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Exceptions to the Rule of Dominance & Recessiveness

Exceptions to the Rule of Dominance & Recessiveness. Incomplete Dominance, Codominance, & Sex-Linked Traits. Incomplete Dominance and Codominance. Incomplete Dominance. R. R. r. r.

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Exceptions to the Rule of Dominance & Recessiveness

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  1. Exceptions to the Rule of Dominance & Recessiveness Incomplete Dominance, Codominance, & Sex-Linked Traits

  2. Incomplete DominanceandCodominance

  3. Incomplete Dominance R R r r • Heterozygotes have an appearance somewhat in between the phenotypes of the dominant and the recessive. (Mix) • Example:snapdragons (flower) • red (RR) x white (rr) • RR = red flower • rr = white flower

  4. Incomplete Dominance R R All Rr = pink (heterozygous pink) Rr Rr r r Rr Rr

  5. Incomplete Dominance

  6. Codominance • Both dominant and recessive alleles are expressed (multiple alleles) in heterozygous individuals. • Ex. Erminette chicken feathers • Both black and white feathers

  7. Codominance in Erminette Chickens

  8. Codominance • Example: blood type • 1. type A = IAIA or IAi • 2. type B = IBIB or IBi • 3. type AB = IAIB • 4. type O = ii

  9. Codominance Problem IB IB IAIB IAIB IA 1/2 = IAIB 1/2 = IBi i IBi IBi • Example: homozygous male Type B (IBIB) • x heterozygous female Type A (IAi)

  10. Another Codominance Problem IA IB IAi IBi i 1/2 = IAi 1/2 = IBi i IAi IBi • Example:male Type O (ii) x female type AB (IAIB)

  11. Codominance • Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? • boy - type O (ii) X girl - type AB (IAIB)

  12. Codominance IA i IAIB IB i ii • Answer: Parents: genotypes = IAi and IBi phenotypes = A and B

  13. Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XX genotype for females • XY genotype for males • Many sex-linked traits carried on X chromosome

  14. Sex-linked Traits fruit fly eye color XX chromosome - female Xy chromosome - male Example: Eye color in fruit flies Sex Chromosomes

  15. Sex-linked Trait Problem XR y XR Xr • Example: Eye color in fruit flies • (red-eyed male) x (white-eyed female)XRY x XrXr • Remember: the Y chromosome in males does not carry traits. • RR = red eyed • Rr = red eyed • rr = white eyed • Xy = male • XX = female

  16. Sex-linked Trait Solution: XR y XR Xr Xr y Xr Xr XR Xr Xr y 50% red eyedfemale 50% white eyed male

  17. Female Carriers

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