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Genetics
Why do we look the way we do?
Honors Biology Chapters 9 & 12
Inheritance of chromosomes
 Egg + sperm  zygote
meiosis
egg
zygote
sperm
fertilization
mitosis &
development
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?)
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
Genes affect what you look like
X
bb
Bb
BB
Bb
Bb
Where did the blue eyes go??
Bb
Genes affect what you look like…
X
bb
Bb
Bb
Bb
bb
Why did the blue eyes stay??
bb
Genes affect what you look like…
X
Bb
BB or Bb BB or Bb BB or Bb
Bb
bb
Where did the blue eyes come from??
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
How does this work?
 Homologous chromosomes have same
genes…

…but maybe different alleles
eye
color
(blue?)
hair
color
eye
color
(brown?)
hair
color
Traits are inherited as separate units
 For each trait, an organism inherits
2 copies of a gene (2 alleles), 1 from
each parent
1 from Mom
homologous chromosomes
1 from Dad
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
Genetics vs. appearance
How were these
brown eyes made?
eye
color
(brown)
eye
color
(brown)
eye
color
(brown)
eye
color
(blue)
vs.
B
BB
B
Bb
B
b
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 ___________
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 _______
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

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

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

Genetics
&
The Work of Mendel
Gregor Mendel
 Modern genetics began in the mid1800s 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
Mendel’s work
Pollen transferred from white
flower to stigma of purple flower
 Bred pea plants

cross-pollinate
true breeding parents

allowed offspring
to self-pollinate
& observed next
generation
all purple flowers result
self-pollinate
?
Mendel collected data for 7 pea traits
Looking closer at Mendel’s work
Parents
1st
F1
true-breeding
true-breeding
X
purple-flower peas
white-flower peas
100%
purple-flower peas
generation
(hybrids)
100%
self-pollinate
2nd
F2
generation
75%
purple-flower peas
25%
white-flower peas
3:1
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
allele producing
functional protein
mutant allele
malfunctioning
protein
homologous
chromosomes
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.
Mendel’s Results
and Conclusions
 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
 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 Legacy
 DNA and chromosomes
weren’t discovered until
many decades after
Mendel’s death
 Today, we understand the
genetic mechanisms that
underlie his mathematical
discoveries…
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.
He can make
 Meiosis produces his gametes…
gametes that are
B
BB
Normal cell
in G1
BB
Cytokinesis
bb
bb
2nd
b
b
1st Cytokinesis
S Phase

b
B
B
Four Gametes
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.
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?
B
How do we say it?
2 of the same allele=
Homozygous
BB = brown eyes
bb = blues eyes
homozygous dominant
homozygous recessive
2 different alleles=
Heterozygous
Bb = brown eyes
BB
B
b
bb
b
B
Bb
b
Practice

Identify each of these genotypes as
being homozygous or heterozygous.
GG ____________
 Yy ___________
 kk ____________

Ss ________
Vv ________
Practice

Identify each of these genotypes as
being homozygous dominant,
homozygous recessive, or
heterozygous.
ee ____________
 QQ ___________
 Ll ____________

CC ________
pp ________
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 ___________________.
Punnett squares
x
Bb
male / sperm
X
female / eggs
Bb
B
b
BB
Bb
Bb
bb
B
b
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:
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)?
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?
Punnett Squares

If you flip TWO normal coins, what are
the odds that you will get heads (H)
on both flips?
Punnett Squares

The first flip will be either heads (H) or
tails (h):
Punnett Squares

The second flip will also be either
heads (H) or tails (h):
Punnett Squares

These are the possible combinations
that you could have produced:
Punnett Squares

These are the possible combinations
that you could have produced:
Punnett Squares

These are the possible combinations
that you could have produced:
Punnett Squares

These are the possible combinations
that you could have produced:
Punnett Squares

These are the possible combinations
that he could have produced:
H
H
H
H
H
H
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
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
Punnett Squares

2 of 4 possible outcomes = 1/2 = 50%
h
H
H
h
H
H
H
H
h
h
h
h
Punnett Squares

What if you flip two different coins:
One coin has two heads
 The other is normal, one heads and
one tails


What are the odds of getting heads on
both flips?
Punnett Squares
H
H
H
H H
H h
h
H H
H h

2/4 = 1/2 = 50%

This is called a Punnett Square.

Punnett Squares display all possible gametes and
possible offspring.
Punnett Squares

The top and side boxes show possible gametes.
The middle boxes show possible zygotes
(offspring) they would create.
Step-By-Step Instructions
Sample problem: What are the
chances that a heterozygous browneyed 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 = __
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.
Punnett Squares

4. 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.
Punnett Squares

4. (continued)
 Like a genotype, if there’s a dominant allele, put it first.
A
A
A
a
a
A
a
a
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
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.
Punnett Squares

b
b
If you did the problem correctly, it
should EITHER look like this, OR…
B
b
Bb
bb
Bb
Genotype Probabilities
BB = 0/4 = 0%
Bb = 2/4 = 1/2 = 50%
bb = 2/4 = 1/2 = 50%
Phenotype Probabilities
Brown eyes = 2/4 = 1/2 = 50%
Blue eyes = 2/4 = 1/2 = 50%
bb
Final Answer
50% probability of a blue-eyed child
Punnett Squares

…OR like this.

Notice that results are the same.
b
b
B
Bb
Bb
b
bb
bb
Genotype Probabilities
BB = 0/4 = 0%
Bb = 2/4 = 1/2 = 50%
bb = 2/4 = 1/2 = 50%
Phenotype Probabilities
Brown eyes = 2/4 = 1/2 = 50%
Blue eyes = 2/4 = 1/2 = 50%
Final Answer
50% probability of a blue-eyed child
Practicing with Punnett Squares
Parents – Tt and tt
T
t
Parents – TT and Tt
T
T
t
Tt
tt
T
TT
TT
t
Tt
tt
t
Tt
Tt
Offspring:
50% Tt – tall
50% tt – short
Offspring:
100% TT or Tt – tall
Practicing Punnett Squares

Show a monohybrid cross, with all
genotype and phenotype probabilities,
for parents who are HH and hh.
Practicing Punnett Squares

Show a monohybrid cross, with all
genotype and phenotype probabilities,
for parents who are Ll and Ll.
Practicing Punnett Squares

If Dad’s genotype is Rr and Mom is
homozygous recessive, what are the
odds of having homozygous dominant
offspring?
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?
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 whitefurred. 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.
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.
