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Genetics Why do we look the way we do?
Honors Biology Chapters 9 & 12
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Inheritance of chromosomes Egg + sperm zygote
egg
sperm
zygote
fertilizationmitosis &
development
meiosis
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Inheritance of genes Chromosomes passed from Mom & Dad
to offspring are comprised of genes may be same information may be different information
eye color(blue or brown?)
eye color(blue or brown?)
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Effect of genes Gene = region of a chromosome that codes for
a trait Genes come in different versions for each trait
brown vs. blue eyes brown vs. blonde hair A version of a gene = an allele
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Genes affect what you look like
XBBbb
Bb Bb Bb Bb
Where did the blue eyes go??
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Genes affect what you look like…
XBbbb
Bb Bb bb bb
Why did the blue eyes stay??
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Genes affect what you look like…
XBbBb
BB or Bb BB or Bb BB or Bb bb
Where did the blue eyes come from??
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What did we show here? Genes come in “versions”
brown vs. blue eye color alleles
Alleles are inherited separately from each parent brown & blue eye colors are separate &
do not blend either have brown or blue eyes, not a blend
Some alleles mask others brown eye color masked blue
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How does this work?
eye color(brown?)
hair color
hair color
eye color
(blue?)
Homologous chromosomes have same genes… …but maybe different alleles
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Traits are inherited as separate units
1 from Mom
1 from Dad
For each trait, an organism inherits 2 copies of a gene (2 alleles), 1 from each parent
homologous chromosomes
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Genetics vs. appearance There can be a difference between
how an organism looks & its genetics Its expressed trait/s = phenotype
brown eyes vs. blue eyes Its alleles, or genetic makeup =
genotype BB, Bb, bb
2 people can have the same phenotype but have different genotypes: BB vs Bb
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Genetics vs. appearance
eye color(brown)
eye color
(brown)
eye color(blue)
eye color
(brown)
vs.
BB
B
BBb
B
b
How were these brown eyes made?How were these brown eyes made?
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Practice
If G is the allele for pointy ears and g is the allele for floppy ears, what will be the ear shape phenotypes of the puppies with these genotypes?
The dominant allele is _ for the trait ___________
The recessive allele is _ for the trait ___________
Genotype GG = Phenotype __________
Genotype Gg = Phenotype __________
Genotype gg = Phenotype ___________
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Practice
G is for pointy ears and g is for floppy ears. Also, H is for a pink nose and h is for a black nose.
Genotype GGHH = Phenotype ______ and ______
Genotype GgHh = Phenotype ______ and ______
Genotype gghh = Phenotype ______ and _______
Genotype GGhh = Phenotype ______ and _______
Genotype Gghh = Phenotype ______ and _______
Genotype ggHH = Phenotype ______ and _______
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Practice
Which of these are traits and which are phenotypes?
1. Finger length 2. Blue eyes 3. Long hair 4. Number of leaves 5. Shape of tentacles 6. Warbling song
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Practice
Which of these are alleles and which are traits? 1. Eye color 2. Bone integrity 3. i 4. Insulin shape 5. B 6. Na
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Practice
Which of these are phenotypes and which are genotypes?
1. Curly hair 2. Jj 3. PP 4. Arthritic knees 5. Type B blood 6. Spotted fur and a pink nose 7. HHGg 8. Purple leaves and spiny stem
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Genetics&
The Work of Mendel
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Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used good experimental design used mathematical analysis
collected data & counted them excellent example of scientific
method
Gregor Mendel
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Pollen transferred from white flower to stigma of purple flower
all purple flowers result
Mendel’s work
?
self-pollinate
Bred pea plants cross-pollinate
true breeding parents
allowed offspring to self-pollinate & observed next generation
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Mendel collected data for 7 pea traits
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2nd
generation
3:175%purple-flower peas
25%white-flower peas
Looking closer at Mendel’s work
Parents
100%1st
generation(hybrids)
100%purple-flower peas
Xtrue-breedingpurple-flower peas
true-breeding white-flower peas
self-pollinate
F1
F2
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What did Mendel’s findings mean? Some traits mask others
purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white
dominant allele functional protein
affects characteristic masks other alleles
recessive allele no noticeable effect allele makes a
non-functioning protein
homologouschromosomes
allele producingfunctional protein
mutant allele malfunctioning
protein
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Mendel’s Results and Conclusions
RESULT: Whenever Mendel crossed two P
plants, one of the traits disappeared in the F1 plants.
