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Introduction to Genetics
Life Science
Traits survey
Why causes these differences?
Genetics - The study of heredity (how
traits are passed on)
or
x
=
or
The study of heredity started
with the work of Gregor Mendel and his
pea plant garden
Mendel was an Austrian Monk that lived
in the mid 1800’s
Mendel noted that the size of pea
plants varied. He cross-bred these
pea plants to find some surprising
results.
Mendel’s cross between tall pea plants
yielded all tall pea plants.
X
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His cross between small pea plants yielded all
small pea plants.
X
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Mendels’ cross between tall pea plants and small pea
plants yielded all tall pea plants.
x
=
Mendel then crossed these second generation tall pea
plants and ended up with 1 out 4 being small.
x
=
Mendel’s work led him to the understanding that
traits such as plant height are carried in pairs of
information not by single sets of information.
Hereditary information (DNA) is carried in
Chromosomes!
DNA
DNA is found in all living cells
– It controls all functions
inside a cell
– DNA stores all the genetic
information for a
living organism
– Single cell like an amoeba
– Multi cell like a human
Genetics
Small sections of DNA are responsible for a
“trait”. These small sections are called
“Genes”.
– Gene - A segment of DNA that codes for
a specific trait
– Trait - A characteristic an organism
can pass on to it’s offspring
through DNA
Gene
Genes
There are two main kinds of genes:
– Dominant - A gene that is ALWAYS
expressed and hides others
– Recessive - A gene that is only expressed
when a dominant gene isn’t present (hidden)
Dominant and Recessive Genes
• A dominant gene will hide a
recessive gene!
Example:
• A “widows peak” is dominant,
not having a widows peak is
recessive.
• If one parent contributes a
gene for a widows peak, and the
other parent doesn’t, the offspring will have a widows peak.
Widows Peak
Alleles
All organisms have two copies of each gene
(one from female, one from male)
Homozygous- Two copies of the same
gene
Heterozygous - Two different genes
Example:
A Widows Peak, dominant trait, would be symbolized
with a capital “W”, while no widows peak,
recessive trait, would be symbolized with a
lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W
Example:
For the widows peak:
WW - has a widows peak
Ww - has a widows peak
ww - no widows peak
Homozygous dominant
Heterozygous
Homozygous recessive
Genotype vs Phenotype
• Genotype- Genes you have
Ex: WW, Ww, or ww
• Phenotype- physical features SHOWN
Ex:
WW Widows peak
Ww Widows peak
ww NO widows peak
Example:
Since Herman has no widows peak, he must
be ______, since Lilly has a widows peak
she could be EITHER ______ or ______.
ww
WW or Ww
Punnett Square
Punnett Square - A tool we use for predicting
the traits of an offspring
–
–
–
–
Letters used as symbols to represent genes
Capital letters= dominant genes
Lower case letters= recessive genes
Genes always exist in pairs
Punnett Squares
We can use a “Punnet Square” to determine
what pairs of genes Lilly has
• A Punnet Square
begins with a box 2 x 2
• One gene is called an
“allele”
Assume Lilly is heterozygous
• One parents pair is
split into alleles on top,
the other along the side
Ww
Assume Herman is homozygous
recessive
ww
W
w
w
Ww
ww
w
Ww
ww
• Each allele is crossed
with the other allele to
predict the traits of the
offspring
Punnett Squares
Notice that when Lilly is crossed with
Herman, we would predict that half the
offspring would be “Ww”, the other half
would be “ww”
Half “Ww”, Heterozygous, and will
have a widows peak
Half “ww”, Homozygous, and
not have a widows peak
W
w
w
Ww will
ww
w
Ww
ww
Try it yourself!
Create a punnett square assuming Lilly is
homozygous dominant (WW)
W
w
w
W
What is the chance of
the offspring having a
widows peak?
Try it yourself!
All of the offspring will have a widows peak!
W
W
w
Ww Ww
w
Ww Ww
100%
Genetics
Recall that Herman and Lilly had another
offspring, Marylin. She had NO widows
peak.
Genetics
So which is true? Is Lilly homozygous
dominant (WW) or is she heterozygous
(Ww)?
W
w
W
W
w
Ww
ww
w
Ww Ww
w
Ww
ww
w
Ww Ww
Genetics
If Lilly were heterozygous,
then 1/2 of their offspring
should have a widows peak,
1/ shouldn’t
2
If Lilly were homozygous, all
of their children will have a
widows peak
W
w
W
W
w
Ww
ww
w
Ww Ww
w
Ww
ww
w
Ww Ww