Exceptions to the Rule of Dominance Recessiveness Incomplete

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

Incomplete Dominance and Codominance

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

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

Dalmatians

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

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

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

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)

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

What number do you see?

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 Trait Colorblind: Father is normal/Mom is a carrier C=Normal c=Colorblind XC XC Xc y 25% chance of having a female carrier 25% chance of having a male Colorblind male

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

Sex-linked Trait Hemophilia: Father has hemophilia/Mom is a carrier H=Normal h=Hemophilia Xh XH Xh y

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

Polygenic Traits that involve multiple genes for a single trait. -Skin Color -Eye Color

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