Download Punnett Square

Objective:
• The learner will realize that certain
characteristics are passed from parent to
offspring
What is Heredity
• Heredity is the passing of traits
from parent to offspring
• You inherit traits from your
parents
What are traits?
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Eye color
Hair color
Height
Weight
Body structure
Facial features
Skin color
Body Shape
Skin Color
Hair Texture
Hair color
Eye Color
What traits did you inherit from your
parents?
• Complete the genetic scavenger hunt with
the people at your table. This is today’s
science starter. Put your answers in your
starter section.
• List any unusual traits that your group
members have on your starter.
• You have 10 minutes to complete this
assignment
• Be ready to share with class
How are traits passed?
• When organisms reproduce,
traits are passed from parent
to offspring
• These traits are carried in
DNA, the genetic material
found in a cell’s nucleus
• DNA acts like a blueprint
What are genes?
• Genes are a segment of DNA
on a chromosome that
controls a particular trait.
• Genes are located on the
chromosomes in the nuclei.
Each organism has a fixed
number of chromosomes.
• Humans have 23 pairs (46)
chromosomes.
• Genetics is the study of how
traits are passed on from one
generation to another
Baker 2003/2004
Genetics
Hair color is a perfect example of a
trait
What color hair
should their children
have?
Prince Charming
is blond
Snow White
has dark hair
Notes
• A. Heredity- passing of traits from parent
to offspring.
– 1. Genes on chromosomes control the trait
that show up in an organism.
– 2. The different forms a trait that a gene may
have are alleles (inherit one from each parent)
-3. During meiosis a pair of chromosomes
separate and alleles move into separate cells.
4. Each chromosome now contains one gene
for each trait.
5. The study of how traits are inherited is
genetics.
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
Father of Genetics
 Monk and teacher.
 Experimented with purebred tall and short
peas.
 Discovered some of the basic laws of
heredity.
 Studied seven purebred traits in peas.
 Called the stronger hereditary factor
dominant.
 Called the weaker hereditary factor
recessive.
 Presentation to the Science Society
in1866 went unnoticed.
 He died in 1884 with his work still
unnoticed.
 His work rediscovered in 1900.
 Known as the “Father of Genetics”.
B. Gregor Mendel- the father of genetics
• 1. Mendel was the first to use the
mathematics of probability to explain
heredity and to trace one trait for several
generations.
• 2. Made careful use of scientific methods,
which resulted in the first recorded study
of how traits pass from one generation to
the next
3. Hybrid- receives different genetic
information for a trait from each
parent.
A. dominant allele- A trait that covers
over, or dominates, another form of
that trait
–Trait that always shows up, even
when only one of the two alleles is
in the dominant form
–Shown by a capital letter
B. Recessive
Allele- A trait that is
covered over, or dominated, by
another form of that trait and
seems to disappear
–Hidden when the other copy of the
gene contains the dominant allele.
–A recessive allele shows up only
when there is no dominant allele
present
–Shown with a lower-case letter
Steps of
Mendel's
Experiment
Mendel then crossed these second generation tall pea
plants and ended up with 1 out 4 being small.
x
=
Mendel’s cross between tall pea plants yielded all
tall pea plants. His cross between small pea plants
yielded all small pea plants.
X
=
X
=
Mendels’ cross between tall pea plants and small pea
plants yielded all tall pea plants.
x
=
4. What is a Punnett Square?
• A. A tool to predict the probability of certain
traits in offspring that shows the different
ways alleles can combine
• B. Probability helps you determine the
chance that something will happen.
• C. A way to show phenotype & genotype
• D. A chart that shows all the possible
combinations of alleles that can result when
genes are crossed
E. Letters stand for dominant
and recessive alleles
F. An uppercase letter stands
for a dominant allele
G. Lowercase letters stand
for recessive alleles
5. Genotype- the genetic makeup of an
organism (genes inherited from parents)
• a. homozygous- Both alleles [forms of
the gene] are the same
• When offspring inherit two dominant
genes, (one dominant gene from each
parent) they are said to be homozygous
dominant
• When offspring inherit two recessive
genes, (one recessive gene from each
parent) they are said to be homozygous
recessive
• b. heterozygous-When alleles occur
in different forms
–When offspring inherit one
dominant gene and one recessive
gene, they are said to be
heterozygous
–Since the dominant gene will be
expressed, they are said to be
heterozygous dominant
• 6. PHENOTYPE- Outward physical
appearance and behavior of an organism
• Part the genotype that is visible.
• Example- color of hair, eye, shape of body, left or
right handed.
7. Three Principles of Heredity
• 1. Traits are controlled by alleles on
chromosomes
• 2. An allele’s effect is dominant or
recessive
• 3. When a pair of chromosomes separate
during meiosis the different alleles for a
trait move into separate sex cells
Introduction to
Punnett Squares:
• A Punnett Square is a chart drawn to
determine the probable results of a genetic
cross. To be able to draw a Punnett
Square, you must know the genotype of
both parents.
R
r
R
RR
Rr
r
RR
rr
Punnett Squares
• Sample Problem 1. In roses, red flowers
are dominant over white flowers. What are
the possible offspring when a homozygous
red rose is crossed with a homozygous
white rose.
Baker 2003/2004
Generation 1
homozygous red
rose = RR
homozygous white
rose = ww
Baker 2003/2004
Generation 2
homozygous red
rose = RR
heterozygous red
rose = Rw
homozygous white
rose = ww
Baker 2003/2004
Generation 3
homozygous red
rose = RR
heterozygous red
rose = Rw
homozygous white
rose = ww
Baker 2003/2004