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 trait Determining # 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 color Light Right Number of individuals

Skin Color  Skin color is actually due to 5 genes  Genotypes darkest to lightest:  AABBCCDDEE would be darkest  AaBbCcDdEe would be medium  aabbccddee 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 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

Thomas Hunt Morgan  American biologist  Worked with fruit flies also called Drosophila  Found that the gene for eye color is located on the X chromosome  Discovered sex-linked inheritance

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

Sex-linked Trait Problem  Example: Eye color in fruit flies (red-eyed male) x (white-eyed female) X R Y x X r X r Remember: the Y chromosome in males does not carry traits. RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female XRXR XrXr XrXr Y

Sex-linked Trait Solution: X R X r X r Y X R X r X r Y 50% red eyed female 50% white eyed male XRXR XrXr XrXr Y

Female Carriers  Females 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 colorblindness  Male Pattern Baldness  Hemophilia  Duchenne Muscular Dystrophy  Hairy Ears  Y Chromosome