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UNIT 7 – HEREDITY
PATTERNS OF INHERITANCE
genes
proteins
cells
tissues & organs
ORGANISMS!!!
ESSENTIAL QUESTIONS
1. How are traits passed from one
generation to the next?
2. What is the relationship
between genotype and phenotype?
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GREGOR MENDEL
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
“Father” of Genetics
Lived 1822-1884
Austrian monk
Gardener and math
teacher
His work with pea plants
led us to Mendel’s Laws of
Inheritance
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WHAT DID MENDEL DO?
LET’S WATCH!
Mendel experimented with pea plants by
“crossing” specific plants and then
analyzing their offspring.
OUTCOMES OF MENDEL’S WORK
Genes (segments
of DNA that code
for one protein)
are located on
chromosomes.
Different forms of genes are
called alleles.
Example: The A blood protein or
the B blood protein

For each protein (that determines a trait), an organism
inherits two alleles, one from each parent – one from the
egg and one from the sperm.

Egg and sperm are called “gametes”
Mitosis
Meiosis
(gametes)
(non-reproductive cells)
Perfect amount of DNA
(4=2N)
Full set of DNA – 2 copies of each
chromosome
½ set of DNA – 1 copy of each
chromosome
THE LAW OF SEGREGATION

Alleles separate during meiosis (making of
egg and sperm)

Each gamete carries only one allele for each
protein (haploid).

Fertilization restores
allele pairs in the
offspring (diploid) so
that the baby will
have a complete
set of DNA.

If the alleles you get from each parent are
the same, the individual is homozygous.

If the alleles you get from each parent are
different, the individual is heterozygous.
Genotype- genetic makeup or allele
combination (Pp)
Phenotype- observable trait (purple or
white)
HOW DO ALLELES INTERACT TO CREATE A
TRAIT?
There are many types of interactions
between alleles that create traits…

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Complete dominance
Incomplete dominance
Co-dominance
Polygenic traits
Sex-linked traits
Punnett Squares help us figure out that
interaction.
COMPLETE DOMINANCE
In a heterozygous (Bb) individual…


The allele that observed (expressed) is called
dominant.

An upper case letter is used to represent the
dominant allele (B)

The “hidden” allele in a heterozygous (Bb)
individual is called recessive.

A lower case letter represents the recessive
allele (b)
USING A PUNNETT SQUARE
Alleles= Alternative forms of the same gene (T or t)
Suppose Tt X Tt
Homozygous
dominant
Eggs
T
t
T
TT
Tt
t
tT
tt
Heterozygous
Sperm
Heterozygous
Homozygous
recessive
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LET’S PLAY NAME THAT WHATEVER!
?
allele
?
hromosome
BB
What 3 genetic terms can
be used to describe this?
genotype
homozygous
dominant
Who is this?
Gregor Mendel
What genetic term would
describe the way he looks?
Phenotype
TIME TO PRACTICE
DO TRAITS ALWAYS APPEAR IN JUST 2 FORMS?
Nooooooooooooooo
oooooooooooooooo
oooo
• Some traits are determined by multiple alleles
• Example: ABO blood groups
MULTIPLE ALLELES AND CODOMINANCE
Multiple Alleles

More than 2 alleles for
a given trait

A, B, and O alleles
Codominance
 Heterozygotes
express both
alleles
(not blended or dominant/recessive)
 Type
AB blood
ANOTHER EXAMPLE OF CO-DOMINANCE
Cattle can be
 red (RR = all red hairs)
 white (R’ = all white hairs)
 roan (RR’ = red & white hairs
together).
INCOMPLETE DOMINANCE

Neither allele is
dominant

Heterozygotes
have a phenotype
that is the
intermediate of
the two
phenotypes
• Actual color (phenotype) probably results from varying amounts of
production of red pigment (RR- a lot of pigment, rr no pigment, and Rr
half as pigment as RR.
INTERMEDIATE INHERITANCE IN CHICKENS
POLYGENIC INHERITANCE
 Traits
coded by more than
one gene
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Eye color (11, 12)
Skin color (1,2 and 4)
Height
 Phenotypes
are the
combined effect of more
than 1 pair of alleles
 Usually
a spectrum
(range) of phenotypes
 Lets
learn a little more!
TWO GENES FOR EYE COLOR
MULTIFACTORIAL INHERITANCE

