Mendelian
Genetics
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Gregor Mendel
The Father of Genetics
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Gene
• A discrete unit of hereditary
information consisting of a
specific nucleotide sequence in
DNA (or RNA in some viruses)
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Alleles
• •Alleles are alternate forms of a gene.
• •Examples: tall and short for plant height or purple
or white for flower color.
• •Every trait has at least two alleles- one from each
parent.
• •The location of an allele on a chromosome is
known as its locus (loci = plural form).
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Genotype
• •The letters that represent
(symbolize) the trait being
investigated. The genetic makeup of an organism.
• •Examples: Bb, BB, bb
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Phenotype
• The actual representation of the genes.
The Physical appearance or traits in an
organism resulting from its genetic
makeup (what you see).
• Examples: tall, purple flower or white
flower, blond hair, freckles, etc.
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Dominant
• The allele that is fully expressed
in an organism (observed).
• •Represented by capital letters.
• •Tall = T
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Recessive
• •The allele that is masked by
the dominant allele.
• •Represented by lower case
letters.
• •Short = t
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Homozygous
• When both alleles (letters) are
the same.
• BB = Homozygous Dominant
• bb = Homozygous recessive
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Heterozygous
• When the alleles (letters) are
different.
• One upper case letter and one
that is lower case.
• Bb = Heterozygous
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Example Problem
• Round = R
• wrinkled = r
• If a plant has round seeds, do we
know what its genotype is?
• It could be RR or Rr
• If a plant has wrinkled seeds, do
we know what its genotype is?
• Yes, it is rr.
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Punnett Square
• A method for finding predicted
outcomes and probabilities for
offspring from any cross.
• A chart for predicting the traits
of offspring.
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Some more terms:
• P-generation is the parental generation.
• The p-generation produce the F1
generation.
• The F1 generation crossed with itself
produces the F2 generation.
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Example Problem
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• In foxes, red coat color is determined by
the dominant gene R; silver-black coat is
determined by the recessive gene r. A
homozygous (pure) red male is crossed
with a silver-black female. (The P
generation).
1.What is the genotype of the
female?
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What are the genotype
percentages of their offspring?
•First…make a Punnett square
for showing your work
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Example Problem
R
R
r
Rr
Rr
r
Rr
Rr
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Assignment
• Section 32-3
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Assignment
33-4 & 33-5
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B = Brown
b = blue
#1
B
b
b
Bb
bb
b
Bb
bb
1 point
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1 point
#1
• 1/2 or 50% chance of blue-eyed.
• 1/2 or 50% chance of Brown-eyed.
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T = Tall
#2
1 point
t = short
T
t
t
Tt
tt
t
Tt
tt
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1 point
#2 What fraction of
offspring would be tall?
• 1/2 or 50% would be Tall.
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W = White
#3
1 point
w = black
W
w
W
WW
Ww
w
Ww
ww
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1 point
#3a What fraction of the
offspring will be white?
• 3/4 or 75% will be white.
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1 point
#3b What fraction of the
offspring will be black?
• 1/4 or 25% will be black.
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1 point
#3c What fraction of each
genotype will you get?
• 1/4 or 25% will be WW
• 1/2 or 50% will be Ww
• 1/4 or 25% will be ww.
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1 point
#3d What fraction of each
phenotype will you get?
• 3/4 or 75% will be White.
• 1/4 or 25% will be black.
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#4
RR = Red
WW = White
RW = Roan
1 point
R
R
W
RW
RW
W
RW
RW
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1 point
#4 Give the fraction of
each color of offspring?
• 1/1 or 100% will be Roan - RW.
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#5
RR = Red
WW = White
RW = Roan
1 point
R
W
W
RW
WW
W
RW
WW
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1 point
#5 Give the fraction of
each color of offspring?
• 1/2 or 50% will be Roan - RW.
• 1/2 or 50% will be White - WW.
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#1a
1 point
G = Green
g = red
G
g
G
GG
Gg
g
Gg
gg
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1 point
#1a Give the fraction of
each genotype of
offspring?
• 1/4 or 25% will be GG.
• 1/2 or 50% will be Gg.
• 1/4 or 25% will be gg.
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#1b
1 point
G = Green
g = red
G
g
g
Gg
gg
g
Gg
gg
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1 point
#1b Give the fraction of
each genotype of
offspring?
• 1/2 or 50% will be Gg.
• 1/2 or 50% will be gg.
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1 point
#2a What is the fraction
of each phenotype?
• 3/4 or 75% will be Green.
• 1/4 or 25% will be red.
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1 point
#2b What is the fraction
of each phenotype.
• 1/2 or 50% will be Green
• 1/2 or 50% will be red.
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b
b
B
Bb
Bb
b
bb
bb
#3
1 point
•The mother had to be heterozygous or Bb
since the couple had a blue eyed child.
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1 point
#4
•There would be a 50% chance that the
2nd child from the couple would have a
brown eyes.
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#5
• Spotted = S
• white = s
2 points
The couple had two
spotted and two
white kittens.
• Mother = ss since she is white.
• Father’s Genotype would be Ss.
• Father’s Phenotype would be
Spotted
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#6
5 points
• Man has blue eyes - bb.
• G-Ma has blue eyes - bb.
• Woman has brown eyes - Bb.
• 50% of children would be Bb. (Brown)
• 50% of children would be bb. (Blue)
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Please put a score on top
of their paper.
• Put the number correct out of
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Punnett Squares
• Why are punnett squares useful?
• We can use a punnett square to predict the
probable genotypes and phenotypes for
offspring from a genetic cross.
• Genotype = What is inside the genes, the
make-up.
• Phenotype = The outward expression of
the genes.
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