The missing trait reappeared in the F2 plants in a 3:1 ratio pattern
CONCLUSION: LAW OF DOMINANCE One trait is dominant because it
masked or dominated the other trait One trait is recessive because it “hid”
behind the dominant one. It can only be seen when the plant has no dominant alleles.
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CONCLUSION: LAW OF SEGREGATION
Pairs of alleles segregate (separate) during the formation of gametes (meiosis—homologous pairs separate)
A parent only passes one allele for each gene onto a zygote
Mendel’s Results and Conclusions
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CONCLUSION: LAW OF INDEPENDENT ASSORTMENT
Factors for different characteristics are distributed to gametes independently or randomly.
Which allele is passed for one one gene doesn’t affect which allele is passed down from other genes
Mendel’s Results and Conclusions
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DNA and chromosomes weren’t discovered until many decades after Mendel’s death
Today, we understand the genetic mechanisms that underlie his mathematical discoveries…
Mendel’s Legacy
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Gamete Formation Suppose there’s a gene for eye color, with the
alleles B for brown eyes or b for blue eyes. A man has the genotype Bb, which gives him
the phenotype brown eyes. Meiosis produces his gametes…
bb
bbb
b
BB
BBB
B
He can make gametes that are EITHER B or b.Half of his gametes will be one, half will be the other.We simplify, saying that he produces either B or b allele sperm. Equal chance of each.
bB
S P
hase
1st C
ytok
ines
is
2nd C
ytok
ines
is
Normal cell in G1
Four Gametes
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Practicing the Law of Segregation
(Some gametes are written with more than one letter. If Dad’s genotype is LTLt, he will make a sperm that has the LT allele or a sperm that has the Lt allele.)
Genotype YY makes what gamete/s?
Genotype Tt makes what gamete/s?
Genotype bb makes what gamete/s?
Genotype Ii makes what gamete/s?
Genotype K1K2 makes what gamete/s?
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How do we say it?
BB = brown eyesbb = blues eyes
Bb = brown eyes
2 of the same allele=Homozygous
2 different alleles=Heterozygous
BB
B
B
bb
b
b
Bb
B
b
homozygous dominanthomozygous recessive
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Practice
Identify each of these genotypes as being homozygous or heterozygous.GG ____________ Ss ________Yy ___________ Vv
________kk ____________
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Practice
Identify each of these genotypes as being homozygous dominant, homozygous recessive, or heterozygous.ee ____________ CC
________QQ ___________ pp ________Ll ____________
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Practice
Suppose that the I allele codes for orange fins and the i allele codes for yellow fins. The heterozygous genotype: __ The homozygous dominant genotype: __ The homozygous recessive genotype: __ A fish with yellow fins must have a _____________
genotype. A fish with orange fins could be either
_____________ or ___________________.
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Punnett squaresBb x Bb
male / sperm
fem
ale
/ eg
gs
X
BB
Bb bb
BbB
b
B b
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Genetics and Probability
Figuring out offspring is a matter of chance.A Punnett Square provides the
probabilities of two parents producing particular zygotes.
An example, using coins:
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Punnett Squares
Using the letter H to stand for heads…If you flip a coin that’s heads (H) on both sides,
what are the chances that it will come up heads (H)?
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Punnett Squares
If it’s a normal coin, heads (H) on one side and tails (h) on the other…What are the odds that it will come up heads
(H) on a flip?
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Punnett Squares
If you flip TWO normal coins, what are the odds that you will get heads (H) on both flips?
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Punnett Squares
The first flip will be either heads (H) or tails (h):
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Punnett Squares
The second flip will also be either heads (H) or tails (h):
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Punnett Squares
These are the possible combinations that you could have produced:
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Punnett Squares
These are the possible combinations that you could have produced:
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Punnett Squares
These are the possible combinations that you could have produced:
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Punnett Squares
These are the possible combinations that you could have produced:
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Punnett Squares
These are the possible combinations that he could have produced:
H
H H H H
H
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Punnett Squares 1 in 4 possible outcomes would be
both heads (HH). The chance of getting heads on both flips = 1/4 = 25%
H h
H
h
H H H h
H h h h
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Punnett Squares What are the odds of getting heads on
one flip, tails on the other?
