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Genetics
Punnett Squares
PA State Standards
3.3.7.C
Genetics
The study of heredity or the passing on
of traits from an organism to its
offspring.
In your cells you have chromosomes.
These chromosomes contain DNA
This DNA contains all of the
information that makes you …YOU!
Gregor Mendel
(1822 – 1884)
Mendel, the father of
genetics, was an Augustinian
monk who taught natural
science to high school
students. He was working on
pea plants
His work was so brilliant at the
time it took thirty-four years
for the rest of the scientific
community to catch up to it.
Gregor Mendel
(1822 – 1884)
The impact of genetic theory is
great. Many diseases are known
to be inherited. Determining
the probability of passing along
a hereditary disease is an
important area of study.
Mendel’s Pea Experiment
Mendel worked with pea
plants.
The practical results of
Mendel's research has not
only changed the way we
perceive the world, but
also the way we live in it.
He wanted to see how
certain traits or
characteristics were
transferred from parent
pea plant to offspring.
How did he breed pea plants?
Most flowering plants
contain a STAMEN, or
the male reproductive
structure of the plant.
These stamen produce
POLLEN, which are the
male sex cells (much
like sperm cells)
Plant reproduction
Plants also contain
a PISTIL, which is
the female
reproductive
structure in a plant.
This pistil also
contains the female
sex cell, the EGG
(ovule).
Pollination
Occurs when the
pollen lands on top
of the pistil of the
flower.
Pollination produces
seeds for the next
generation.
Cross pollination
Taking pollen from
one plant with a
certain trait and
placing it on a plant
with a different trait.
For example, taking
pollen from a short
plant and placing it
on a plant that is
tall.
Terms to know…
Alleles each member of a gene pair
that determines a specific trait
Dominant Trait the “more forceful”,
the “stronger” of the two traits
Recessive Trait the “less forceful”, the
“weaker” of the two traits; this trait
seems to disappear when the dominant
trait is present
More terms to know…
Homozygous (pure) a trait that has two of
the same members of individual genes
Heterozygous (hybrid) a trait that has two
different members of a gene
Phenotype the physical appearance of an
organism
Genotype the actual gene make up of an
organism
Dominant Genes
Certain genes are
dominant, you only
need one of the pair
to give you its trait.
Dominant genes are
represented as capital
letters
Have a partner
examine your
earlobes. If they hang
free at the bottom,
you are dominant.
(Alleles)
(E __ )
Recessive Genes
Other genes are
recessive, you both
genes in the pair as
recessive to give you
the trait.
Recessive genes are
represented as lower
case letters
If your earlobes are
attached, you are
recessive.
(Alleles)
(e e)
Pure Trait
(Homozygous)
Brown Eyes
Having two identical
genes (alleles) for a
given trait.
Individuals homozygous
for a trait always breed
true/pure; that is, they
produce offspring that
resemble them in
appearance.
BB
Blue Eyes
bb
Hybrid Trait
(Heterozygous)
Brown Eyes
Having dominant and
recessive (alleles) for a
given trait.
Only the dominant trait
is expresses in the
phenotype.
Offspring of
heterozygouts can show
the trait or the trait can
remain recessive.
Bb
Detached Ear Lobe
Ee
Genotype & Phenotype
The genes of an
individual are called
its genotype.
Genotype
Genotype & Phenotype
The genes of an
individual are called
its genotype.
These genes
determine what an
individual will look
like, which is called
its phenotype.
Phenotype
Mendel’s experiment
He took two plants
that were short and
cross pollinated
them.
In this next
generation he
discovered that all of
the offspring were
short.
Mendel’s experiment
He then decided to
take a plant that
was short and mix it
with a plant that
was tall.
All of the
offspring were
TALL. There were
no short plants.
Mendel’s experiment
He then took one of the
offspring he had just
worked with (tall and
short parent) and allowed
this plant to selfpollinate (pollinate
itself).
The result = some tall
and some short plants.
The short gene had
reappeared.
Mendel concluded there
must be dominant and
recessive genes.
Punnett Squares
The easiest way to
calculate the
mathematical
probability of
inheriting a specific
trait is the Punnett
Square.
In a graphical way
you can determine
the possible
genotypes of the
offspring produced.
Reginald Punnett
(1875 – 1967)
Punnett Squares 1
A true (homozygous) blue eyed
b father
b and a
true (homozygous) brown eyed mother want to
see the probability for their children’s eye color.
B
Bb
Bb
1. How can we show this “crossing” of traits?
2. What would be theBgenotype
of
the
children?
Bb Bb
3. What would be the phenotype of the children?
Genotype: Heterozygous
Phenotype: 100% Brown Eyes
Punnett Squares 2
A true (homozygous) blue eyed
b father
b and a
hybrid (heterozygous) brown eyed mother want
to see the probability for their children’s eye
B Bb Bb
color.
1. What would be the genotype of the children?
b
b
b
b
b
2. What would be the phenotype of the children?
Genotype(s): Heterozygous & Homozygous
Phenotype: 50% Brown Eyes : 50% Blue Eyes
Punnett Squares 3
A hybrid (heterozygous) brown
B eyedbfather and
a hybrid (heterozygous) brown eyed mother
want to see the probability for their children’s
B BB B b
eye color.
1. What would be the genotype of the children?
b
B
b
b
b
2. What would be the phenotype of the children?
Genotype(s): Heterozygous & Homozygous
Phenotype: 75% Brown Eyes : 25% Blue Eyes
Punnett Squares (Monohybrid
Cross)
Genotype(s):
___________________________
Phenotype:
___________________________
Punnett Squares (Dihybrid
Cross)
Genotypes:
Phenotypes:
Credits
http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.htm
l
http://www.accessexcellence.org/RC/VL/GG/mendel.html
http://www.mendel-museum.org/eng/8garden/
http://www.dushkin.com/connectext/psy/ch02/eyes.mhtml
http://www.anistonavenue.com/new_pics_13.htm
http://biology.about.com/library/glossary/bldefalleles.htm
http://www.windows.ucar.edu/tour/link=/earth/Life/genetics_in
heritance.html
http://student.biology.arizona.edu/sciconn/heredity/worksheet_
heredity.html
http://anthro.palomar.edu/mendel/mendel_2.htm
www.scienceatclifton.co.uk/clifton/punnett.htm