Exceptions to the Rule of Dominance Recessiveness Incomplete

Incomplete Dominance • Heterozygotes have an appearance somewhat in between the phenotypes of the

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

Codominance • Both dominant and recessive alleles are expressed (multiple alleles) in heterozygous individuals.

Codominance • Example: blood type • • 1. 2. 3. 4. type A type

Codominance Problem • Example: • homozygous male Type B (IBIB) x heterozygous female Type

Another Codominance Problem • Example: male Type O (ii) x female type AB (IAIB)

Codominance • Question: If a boy has a blood type O and his sister

Codominance • Answer: IA IB i i I AI B ii Parents: genotypes =

Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are

Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color

Sex-linked Trait Problem • Example: Eye color in fruit flies • (red-eyed male) x

Sex-linked Trait Solution: XR Xr y Xr XR Xr Xr y 50% red eyed

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

Incomplete Dominance and Codominance

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

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

Incomplete Dominance

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

Codominance in Erminette Chickens

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

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

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

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)

Codominance • Answer: IA IB i i I AI B ii Parents: genotypes = IAi and IBi phenotypes = A and B

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

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

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

Sex-linked Trait Solution: XR Xr y Xr XR Xr Xr y 50% red eyed female 50% white eyed male

Female Carriers