fur Colors • Fur colors (determined by 4 alleles): black agouti yellow

  15. Multiple Alleles:Rabbit Fur Colors • Fur colors (determined by 4 alleles): full, chinchilla, himalayan, albino

  16. Multiple Alleles:Blood Types (A, B, AB, O) • Rules for Blood Types: A and B are co-dominant (Both show) AA or IAIA = type A BB or IBIB = type B AB or IAIB = type AB A and B are dominant over O (Regular dom/rec) AO or IAi = type A BO or IBi = type B OO or ii = type O

  17. Multiple Alleles:Blood Types (A, B, AB, O)

  18. Ai Ai Bi Bi Problem: Multiple Alleles • Show the cross between a mother who has type O blood and a father who has type AB blood. GENOTYPES: i i - Ai (50%) Bi (50%) - ratio 1:1 A B PHENOTYPES: - type A (50%) type B (50%) - ratio 1:1

  19. Bi AB Ai ii 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. GENOTYPES: A i • AB (25%); Bi (25%); Ai (25%); ii (25%) - ratio 1:1:1:1 B i PHENOTYPES: • type AB (25%); type B (25%) type A (25%); type O (25%) - ratio 1:1:1:1

  20. Polygenic Traits • traits produced by multiple genes • example: skin color

  21. Sex-Linked Traits • Gene is attached to the X chromosome only, not found on the Y chromosome at all. (women have XX, men have XY chromosomes). These disorders are more common in boys. • examples: red-green colorblindness, hemophilia

  22. Sex-Linked Traits • in males, there is no second X chromosome to “mask” a recessive gene. If they get an X with the disorder, they have it. Girls must inherit defective X’s from both parents.

  23. Sex-Linked Traits A: 29, B: 45, C: --, D: 26  Normal vision A: 70, B: --, C: 5, D: --  Red-green color blind A: 70, B: --, C: 5, D: 6  Red color blind A: 70, B: --, C: 5, D: 2  Green color blind

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