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GENETICS
Mendel’s Laws of Heredity
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Mendel’s Laws
Law of segregation
The two alleles for each trait must
separate when gametes are formed
A parent will pass down AT RANDOM,
only one allele for each trait
Law of independent assortment
Genes for different traits are inherited
independently of one another
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Some Needed Vocabulary
Heredity
Passing on of characteristics from parent to
offspring
Traits
Characteristics that are inherited
Genetics
The study of Heredity
Gametes
Sex cells
Sperm = male gamete
Egg = female gamete
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Gregor Mendel
A monk who studied heredity using garden
pea plants
Why garden peas?
They reproduce sexually
They have both male & female gametes on the
same plant
What did he do?
He transferred pollen from one plant to another
with different traits
This is called “making a cross”
Mendel first “crossed” tall plants with short plants
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Mendel’s Monohybrid crosses
Monohybrid cross
Focuses on one (mono) trait
1st generation
Crossed a tall pea plant with a short pea plant
All of the offspring were tall
2nd generation
Crossing the offspring of the 1st generation
¾ of the offspring were tall; ¼ of the offspring were
short
3:1 ratio (tall to short)
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Mendel’s Findings
Mendel found that each chromosome has
two factors for each traits… called alleles
Alleles = forms of genes
Types of alleles
Dominant
The trait that has the ability to mask the other
Represented with a capital letter (A)
Recessive
The trait that will be masked by the dominant
trait… it will be there, but will not be expressed
Represented with a lowercase letter (a)
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Phenotype vs. Genotype
Phenotype
The way that an offspring looks; how the
trait appears
Tall, short
Genotype
The offspring’s genetic combination
TT, Tt, tt
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Homozygous vs. Heterozygous
Homozygous (“homo” means “same”)
Having two identical alleles for a trait
TT, tt
Heterozygous (“hetero” means “different”)
Having two different alleles for a trait
Tt
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Punnett square
Short way to find the expected
proportions of possible genotypes in the
offspring of a testcross
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Mendel’s Punnett Square
st
1 generation
T = Tall
t = short
TT x tt
T T
t Tt Tt
t Tt Tt
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Interpreting the Punnett Square
Tall = T
Short = t
T T
t Tt Tt
t Tt Tt
All offspring are
heterozygous tall (Tt)
Ratio
4:0 (Tall : Short)
Percentage
100% Tall
Each offspring box
represents 25% of the
population of offspring
produced
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Mendel’s Punnett Square
nd
2 generation
T
Tall = ____
t
Short = ___
Tt
Tt
Parents: _______ x _______
T t
T TT Tt
t Tt tt
oHomozygous Tall:
TT
oGenotype: _______
1
oHow many? ______
oHeterozygous Tall:
Tt
oGenotype: _______
2
oHow many? ______
oShort:
tt
oGenotype: _______
1
oHow many? ______
3
oTOTAL # OF TALL: _____
1
oTOTAL # OF SHORT: _____
3:1
oRatio of Tall : Short  __________
oPercentages:
75%
oPercent Tall: ___________
25%
oPercent Short: __________
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GENETICS
Patterns of Heredity
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Simple Mendelian Inheritance
The type of inheritance we have
discussed thus far (Mendel’s peas)
Not all patterns in heredity are as
simple…
There is more than just dominant and
recessive
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Incomplete Dominance
The phenotype of the heterozygote is
intermediate (BLENDED) between
those of the two homozygotes
For Example:
A cross between a particular Red flower
and a White flower will produce a Pink
flower
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Punnett Square for
Incomplete Dominance
Cross:
(RR) Red flower X (R’R’) White flower
RR = Red
R’R’ = White
RR’ = Pink
R R
R’
R’
RR’ R R’
RR’ RR’
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Codominance
The phenotypes of both homozygotes is
produced (SPOTTED) in the
heterozygous individual
For Example:
A black rooster and a white hen might
produce a chick with some black feathers
and some white feathers (stripes, polka
dots and patches are co-dominant)
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Multiple Alleles
Traits controlled by more than 2 alleles …
For Example:
Multiple alleles are commonly seen in the
coat color of rabbits
Also… Blood Typing…
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Multiple Alleles – Blood Types
Human blood types are determined by the
presence or absence of certain molecules on
the surfaces of red blood cells
There are three alleles for blood type…
IA …………………… written as “A”
IB …………………… written as “B”
i …………………… written as “O”
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Sex-linked traits
Traits that are controlled by genes located
on sex chromosomes
These traits help explain why some
characteristics and disorders are more
common in one sex than in the other
Commonly referred to as “X-linked”
because the trait is usually linked to the Xchromosome
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Background Information
Humans have 46 chromosomes; or 23 pairs
of chromosomes
Of these, 22 pairs are autosomes
(self chromosomes)
The last (23rd) pair is different in males &
females… these are the sex chromosomes
Females have two “X” chromosomes (XX)
Males have one “X” and one “Y” chromosome
(XY)
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“Sex-linked” traits
Green-eyed male (XbY)
X
Heterozygous Blue-eyed female (XBXb)
B = Blue eyes
b = Green eyes
b
X
Y
B
X
B
b
X X
B
X Y
b
X
b
b
X X
b
X Y
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“Sex-linked” traits
Green-eyed male
(XbY)
X
Heterozygous Blueeyed female (XBXb)
B = Blue eyes
b = Green eyes
b
X
Y
XB XBXb XBY
b
b
b
b
X
X
X
Y
X
OFFSPRING
• MALES (XY)
•Blue Eyes
XBY
•Genotype _______
1
•How many? _____
•Green Eyes
XbY
•Genotype _______
1
•How many? _____
•FEMALES (XX)
•Blue Eyes
XBXb
•Genotype _______
1
•How many? _____
•Green Eyes
XbXb
•Genotype _______
1
•How many? _____
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Sex-linked percentages
Green-eyed male
(XbY)
X
Heterozygous Blueeyed female (XBXb)
B = Blue eyes
b = Green eyes
B
X
Xb
Y
X BX b
XBY
X b X bX b X bY
Females
Males
oPercentages
oWhen figuring percents
for sex-linked punnett
squares, you must be
careful to only count what
is asked for…
% of females with blue eyes
50%
% of males with blue eyes
50%
% of females with green eyes
50%
% of males with green eyes
50%
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Polygenic Inheritance
When a trait is controlled by more than
one gene
For Example:
Skin color is actually the accumulation of
dominant genes for pigment in the skin
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