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HEREDITY

2. HEREDITY
Passing of traits from parents to offspring

Trait: distinguishing quality

Why then don’t you look exactly like your

parents?
About 150 years ago Gregor Mendel

performed some experiments that began to
help us find the answers

3. GREGOR MENDEL
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

garden
Interested in how traits are passed from parents

to offspring

4. GREGOR MENDEL
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.


5. SELF-POLLINATING PLANTS
Garden peas were a good choice because

They grow quickly

Usually self-pollinating

Come in many varieties

Self-pollinating: contains both male and

female reproductive structures

6. TRUE-BREEDING PLANTS
Mendel chose to study only one

characteristic, like plant height or pea color, at
a time
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.

7. CROSS-POLLINATION
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.

8. MENDEL’S FIRST EXPERIMENT
Performed crosses to study 7 different

characteristics
Each cross was between the two traits of each

characteristic

9. 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.

10. TYPES TRAITS
OF
Dominant trait: observable trait when at least

one dominant allele for a characteristic is
inherited
Recessive trait: trait that is observable only

when two recessive alleles for the
characteristic are inherited

11. SCIENTIFIC METHOD
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
offspring.
Analyze the Results: Identify patterns in inherited traits.
Draw Conclusions: Traits are inherited in predictable
patterns.

12. MENDEL’S SECOND
EXPERIMENT

13. MENDEL’S SECOND
EXPERIMENT
Mendel allowed the 1st generation to self-

pollinate.
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.

14. MENDEL’S SECOND
EXPERIMENT
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 ___________

15. GENES ALLELES
AND
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.


16. PUNNETT SQUARES
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.

17. PUNNETT SQUARES
Make a Punnett Square for a cross between a
purebreed round seed plant(R) and a purebreed
wrinkled seed plant(r).
R R
Rr
Rr
r
Rr Rr
r

18. GENOTYPES AND
PHENOTYPES
Dominant alleles are symbolized with capital

letters. Recessive alleles are symbolized with
lowercase letters.
Genotype: Offspring’s inherited combination

of alleles
Phenotype: Organism’s appearance

An organism’s phenotype is due to its

genotype

19. ALLELES
Heterozygous: having two alleles that are

different for a given gene
Homozygous: having identical alleles for a

given gene