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Human Genetics
What Genes do you have?
What is a trait?
• “Your Mom gives you
½ of the instructions,
your Dad gives you
the other ½.”
• In science jargon:
Your Mom gives you
one allele of that gene
and your Dad gives
you one allele of that
gene and you put
them together to make
a chromosome.
Genotype vs Phenotype
• What instructions or genes you have from your
parents are said to be your genotype
• What people see on the outside is your
phenotype
• So your genotype determines your phenotype!
Dominant Traits
• When alleles get together, some alleles are
“stronger” than other alleles.
• We call these alleles dominant and then the trait
that they express the dominant trait.
• Both parents might give you instructions on how
your ear lobes will look, but some looks are
dominant (stronger) than others
• Let’s try a simulation…
Male or Female?
• Your sex is
determined by
whether
• you get an X or Y
chromosome from
your father.
• Everyone receives an
X from their mother.
• Women = XX
• Men = XY
Ear Lobes
• Free earlobes
•
(dominant trait) hang
below the point of
attachment to the
head.
Attached ear lobes
(recessive trait) are
attached directly to the
side of the head.
Traits inherited
The physical characteristics below
are common genetic traits inherited
from one generation to the next:
 Free vs. Attached Earlobes
 Straight vs. Curved Thumbs
 Bent vs. Straight Pinky
 With vs. Without White Forelock
Thumbs
• Straight thumbs
•
(dominant trait) can be
seen as nearly a
straight line and may
contain a slight arch
when viewed from the
side as in the
illustrations.
Curved thumbs
(recessive trait) can be
seen as part of a circle.
Pinky?
Bent pinky (dominant
trait) vs. Straight pinky
(recessive trait):
1. Hold your hands
together as if you are
covering your face.
2. If the tips of the pinkies
(or baby fingers) point
away from one another,
the pinkies are bent
(recessive trait).
Forelock
• A White Forelock
•
(dominant trait) is a
patch of white hair,
usually located at the
hairline just above the
forehead. The photo to
the left clearly shows
an exaggerated white
forelock.
No White Forelock is
the recessive trait.
Cleft Chin
• No cleft dominant,
• cleft recessive
Widows Peak
• Widow peak dominant
• straight hairline
recessive
Freckles
•
Freckles dominant,
no freckles recessive
Tongue Roller vs Non Roller
•
Roller dominant,
nonroller recessive
• Folding Inability
dominant, ability
recessive
Dimples vs No Dimples
• Dimples are thought to be Dominant
• No dimples is recessive.
Hand Clasping
• Hand clasping is
believed to be
determined by genetic
factors,
• although it is unclear if
one trait is dominant
over the other.
• 55%= Left
• 45% right
Bent Finger
• Having a bent little finger is a
dominant trait
Multiple Alleles •

Eye color is determined by more
than one gene
Thus eye color appears to vary on an
almost continuous scale from brown to
green to gray to blue
 Eye color is determined by two genes,
 one controls texture of the iris
which refracts light to make blue.
 A second determines relative
abundance of melanin.
 When a small amount of melanin
is present, green eyes result
while brown and black eyes
result from relatively increasing
amounts of melanin