Have you ever wondered…
… why you inherited some traits from your mom & other
traits from your dad?
… or maybe you have characteristics that neither of your
parents have?
… why do people with red hair usually have freckles, too?
Introduction to
Genetics
Genetics
• The scientific study of
heredity that involves
how genes are passed
from parent to offspring
History of Genetics
• Gregor Mendel –
Austrian monk &
scientist who has been
named the “Father of
Genetics” due to his
work on the inheritance
of traits found in pea
plants in the 1850-60s
Gregor Mendel
• Worked with pea plants in the monastery garden
• Pea plants are a good choice to study genetics, because
they are self-pollinating (asexual reproduction).
– All offspring would be genetically identical to the parents.
Pea Plant Reproduction
• Each pea plant has male
sexual organs & female
sexual organs.
• In nature, pea plants
typically self-fertilize
(fertilizing eggs with their
own sperm).
Gregor Mendel
• Developed a technique of producing seeds from a
process called cross-pollination, in which he dusted the
pollen of 1 pea plant onto another plant.
• He was in control of which plants crossed with each
other & prevented self-pollination.
Traits, Genes, & Dominance
• Trait: a specific characteristic that varies from 1
individual to another
– Examples: eye color, hair color, height
Fun Fact!
About 30% of people in the
US have a widow’s peak.
Pea Plant Traits
• Mendel studied 7 different
pea plant traits, including:
–
–
–
–
–
–
seed shape
seed color
pod shape
pod color
flower position
plant height
Traits, Genes, & Dominance
• There is 1 gene for each of
these pea plant traits.
• For example, there is 1 gene
that controls the plant’s
height.
– BUT, there are 2 versions of
this gene – “tall” & “short”
Gregor Mendel & Genes
• Mendel studied the
different versions of each
trait
– Wrinkled or smooth pea
shape, green or yellow
seed color, etc.
Alleles
• Different versions of the same gene
– Genes, which are made of DNA, are used to make proteins.
– Each allele contains the DNA that codes for a slightly
different version of the same protein.
– This is what gives all of us different characteristics.
There are several different
alleles (versions) of genes
that give us our eye color.
These are the different
alleles (versions) of the pea
plant traits that Mendel
studied.
Mendel’s Investigations
• Remember that pea plants typically self-fertilize, so
many of Mendel’s plants were purebred.
• A purebred plant doesn’t contain the genes from any
other plant.
Mendel’s Investigations
• Mendel crossed (bred) 2 purebred plants with different
versions of the same trait (different alleles).
– Example: he bred a short plant with a tall plant.
Keeping Track of the Generations in a Cross
• When crossing pea plants (or
any other cross), we keep track
of the generations by labeling
them.
– P Generation: the parental
generation
– F1 Generation: the offspring of
the 2 parental plants
– F2 Generation: a cross between
F1 organisms
Mendel’s Investigations
• Mendel crossed a purebred tall
plant with a purebred short
plant.
– This is the P generation.
• He found that all of the offspring
were tall.
– The F1 generation
There were no medium height or short plants in the offspring.
Principal of Dominance
• Some alleles (versions of a
gene) are dominant & some
are recessive.
– Recessive Alleles: are able
to be masked (not always
observed in the organism)
– Dominant Alleles: mask
recessive alleles (can be
observed)
Chromosome
from mom
Allele
for free
earlobes
Chromosome
from dad
Allele for
attached
earlobes
• Remember that you have 2
copies of each gene – 1
from mom & 1 from dad.
• What if mom gives you 1
version of a gene (free
earlobes), but dad gives you
a different version
(attached earlobes)?
• You will express the allele
that is dominant.
• In this case, you’ll have free
earlobes, but you still carry
the allele for attached
earlobes.
Principal of Dominance
There are 2 alleles for the widow’s peak
gene – 1 allele (A) codes for the widow’s
peak, but the other (a) does not.
If you get the A allele from mom, but the
a allele from dad, you will have a
widow’s peak, because the A allele is
dominant.
You only need 1 copy of the dominant
allele in order to express that trait.
Principal of Dominance
• The trait that was represented in the offspring (F1)
was the dominant allele.
– Tall is dominant, short is recessive
• How many alleles do YOU have for each of your
genes?
TWO!
• Where do your alleles (versions of your genes)
come from?
You get 1 from your mother & 1 from your father!
Mendel’s Investigations
• Mendel’s Observation: after seeing that his F1 plants
only looked like 1 parent (tall), Mendel wanted to know
what happened to the recessive alleles (the short
version).
• Mendel’s Question: did the recessive alleles disappear?
Mendel’s Investigations
• Mendel allowed the F1 plants to self-pollinate to produce
the F2 generation. From this, he observed:
– The recessive allele (short) reappeared in the F2 plants.
– The recessive allele was still there; it had not disappeared.
P Generation
F1 Generation
Mendel crossed a tall &
short plant together &
got all tall offspring.
To understand why none
were short, he crossed
the F1 generation
together.
In the F2 generation,
there were tall & short
plants.
The short allele (t) had
been present all along; it
just wasn’t expressed.
F1 Generation
F2 Generation
Mendel’s Results
• About 25% (or ¼ ) of the F2 plants showed the
recessive allele of the trait (short plant).
• The dominant allele (tall) was found in 75% (or ¾)
of the F2 plants.
Let’s say Mendel crosses a
purple plant with a white
plant.
The F1 plants are all
purple.
What happened to
the white allele?
In the F2 generation
75% of the flowers
were purple, but 25%
were white.
He knows that these plants
are purebred (only contain Mendel allows the F1 The white allele must be
purple alleles or only white
recessive; it was hidden
plants to selfalleles)
in the F1 generation!
pollinate.
Conclusions
Law of Segregation
During meiosis, the 2
alleles for the same gene
separate.
This is a cell from a plant with
yellow seeds. The plant carries a
yellow allele & a green allele (Yy) 1 copy of the gene came from
mom & 1 from dad.
At the end of meiosis, the gametes
(egg or sperm) carry only 1 copy of
the allele.
Half of the gametes carry the Y
allele (yellow seeds), while the
other half carry the y allele (green
seeds).
These tall plants (TT) can only produce
gametes that carry T; the short plants
(tt) can only produce gametes that
carry t.
When these gametes combine, they
produce tall plants (Tt) in the F1 gener.
T + t = Tt
These tall plants (Tt) can produce
gametes that carry T or gametes that
carry t.
So, depending on which gametes
combine, the F2 generation could be
tall or short!
T + t = Tt
T + T = TT
t + t = tt
Law of Independent Assortment
• Genes for each trait can be inherited independently
of each other.
– For example, not all tall plants have green pea pods.
– Another example, not all people with brown hair have
brown eyes.
You can have blue
eyes, no matter what
color hair you have!
Genes are inherited
separately!