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Genetics
The study of traits and
how they are passed on
from one generation to
the next
What is a Trait?
Characteristics of living
things that are inherited
from their parents
Traits are expressed in
many different ways
Examples:
hair color and texture,
skin color, etc…
Tongue
Rolling
Unattached Earlobe
Hitchhiker’s
Thumb
Widow’s Peak
Attached Earlobe
Bent
Little
Finger
How are traits inherited?
Traits are controlled
by chromosomes
found in the nucleus
of cells
Chromosomes are
made of DNA, which
contain the genes that
control each trait
DNA is the blueprint for how living
things will develop and function
The DNA molecule looks
like a twisted ladder
The steps of the ladder
are made of chemical
compounds called bases
The 4 bases are adenine,
guanine, cytosine
and thymine
The bases are arranged
in different ways to
form different chemical
messages that control
different traits
Adenine always pairs
with thymine (A-T)
Guanine always pairs
with Cytosine (G-C)
Identical twins are the only
human beings who have identical
DNA
Everyone else on Earth has
different DNA combinations that
make us each UNIQUE
Gregor Mendel
The father of modern genetics
Mendel studied pea plants due to their great variation
Mendel concluded that traits
occur according to patterns, not
at random
Each parent contributes one gene
per trait to the offspring
Therefore, offspring are not
identical to their parents
Mendel found that
when he crossed a
purple-flowered
plant with a plant
that had whiteflowers, he produced
plants with purple
flowers 100% of the
time on the first
cross and plants
with purple flowers
75% of the time on
the second cross
Parents can pass on one of
two types of traits:
1. Dominant trait- the trait that
dominates (covers up) the
recessive trait
2. Recessive trait-the hidden trait
that does not appear
DOMINANT
TRAITS
RECESSIVE
TRAITS
Eye Coloring
brown eyes
grey, green, hazel,
blue eyes
Vision
normal vision
normal vision
night blindness
color blindness*
Hair
dark hair
non-red hair
curly hair
full head of hair
widow's peak
blonde, light, red hair
red hair
straight hair
baldness*
normal hairline
Facial Features
dimples
unattached earlobes
freckles
broad lips
no dimples
attached earlobes
no freckles
thin lips
A capital letter represents
a dominant trait
Tallness = T
A small letter represents
a recessive trait
Shortness = t
Remember, each
parent provides one
gene…
Types of Traits
Pure Dominant- When the offspring
receives the dominant form of a trait from
each parent (TT)
Pure Recessive- When the offspring
receives the recessive form of a trait from
each parent (tt)
Heterozygote (hybrid)- when the offspring
receives one dominant gene and one recessive
gene for a trait (Tt)
Genotype= Phenotype=
The gene
combinations
TT
The physical
appearance
Tall
Tt
Tall
tt
Short
Tall female paired
with a short male
Female
Male
TT
tt
T
t
Tt
This child will be Tall, but…
Can we ever know if a Tall
person has pure dominant tall
genes (TT) or is a hybrid (Tt) for
tallness?
It is impossible to know if a gene is pure
dominant or heterozygous simply by
looking at a person.
Pedigree Chart
A diagram that traces a
genetic trait from one
generation to the next
Reading a pedigree chart
=Female
= Male
Shaded circles or squares show the
dominant trait
Unshaded circles or squares show
the non-dominant trait
Here’s a pedigree
for curly hair,
which is a
dominant trait…
Curly
hair
Curly
hair
Curly
hair
Curly
hair
Curly
hair
Another Pedigree
Punnett Squares
Shows the probability that a child will
have a specific trait
Each box represents a 25% chance of
inheriting a particular trait
First, the father’s genes (genotype) are listed
along the top of the Punnett Square and the
mother’s genes (genotype) are listed along the
left side (Freckles (F) vs. No Freckles (f))
F
f
f
F
Next, we fill in the boxes with the letters
that represent the potential genotype of
the offspring (bring genes down & over)
F
f
f
F
F
F
f
Ff
f
Ff
Ff
Ff
RESULT: offspring have a 100% chance
of having freckles
In this example, what percentage of the
offspring would have freckles?
f
f
F
f
Ff
ff
Ff
ff
Father is heterozygous dominant (has hidden gene for no freckles)
Mother is homozygous recessive (no freckles)
Result: there is only a 50% chance the child will have freckles!
Blue-Eyed Father and BlueEyed Mother
b
b
bb
b
bb
b
bb
bb
There is a 100% chance the baby will
have blue eyes
What is the probability that the
offspring will be blue??
All healthy humans have 23
pairs of chromosomes
The 23rd pair
determines
the sex of the
organism
Gender
In Females, the chromosome pair is XX
In Males, the chromosome pair is XY
Some human diseases are a result of
genetic defects:
– Color blindness
– Tourette Syndrome
– Tay Sachs Disease
– Muscular Dystrophy
– Cystic Fibrosis
– Hemophilia
– Down Syndrome
– Dwarfism
– Albinism
Mutations
• Genetic “mistakes” that effect the way
traits are inherited. (A change in a gene
or chromosome)
• If a mutation takes place in a body cell,
it affects only the individual.
• If it occurs in a sex cell, the mutation
can be passed onto the offspring.
• Usually caused by environmental factors
such as chemicals, x-rays or radiation.
• Mutations may reduce the organisms’
chance of survival or reproduction.
• Some mutations are actually helpful
and improve the chances of survival
• Seedless navel oranges are sweeter and juicier
• Potatoes that resist diseases that attack other
potatoes
• Mutations provide variations in
species and can result in brand new
species (this is called evolution)
• Before body cells divide, the DNA code
of the parent is duplicated and a copy
is passed on to the new cells during
the process of mitosis.
• The new cells recognize the DNA code
and determine how each trait is
expressed. (phenotype- physical
expression)