Traits controlled by more than one
factor

Factors include genes,
environment, diet, exercise,
medication
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Diabetes (chromosome 6)
Asthma (chromosome 5)
Obesity (chromosome 7)
LINKED TRAITS
 Traits
coded by genes
located on the same
chromosome
 The
closer they are on
the same
chromosome, the
more likely they’ll be
inherited together.
 i.e.
body color & wing
length in fruit flies
SEX-LINKED TRAITS
 Traits
coded by genes
on the X chromosome
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Females have 2 alleles
Males have 1 only
Males are more likely to
express recessive sexlinked traits because
only 1 recessive allele is
required for expression.
Examples: hemophilia
and red-green
colorblindness
More about sex-linked traits!
• Some genes of the HLA (human leukocyte
antigen system) involved in the immune
response have over 200 alleles.
• Organ transplants have a much higher
success rate when donor and recipient are
matched for their HLA genes, but the high
level of variation makes this difficult.
USING A TEST CROSS
Test crosses help determine the genotype of a heterozygous phenotype by
crossing it with a homozygous recessive
CAN YOU ANALYZE OFFSPRING FOR MORE
THAN ONE TRAIT AT A TIME? YES!
DIHYBRID
CROSS
2
23
=
8,388,608
possible eggs or
sperm
DIHYBRID CROSSES
 Cross
 Each
of 2 heterozygotes - 9:3:3:1 phenotype ratio
trait maintains its 3:1 phenotype ratio

3 yellow : 1 green

3 smooth : 1 wrinkled
THE PRINCIPLE OF INDEPENDENT ASSORTMENT

Alleles of different characteristics are inherited
independently of one another (e.g. color and tail length)
FIGURING OUT THE POSSIBLE GAMETES FOR TWO TRAITS
FIGURING OUT THE POSSIBLE GAMETES FOR TWO TRAITS
Rr
Yy
Round is dominant
to wrinkled.
Round
Yellow
Yellow is dominant
to green
R
Y
r
y
R
y
r
Y
YOU CAN ALSO “FOIL”!
Round is dominant
to wrinkled.
Yellow is dominant
to green
RrYy
Round
Yellow
First
Outer
Inner
Last
RY
Ry
rY
ry
TIME TO
PRACTICE!!
TIME TO MAKE A
BABY DRAGON!!
When crossing two dragons, both
heterozygous for horns and
freckles, what are the possible
phenotypes of the offspring and
their ratios?
HhFf = HF, Hf, hF, hf
TT & Tt
T
HF
Hf
hF
hf
HF
Hf
hF
hf
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When crossing two dragons, both
heterozygous for horns and freckles, what are
the possible phenotypes of the offspring and
their ratios?
HF
Hf
hF
hf
HF
HHFF
HHFf
HhFF
HhFf
Hf
HHFf
HHff
HhFf
Hhff
hF
HfFf
HhFf
hhFF
hhFf
hf
HhFf
Hhff
hhFf
hhff
1. No horns and Freckles
2. Horns and Freckles
3. No horns and No Freckles
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4. Horns and No Freckles
How many of the offspring will not have horns
but will have freckles?
Genotype for no horns and freckles?
Hh or HH = no horns
Ff or FF = freckles
HF
Hf
hF
hf
HF
HHFF
HHFf
HhFF
HhFf
Hf
HHFf
HHff
HhFf
Hhff
hF
HfFf
HhFf
hhFF
hhFf
hf
HhFf
Hhff
hhFf
hhff
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How many of the offspring will have horns and
freckles?
Genotype for horns and freckles?
hh = horns
Ff or FF = freckles
HF
Hf
hF
hf
HF
HHFF
HHFf
RrYY
RrYy
Hf
HHFf
HHff
HhFf
Hhff
hF
HfFf
HhFf
hhFF
hhFf
hf
HhFf
Hhff
hhFf
hhff
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How many of the offspring will not have horns
or freckles?
Genotype for no horns and no freckles?
Hh or HH = no horns
ff = no freckles
HF
Hf
hF
hf
HF
HHFF
HHFf
HhFF
HhFf
Hf
HHFf
HHff
HhFf
Hhff
hF
HfFf
HhFf
hhFF
hhFf
hf
HhFf
Hhff
hhFf
hhff
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How many of the offspring will have horns but
and freckles?
Genotype for horns and freckles?
hh = horns
ff = no freckles
HF
Hf
hF
hf
HF
HHFF
HHFf
RrYY
RrYy
Hf
HHFf
HHff
HhFf
Hhff
hF
HfFf
HhFf
hhFF
hhFf
hf
HhFf
Hhff
hhFf
hhff
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