Polygenic Inheritance

Trait controlled by 2 or more genes

May be on the same or different chromosomes

Shows a range, intermediate is most common phenotype

Upper and lower case letters used

Many variations in skin color

Skin color: A polygenic traitSkin color: A polygenic traitDetermining # of Genes Involved in Skin Color

Observed distribution of skin color

Expected distribution- 1 gene

Expected distribution- 4 genes

Expected distribution- 3 genes

Range of skin colorLight Right

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Skin ColorSkin color is actually due to 5 genes

Genotypes darkest to lightest:AABBCCDDEE would be darkestAaBbCcDdEe would be mediumaabbccddee would be lightest

Influence of external environment

In arctic foxes temperature has an effect on the expression of coat color. In winter, fur is white.

Arctic Fox in summer coloration: influence of external environment

External influences can also be seen in leaves. Leaves can have different sizes, thicknesses, and shapes depending on the amount of light they receive.

Influence of external environment

The internal environments of males and females are different because of hormones and structural differences.

An organism’s age can also affect gene function.

Influence of internal environment

Peacock (male) Peahen (female)

Sex-linked TraitsSex-linked Traits

Traits Traits (genes) located on the(genes) located on the sex chromosomessex chromosomes

Sex chromosomes are Sex chromosomes are X and X and YY

XXXX genotype for females genotype for femalesXYXY genotype for males genotype for malesMany Many sex-linked traitssex-linked traits carried carried on on XX chromosome chromosome

Thomas Hunt Morgan

American biologistWorked with fruit flies also called DrosophilaFound that the gene for eye color is located on the X chromosome

Discovered sex-linked inheritance

Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies

Sex-linked Trait Sex-linked Trait ProblemProblem

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 eyedRr = red eyedrr = white eyedXY = maleXX = female

XR

Xr Xr

Y

Sex-linked Trait Solution:Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female50% white eyed male

XR

Xr Xr

Y

Female CarriersFemale CarriersFemales are carriers of sex-linked

traits if they have the heterozygous genotype.

Males cannot be carriers because they only have one X chromosome.

Female parents who are carriers, pass sex-linked traits to all children, but males are usually the ones who express the trait.

Examples of Sex-linked Traits: Red-green colorblindnessMale Pattern BaldnessHemophiliaDuchenne Muscular DystrophyHairy Ears

Y Chromosome