Transcript WW - Mrs. Chan

Introduction to Genetics
Who was Gregor Mendel?
• He was known as the “FATHER OF
GENETICS”
• He discovered how traits were inherited
Genetics: The study of heredity,
how traits are passed from parent to offspring
or
x
=
or
Mendel’s Peas
• Mendel did his study on pea plants
• Pea plants have many traits
(tall/short, purple flowers/white
flowers)
• Pea plants can be
self-fertilized or
cross-fertilized
• Mendel crossed (bred) tall pea plants
with short pea plants. Predict what the
offspring were like.
Tall x Short
?
Mendel allowed the F1 plants to
self-pollinate
Types of Plants
1.True-Breeding - these plants
always create plants that look
like themselves
2.Hybrids – offspring of truebreeding plants
Tall x Short = Hybrid
Dominance
Some traits are dominant over
others.
Tall x Short = all tall offspring
(hybrids)
*Tall is the dominant trait
* Short is recessive
Genes and Alleles
Mendel discovered that each trait is
controlled by two factors (alleles)
Genes – factors that determine your
traits
There are three basic 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
–Codominant - Genes that work
together to produce a third trait
GENOTYPE - what genes,
letters, the organism has (TT, Tt,
tt)
PHENOTYPE - what it looks
like (tall or short)
Dominant and Recessive Genes
• A dominant gene will always mask a recessive
gene.
• A “widows peak” is dominant, not Widows Peak
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.
Punnett Square - A tool we use for
predicting the traits of an offspring
• Shows all of the possible combinations of
alleles from the parents
Alleles: different forms of the same gene
• Organisms have two alleles for each gene
(one from each parent)
– Letters are used as symbols to designate
genes
– Capital letters are used for dominant
genes
– Lower case letters are used for
recessive genes
– Genes always exist in pairs
Ex) Tall pea plants x short pea plants
(TT)
(tt)
genotype
TALL
SHORT
T
T
t
tT
tT
t
tT
tT
offspring
100% tall (tT)
Ex) Tall pea plants x tall pea plants
(tT)
(tT)
genotype
TALL
TALL
T
t
T
TT
tT
t
tT
tt
offspring
¾ tall – 75%
¼ short – 25%
All organisms have two copies of each
gene, one contributed by the father,
the other contributed by the mother.
Homozygous – same alleles (TT, tt)
Heterozygous – different alleles (tT)
A Widows Peak, dominant, would be
symbolized with a capital “W”, while no
widows peak, recessive, would be
symbolized with a lower case “w”.
Father - No Widows Peak - w
Mother - Has a Widows Peak - W
For the widows peak:
WW - has a widows peak
Ww - has a widows peak
ww - no widows peak
Homozygous dominant
Heterozygous
Homozygous recessive
Since Herman has no widows peak, he must
be “ww”, since Lilly has a widows peak she
could be either “WW” or “Ww”
Definitely ww
Either Ww
or WW
Homozygous
recessive
Heterozygous
Homozygous
dominant
We can use a “Punnet Square” to determine
what pairs of genes Lilly has
• A Punnet Square
begins with a box 2 x 2
Assume Lilly is heterozygous
Ww
Assume Herman is homoozygous
recessive
ww
• One gene is called an
“allele”
W
w
w
Ww
ww
w
Ww
ww
• One parents pair is
split into alleles on top,
the other along the side
• Each allele is crossed
with the other allele to
predict the traits of the
offspring
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
will not have a widows peak
W
w
w
Ww
ww
w
Ww
ww
Genetics
Another possibility is that Lilly might be
“WW”, homozygous dominant.
Assume Lilly is homozygous
dominant
WW
W
Assume Herman is homoozygous
ww
W
w
Ww Ww
w
Ww Ww
Notice that all the
offspring are
heterozygous and will
have a widows peak
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
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
Recall that Herman and Lilly had another
offspring, Marylin. She had no widows peak,
therefore, Lilly must be heterozygous.
Hair color is a perfect example of a
trait
Prince Charming
is blond
Snow White
has dark hair
What color hair
should their children
have?
Hair color is different from widows
peak, no color is truly dominant.
– Brown and blond are the two, true
traits
– Homozygous conditions produce
either brown or blond hair
– Heterozygous conditions produce
red hair
For Snow White to have brown hair she
must be homozygous dominant, “BB”, a
blond Prince Charming must be
homozygous recessive, “bb”.
B
B
b
Bb
Bb
b
Bb
Bb
All the offspring from Prince Charming
and Snow White will therefore be
heterozygous, “Bb”, and since hair color
is codominant….. all their children will
have red hair.
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