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Exceptions to the Rule of Dominance & Recessiveness

Understand the fascinating genetic phenomena of incomplete dominance, codominance, and sex-linked traits through examples and explanations. Explore how traits manifest with various combinations of alleles.

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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. R R r r Incomplete Dominance • Example:Snapdragons (flower) • red (RR) x white (rr) RR = red flower rr= white flower

  4. Codominance • Both alleles are dominant and expressed • Ex. Erminette chicken feathers • Both black and white feathers

  5. Dalmatians

  6. Codominance • Example: blood type • 1. type A = AA or AO • 2. type B = BB or BO • 3. type AB = AB • 4. type O = OO

  7. A O AB BO B 1/2 = AB Blood 1/2 = B Blood B AB BO Codominance Problem • Example: homozygous B male: Type B (BB) x heterozygous female Type A (AO)

  8. A B AO BO O 1/2 = Type A 1/2 = Type B O AO BO Another Codominance Problem • Example:male Type O x female type AB

  9. 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 (OO) X girl - type AB (AB)

  10. Let’s review: Determine if the picture shows codominance or incomplete dominance.

  11. What number do you see?

  12. 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

  13. XC y XC Xc Sex-linked Trait Colorblind:Father is normal/Mom is a carrierC=Normalc=Colorblind 25% chance of having a female carrier 25% chance of having a male Colorblind male

  14. XX chromosome - female Xy chromosome - male Sex-linked Traits Example: Colorblindness and Hemophilia Sex Chromosomes

  15. Xh y XH Xh Sex-linked Trait Hemophilia:Father has hemophilia/Mom is a carrierH=Normalh=Hemophilia

  16. XR y Xr Xr Sex-linked Trait Problem • 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

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

  18. Female Carriers

  19. Polygenic Traits Traits that involve multiple genesfor a single trait. -Skin Color -Eye Color

  20. Polygenic traits – traits controlled by many genes. Ex. The wide range of skin color in humans.

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