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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
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