H h
H
h
H H H h
H h h h
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Punnett Squares
2 of 4 possible outcomes = 1/2 = 50%
H h
H
h
H H H h
H h h h
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Punnett Squares
What if you flip two different coins:One coin has two headsThe other is normal, one heads and
one tails
What are the odds of getting heads on both flips?
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Punnett Squares
2/4 = 1/2 = 50%
This is called a Punnett Square. Punnett Squares display all possible gametes and
possible offspring.
H H
H
h
H H H H
H h H h
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Punnett Squares
The top and side boxes show possible gametes. The middle boxes show possible zygotes (offspring) they would create.
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Step-By-Step Instructions
Sample problem: What are the chances that a heterozygous brown-eyed father and a blue-eyed mother would have a blue-eyed child? (Use letters B/b)
1. Figure out Mom and Dad’s genotypes.In the example: Dad = ___, Mom = __
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Punnett Squares
2. Figure out Mom and Dad’s gametes. Dad’s gametes = __ and __ Mom’s gametes = __ and __
3. Set up a square. For a monohybrid cross (studying only one gene),
make a normal tic-tac-toe board.
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Punnett Squares4. Write Dad’s gametes on one side, and Mom’s
on the other.• It doesn’t matter whether Mom or Dad is on the side vs top, just
keep both eggs together and both sperm together.
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Punnett Squares
4. (continued) Like a genotype, if there’s a dominant allele, put it first.
A a
A
a
A
A
a a
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Punnett Squares
5. Complete zygote genotypes.Remember to put dominant allele first,
if there is one.
6. Write out all the zygote genotypes as fractions.I.e. 1/4, 2/4, 3/4, 4/4
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Punnett Squares
7. Reduce fractions if possible, and convert fractions to percentages.For instance, if two of the four zygotes
are AA, the probability of genotype AA is 2/4 = 1/2 = 50%
8. If applicable, rewrite offspring genotype results as phenotype results.
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Punnett Squares
If you did the problem correctly, it should EITHER look like this, OR…
B b
b
b
Bb
Bb
bb
bb
Genotype ProbabilitiesBB = 0/4 = 0%Bb = 2/4 = 1/2 = 50%bb = 2/4 = 1/2 = 50%
Phenotype ProbabilitiesBrown eyes = 2/4 = 1/2 = 50%Blue eyes = 2/4 = 1/2 = 50%
Final Answer50% probability of a blue-eyed child
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Punnett Squares
…OR like this. Notice that results are the same.
b b
B
b
Bb
bb
Bb
bb
Genotype ProbabilitiesBB = 0/4 = 0%Bb = 2/4 = 1/2 = 50%bb = 2/4 = 1/2 = 50%
Phenotype ProbabilitiesBrown eyes = 2/4 = 1/2 = 50%Blue eyes = 2/4 = 1/2 = 50%
Final Answer50% probability of a blue-eyed child
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Practicing with Punnett Squares
Parents – Tt and ttT t
t Tt tt
t Tt tt
Offspring:50% Tt – tall50% tt – short
Parents – TT and TtT T
T TT TT
t Tt Tt
Offspring:100% TT or Tt – tall
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Practicing Punnett Squares
Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are HH and hh.
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Practicing Punnett Squares
Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are Ll and Ll.
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Practicing Punnett Squares
If Dad’s genotype is Rr and Mom is homozygous recessive, what are the odds of having homozygous dominant offspring?
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Practicing Punnett Squares
If both parents are heterozygous, and they have ten offspring, how many of those offspring would you predict will be homozygous recessive?
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Practicing Punnett Squares
Suppose black fur is dominant and white fur is recessive. Two parents, one with black fur and one with white fur, have many offspring. Roughly half of their babies are black-furred, and half are white-furred. What were the genotypes of the parents? Hint: when a question asks you to figure out parental
genotypes, make test crosses, Punnett Squares for every possibility, then see which one gives you offspring results that fit.
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Testcross
Dominant phenotypes can have either BB or Bb genotypes. How do you know?
Perform a test cross, or a cross with a known recessive genotype (bb) If any recessive offspring appear,
unknown must be heterozygous If no recessive offspring appear,
unknown can be homozygous.