Passing of traits from parents to offspring
Trait: distinguishing quality
Why then don’t you look exactly like your
About 150 years ago Gregor Mendel
performed some experiments that began to
help us find the answers
Born 1822 in Austria
Grew up on his family’s farm where he learned to
grow flowers and fruit trees
Went to a monastery where he worked in the
Interested in how traits are passed from parents
Sometimes a trait that appeared in one
generation did not show up in any of the
offspring in the next generation.
In the 3rd generation, the trait showed back
up. Mendel noticed similar patterns in
people, plants, and many other living things.
He chose to study garden pea plants.
Garden peas were a good choice because
They grow quickly
Come in many varieties
Self-pollinating: contains both male and
female reproductive structures
Mendel chose to study only one
characteristic, like plant height or pea color, at
He made sure to use true-breeding plants
When true-breeding plants self-pollinate they
always produce offspring with the same trait
the parent plant has.
Mendel wanted to see what would happen if
he crossed two plants that had different
forms of a trait
To do this, he used cross-pollination. This
method removes the male reproductive parts
of a plant so that it can’t self-pollinate.
MENDEL’S FIRST EXPERIMENT
Performed crosses to study 7 different
Each cross was between the two traits of each
MENDEL’S FIRST EXPERIMENT
•Mendel got similar results for
each of the crosses that he made.
•One trait always appeared and
the other trait seemed to vanish.
Dominant trait: observable trait when at least
one dominant allele for a characteristic is
Recessive trait: trait that is observable only
when two recessive alleles for the
characteristic are inherited
IN MENDEL’S WORK
Ask a Question: How are traits inherited?
Form a Hypothesis: Inheritance has a pattern.
Test the Hypothesis: Cross true-breeding plants and
Analyze the Results: Identify patterns in inherited traits.
Draw Conclusions: Traits are inherited in predictable
Mendel allowed the 1st generation to self-
This time the plant with the dominant trait for
purple flowers was allowed to self-pollinate.
As you can see, the recessive trait for white
flowers show up again.
Mendel decided calculate the ratios of
dominant to recessive traits in each
characteristic. Calculate the dominant-to-
recessive ratio for each characteristic.
Characteristic Dominant Recessive Ratio
Flower color 705 purple 224 white 3.15:1
Seed color 6,002 yellow 2,001 green ___________
Seed shape 5,474 round 1,850 wrinkled ___________
Pod color 428 green 158 yellow ___________
Pod shape 882 smooth 299 bumpy ___________
Flower position 651 along stem 207 at tip ___________
Plant height 787 tall 277 short ___________
Mendel realized that his results could be
explained only if each plant had two sets of
instructions for each characteristic
Each parent donates one set of
instructions, genes, to the offspring.
The fertilized egg would then have 2 forms of
the same gene for every characteristic- one
from each parent.
The two forms of a gene are known as alleles.
Used to visualize all possible combinations of
alleles from parents.
To make a Punnett Square, draw a square and
then divide it into 4 sections.
Next, write the letters that represent alleles
from one parent along the top of the box.
Write the letters that represent alleles from
the other parent along the side of the box.
Make a Punnett Square for a cross between a
purebreed round seed plant(R) and a purebreed
wrinkled seed plant(r).
Dominant alleles are symbolized with capital
letters. Recessive alleles are symbolized with
Genotype: Offspring’s inherited combination
Phenotype: Organism’s appearance
An organism’s phenotype is due to its
Heterozygous: having two alleles that are
different for a given gene
Homozygous: having identical alleles for a