Classical Genetics
MENDEL AND INHERITANCE
What determines what characters You
get from your parents and why are all
your siblings different from you?
Before Mendel: The Blending
Hypothesis

Traits an individual has
are a blend of the
traits of the parents
Mendelian Genetics
Predicting the outcome
of crosses between
individuals
Mendel worked with
sweet pea plants to
establish the laws of the
inheritance of traits from
parents to offspring
What did Mendel see?
What Mendel Observed


Mendel observed that the pea plants had traits that occurred
in pairs

Purple/white

Smooth/round

Green/yellow
Mendel hypothesized that each trait was the result of “unit
factors” that occurred in pairs to produce the trait
What Mendel Observed
What Mendel Observed

Mendel found that one of the forms “skipped”
generations

One form was being “covered up” by the other

The form that covered up the other he called
“dominant”

The form that was covered up he called “recessive”
Mendel’s Alleles

With a partner define Allele
From Allele to Trait

The combination of alleles resulted in particular traits

The actual alleles of a trait are called the genotype

The physical result of the genotype, its expression, is called the
phenotype
How did Mendel’s ideas explain his observations?
Mendel’s Conclusions

There are three possible genotypes
 PP
: Homozygous Dominant
 Pp : Heterozygous
 pp : Homozygous recessive
 An
organism that is homozygous for a
trait is called “true breeding”


What is the difference between
phenotype and genotype?
Mendel proposed two laws that govern
inheritance
Mendel’s Laws

Law of Segregation:


During gamete formation the alleles for a trait
separate
Law of Independent Assortment
 During
gamete formation the alleles for a trait are
randomly assorted and distributed to gametes
How Do Mendel’s
Laws Explain His
Observations?
Law of Segregation
How Do Mendel’s Laws Explain His
Observations?
Law of Independent
Assortment
Predicting Outcomes: Try It

Monohybrid cross

What is the genotype
ratio?

What is the chance of
producing a tall
individual?
T = tall, t = short
Tt X Tt
Predicting Outcomes: Try It


Test Cross
Used when the genotype of an
individual is unknown


Expresses dominant phenotype
Cross the unknown with a
homozygous recessive

“True Breeding”

Some of the offspring produced
are tall and some are short.

What is the unknown’s genotype?
T = tall, t = short
Tall Individual
Predicting Outcomes: Try It

Dihybrid cross

Use FOIL to
determine all
possible gametes

What is the
phenotype ratio?
Y = yellow, y = green
R = round, r = wrinkled
YyRr X YyRr
Where in Meiosis are Mendel’s Laws
Shown?

Law of Segregation

Separation of
chromosomes during
Anaphase I
Where in Meiosis are Mendel’s Laws
Shown?

Law of Independent Assortment

Random Alignment of
homologous pairs in Metaphase
Mendel dealt with traits exhibiting
complete dominance, but are
there other patterns? If so what
are they? Do they follow Mendel’s
Laws?
Incomplete Dominance
Co-Dominance and Multiple Alleles
Gene Linkage

If the traits shown are
linked, what caused them
to be inherited in mixed
form?

How do you know if that is
the cause or if its regular
dominance?
Sex-Linked Genes
Sex-Linked Genes





For genes on the X chromosome, sexlinkage effects inheritance patterns
Males more likely to express recessive
traits
Fathers pass sex-linked alleles to all
daughters, but no sons
Mothers can pass to both daughters
and sons
Color-blindness is a recessive sexlinked trait
Evolution Connection




The human Y chromosome passes
intact from father to son.
The Y chromosome has been used in
research on human ancestry, such as
that 8% of males in central Asia can
trace their ancestry to Genghis Khan
who lived 1,000 years ago.
Why is the Y chromosome
particularly useful in tracing
recent human heritage?
Can you think of another
source of DNA in your cells that
is also useful for this?