Download Polygenic/Sex

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
Expected
distribution- 4
genes
Number of individuals
Observed
distribution
of skin color
Expected
distribution1 gene
Light
Expected
distribution- 3
genes
Right
Range of skin color
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
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 internal
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.
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
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 rX r
Remember: the Y chromosome in males
does not carry traits.
Xr
Xr
RR = red eyed
Rr = red eyed
rr = white eyed
XY = male
XX = female
XR
Y
Sex-linked Trait Solution:
50% red eyed female
50% white eyed male
Xr
Xr
XR
XR Xr XR Xr
Y
Xr Y
Xr 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