MENDELIAN GENETICS
Gregor Mendel: Hereditary Genetics
• Three Principles:
• Principle of Dominance
• Some alleles are dominant, others are recessive
• Principle of Segregation
• Alleles are separate from one another; each gamete only carries a single
copy of each gene
• Principle of Independent Assortment
• Genes for different traits segregate independently of one another during
the formation of gametes (sperm/egg)
• Video clip: Alleles & Genes

Mendel and His Beloved Peas
• Garden peas are:
• Self-pollinating
• Easy to experimentally cross-pollinate
• Clear traits—pure purple, pure white, etc.
• Trait: any single feature of an organism

What Mendel Noticed:
• P Generation (“Parent”)
• F1 Generation (“First Filial”) (Kids)
• “Short” trait disappears…
Whoa!
• F2 Generation (“Second Filial”)
(Grandkids)
• …then reappears!
Did You Catch That?
• Where did that short plant
(trait) come from?

Recessive Traits “Hide” Behind Dominant
Traits
• Sexual Organisms
have two of each
trait (homologous
pairs)
• One of each pair is
passed on to
offspring by each
parent.
(p265)

Recessive Traits “Hide” Behind Dominant
Traits
PLANT HEIGHT
• Different forms of
the same trait are
called alleles
• What TRAIT is
being studied in
this experiment?
“Tall”
… and …
“Short”
• What are the TWO
ALLELES (forms)
of the trait being
studied?
Other Traits That Mendel Studied
ALLELES
ALLELES
Try one
• What trait is being studied?
• What’s the dominant allele of the trait?
• What’s the recessive allele of the trait?
Note the Percentages
50:50
100:0
~75:25
50
50 Short
• In P Gen: ____%
Tall, ____%
100
0
• In F1 Gen: ____%
Tall, ____%
Short
~75
~25 Short
• In F2 Gen: ____%
Tall, ____%

Mendel proposed that
“Factors” segregate
(separate) from pairs to
be passed on to
offspring, one from
each parent!
(p265-266)
(Sound familiar?)
(Sound familiar?)
Homologous chromosomes split up in
Anaphase I so that only one allele of each
trait gets passed on in your gametes!

The pair of alleles you have (for any trait) is called
your GENOTYPE
Example Trait: Nose Shape
(Let us pretend that round nose is dominant to pointy nose)
(usually, letters are used to represent alleles, upper case for dominant)
n
N
• Alleles: ___ (dominant), ___ (recessive)
NN Nn nn
• Genotypes: ____, ____, ____
“Homozygous
Dominant”
“Heterozygous”
“Homozygous
Recessive”
The pair of alleles you have (for any trait) is called
your GENOTYPE
Again!
NN Nn nn
• Genotypes: ____, ____, ____
This genotype is
described as
“Homozygous
_______________
Dominant”
___________
(Purebred)
This genotype is
This genotype is
described as
described as
“Heterozygous”
“Homozygous
_____________ _______________
(hybrid or carrier)
Recessive”
___________
(Purebred)
NN
One from Mom
One from Dad

Each set of alleles is a homologous pair, one
from Mom and one from Dad!
What is this person’s
genotype?
• This is called your
 PHENOTYPE (an
expressed feature
you can see!)
NN
________________
Homozygous Dominant
What kind of nose do they have?
Round? Or Pointy?
Nn
• Remember:
Dominant
overshadows
recessive!
________________
Heterozygous
What is this person’s PHENOTYPE?
Round? Or Pointy?
nn
• Recessive alleles
show only if no
dominant is
present!
________________
Homozygous Recessive
What is this person’s phenotype?
Round? Or Pointy?
Another Example:
• What’s the TRAIT?
• What are the ALLELES?
• Which is dominant?
PP
pp
Pp
Remember: Only one of each allele is passed on
from each parent!
• What is Mom’s genotype?

Heterozygous
• What is Dad’s genotype?
Ova
(either)
Heterozygous

GERM
CELL
Pp
Pp
Sperm
(either)
GERM
CELL
Let’s Look At
the
Offspring
Probabilities
Ova
(either)
• What if this
egg meets this
sperm?
Sperm
(either)
What if this egg
meets this sperm?
What if this egg
meets this sperm?
What if this egg
meets this sperm?
What if this egg
meets this sperm?

This Is Called
A Punnett
Square
• Punnett squares
allow us to
predict
probabilities
about offspring
What percentages does this Punnett square predict?
• How many
75% (3/4)
Purple? ______
• How many
White? ______
25% (1/4)
• How many
homozygous
dominant? 25%
___(1/4)
• How many
heterozygous?
______
50%
(2/4)
• How many
homozygous
recessive? ___
25% (1/4)
25%
25%
25%
25%
What is the probability that this couple
will have a baby with the “a” disease?
• Healthy allelle = A
• Disease allele = a
25%
(1/4)
What is the probability that this couple will have a
baby who is a CARRIER for the “a” disease?
• Healthy allelle = A
• Disease allele = a

50%
(2/4)
A CARRIER is a heterozygote
(he or she can “pass on” the
trait but does not exhibit it)

Your Turn! (For disease “A”):
What if a homozygous dominant man marries a
carrier woman?
AA
Aa
____ X ____
Dad
Mom
A
a
A
AA
Aa
A
AA
Aa
Offspring
Genotype:
- HD (AA) = ___%
50
- Het (Aa)= ___%
50
- HR (aa)= ___%
0
Offspring
Phenotype:
- Healthy* = ___%
100
- Disease = ___%
0
- *(Healthy) Carrier =
50
_____%

One More Time! (For disease “A”):
What if a carrier man marries a
woman with the disease?
Aa
____ X aa
____
Dad
Mom
a
a
A
Aa
Aa
a
aa
aa
Offspring
Genotype:
0
- HD (AA) = ___%
50
- Het (Aa)= ___%
50
- HR (aa)= ___%
Offspring
Phenotype:
- Healthy* = 50
___%
50
- Disease = ___%
- *(Healthy) Carrier =
50
_____%