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Introduction to Genetics
Mendel and His Pea Plants
Think About It
Has anyone ever told you that you look
like your Mom? Or your Dad? Or an
uncle, aunt, or a grandparent?
 In what ways do you not look like the
people in your family?

Some Words You Need to Know

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
Genetics – the study of heredity
Heredity – the passing of traits to
offspring (children, young, descendents)
Traits – characteristics that parents can
pass to offspring (ex: eye color, hair
color, shape of eyebrows)
What traits do you share…




with
with
with
with
your parents?
your brothers or sisters?
your cousins?
other people in this class?
Gregor Mendel (1822-1884)

“Father of Genetics”

Austrian monk
Studied the traits
of pea plants

Why pea plants?
Pea plants are easy to grow
 Pea plants have a lot of traits that can be
studied
 A lot of their traits have only two
varieties:




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Tall or short
Round seeds or wrinkled seeds
Yellow seeds or green seeds
Purple flowers or white flowers
Green pods or yellow pods
Some Pea Plant Traits
More about plants
In plants, male and female reproductive
parts are found in every flower. (There
are no boy plants or girl plants.)
 The male reproductive organs produce
pollen (plant sperm).
 The female reproductive organs produce
eggs (ovules).
 Self-pollination – when a plant fertilizes
itself
 Cross-pollination – when a plant is
fertilized by another plant

Mendel’s Experiment
Mendel controlled how plants were fertilized
by:
 removing the male parts from some
flowers so they could not produce pollen.
This means they could not self-pollinate.
 using a brush to move pollen from one
flower to another
In other words, Mendel decided which plants
were going to be the parents of the
offspring.
Mendel’s Experiment Part 1
First, Mendel crossed (bred) these two
plants. He called these plants the P1
Generation.
Purebred tall
plant
X
Purebred short
plant
Mendel’s Experiment Part 1
What size of offspring do you think this
produced?
All of the offspring were…
Mendel’s Experiment Part 1
Mendel called them purebreds because the
short one came from a family of all short
plants, and the tall one came from a
family of all tall plants. You will learn a
different meaning for purebred later.
Mendel’s Experiment Part 2
Next, Mendel self-pollinated the offspring
from the first generation. He called them
the F1 Generation.
Tall plant X Tall plant
X
Mendel’s Experiment Part 2
What size of offspring of the F2 Generation
do you think this produced?
Three out of four of the offspring were…
Mendel’s Experiment Part 2
But, one out of four was…
What did Mendel figure out?
Each parent contributes a substance that
controls the traits of their offspring.
For example, each parent contributes a
substance for the height, the seed color,
the seed shape, etc.
What we now know

Those substances that control traits are
called genes and they are part of
chromosomes.

Chromosomes, you should remember, are
found in the nucleus of cells.
What we now know
We have two genes for every trait. One
comes from the male parent and one
comes from the female parent.
The different forms of a
gene are called alleles.
Some examples of genes & alleles
Mendel’s pea plants had two genes for
their height. The alleles for that gene are
short and tall.
 The pea plants had two genes for flower
color. The alleles for that gene are purple
and white.
 The pea plants had two genes for their
seed shape. The alleles for that gene are
round and wrinkled.

What else did Mendel figure out?
One allele can mask, or hide, the other
allele.
If one parent gives the offspring an allele
that says a plant should have green seeds
and the other parent contributes an allele
that says the offspring should have yellow
seeds, one of them will mask the other
one. (one allele will mask the other)
Mask
Think about someone wearing a mask.
 What do you see?
 The mask.
 Is your face still there?
 Of course, it is just hidden.

More words to know

Dominant allele – an allele that can mask
another allele and always “shows up” if
present (meaning you will see it)

Recessive allele – an allele that can be
masked by a dominant allele and only
“shows up” if both alleles for a trait are
recessive
Symbols used for alleles
Dominant alleles are always represented
by a capital letter. The first letter of the
dominant trait is usually used. For
example, if tall is dominant in pea plants,
then T represents that allele.
 Recessive alleles are always represented
by the lower case letter of the dominant
allele. Since short is recessive in pea
plants, t represents that allele.

More words to know

Remember: dominant alleles can mask
another allele and always “shows up” if
present

this is the BIG GUY
the allele is a
CAPITAL letter
(ex. T)

T
More words to know

Remember: recessive alleles can be
masked by a dominant allele and only
“shows up” if both alleles for a trait are
recessive

this is the little guy
that hides behind the BIG
guy
the allele is a lowercase
letter (ex. t)

t
Symbols used for alleles
Dominant allele for being tall is T.
 Recessive allele for being short is t.
 So, if a pea plant has one allele for tall
and one allele for being short, you would
write T t.

T
t
Symbols used for alleles

Remember any time you have a dominant
allele present, you are going to see him,
not the recessive allele.
T
t
Questions to ask yourself
1. What is the dominant trait?
 2. What is the first letter of that trait?

Are you getting it so far?

How would you write the symbols for a
pea plant that has one allele for purple
flowers and one allele for white flowers if
purple is dominant?

What color flowers would that plant have?
Are you getting it so far?

How would you write the symbols for a
pea plant that has two alleles for purple
flowers?

What color flowers would that plant have?
Are you getting it so far?

How would you write the symbols for a
pea plant that has two alleles for white
flowers?

What color flowers would that plant have?
Phenotypes & Genotypes

Phenotype – a physical trait of an
organism (what it looks like or how you
describe it)


ex. purple flower
Genotype – the genetic makeup of an
organism; it determines the phenotype (or
appearance) of the plant; represented by
letters

ex. PP
Phenotypes & Genotypes
Examples:
Genotype
PP
Pp
pp
Phenotype
PURPLE flowers
PURPLE flowers
WHITE flowers
Try these

Fill in the missing information.
Phenotype
tall
tall
short
Genotype
_t
_T
__
Try these

Fill in the missing information.
Phenotype
tall
tall
short
Genotype
Tt
TT
tt
A few more words
homozygous or purebred –
2 of the same alleles for a trait
examples: P P, p p, T T, t t
(can be homozygous dominant or
homozygous recessive)

heterozygous or hybrid –
2 different alleles for a trait
examples: P p, T t


YouTube - The Gregor Mendel Rap

YouTube - Mr. Lee - Genetics rap
Now, on to Punnett Squares
Punnett squares are tools to help geneticists
predict the probability of certain traits in
offspring by showing all of the ways the
parents’ alleles can combine.
Probability is the relative possibility that an
event will occur. It is usually expressed in
a percentage or fraction.
This website will explain punnetsquares.