Mendelian Genetics

1
Mendelian Genetics

• Chapter 12, part 1

2
Gregor Mendel

• Born in 1822 in Moravia (now part of the Czech
  Republic.
• Son of a tenant farmer joined a monastery to get
  an education.
• Deeply interested in science, particularly
  heredity.

3

• At the monastery in Brno, Moravia, Mendel
  received the support of Abbot Napp.
• From 1851-1855, studied at the University of
  Vienna, but did not receive a degree.

4

• What was understood at the time
• Heredity appeared random and unpredictable.
• Many traits seemed to blend in the offspring,
  suggesting a liquid factor controlled heredity.
• Yet some traits, such as red hair, did not blend
  away.

5

• With Abbot Napps encouragement, Mendel studied
  heredity in peas, carefully choosing traits that
  did not appear to blend. Collected data from 1856
  - 1865.
• Mendels creative contribution he was the first
  to follow single traits from generation to
  generation instead of trying to document and
  follow every trait in the plants.

6

• Mendel presented his findings to the Association
  of Natural Research in Brno in 1865.
• Few people recognized the significance of
  Mendels research. His quantitative methods were
  uncommon at the time, and the blending theory
  was widely accepted.

7

• In 1868, Mendel became abbot of his monastery.
• His religious work left little time for research,
  which he set aside, though he was always
  convinced he had made a valuable contribution to
  science.

8

• Mendel died in 1884. Sixteen years later, in
  1900, his work was rediscovered by Hugo de Vries
  and others looking for clues into the puzzle of
  heredity.
• Though criticized in some details, the main body
  of Mendels work still stands.

9
Mendels Laws

• A scientific law is an evidence-based description
  of a natural phenomenon in a given set of
  circumstances. (Theories are evidence based
  explanations of natural phenomena.)
• Mendels three Laws of Heredity describe what
  Mendel observed in patterns of inherited traits.

10
Three Laws of Heredity

• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

11
D A I L Y W O R K

• Dominant is one of those pesky words with
  multiple meanings that can interfere with our
  understanding of how it is used in science.
  First, list several meanings that you can think
  of for the word dominant.
• Now write out what you believe is meant by a
  dominant trait.

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Law of Dominance

• Traits are controlled by two factors that can be
  called dominant or recessive.
• A dominant trait shows if the offspring
  inherits at least one dominant factor from one
  parent.
• A recessive trait shows only if the offspring
  inherits two recessive factors, one from each
  parent.

13
Mendels Peas

• Mendel chose traits in peas that had an
  either/or condition purple OR white blossoms,
  yellow OR green seeds, and so on.

Blauschokker peas, photo by Territorial Seed
Company.
https//www.jbgorganic.com/blog/2009/03/two-meals-
not-to-be-missed/
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In this cross between two purple-flowered pea
plants, one-quarter of the offspring have white
flowers. Based just on this information, which
is dominant white or purple flowers? How do you
know?
X
Hint Dominance is not based on numbers of
individuals with the trait. It is based on the
number of copies of the allele that must be
inherited to show the trait.
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The offspring of a purple-flowered pea plant and
a white-flowered pea plant all have purple
flowers. The purple trait is dominant. Why?
RR
rr
pollen
Parental generation (P)
pollen
cross-fertilize
true-breeding, purple-flowered plant
true-breeding, white-flowered plant
First-generation offspring (F1)
Rr
16
Offspring of the F1 generation (the hybrids) may
be purple-flowered if they inherit at least one
factor for purple flowers, or may be white
flowered if they inherit the white factor from
both parents.
First- generation offspring (F1)
Rr
Rr
X
RR
Rr
Rr
rr
Second- generation offspring (F2)
1/4 white
3/4 purple
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The purple-flowered trait is dominant because
each an individual who inherits at least one copy
of the purple allele (R) shows the purple
phenotype.
The white-flowered trait is recessive because an
individual must inherit two copies of the white
allele (r) to show the white phenotype.
Same letter, different case same gene,
different allele
genotypes
RR or Rr
rr
purple
white
phenotype
18
What is a dominant trait?

1. A trait that is more common than the recessive
   form.
2. A trait that is stronger or more adaptive than
   the recessive form.
3. A trait that is expressed if only one allele for
   that trait is inherited.
4. All of these are true.

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True or false? A trait that is dominant will
increase in the population, while recessive
traits decrease.

1. True
2. False

20
True or false? A person can be a carrier for a
dominant trait (that is, can have one allele but
not show the trait).

1. True
2. False

21
Solving problems involving dominance
Lets start by making a family tree
Dexter has freckles. So does his wife, Darla.
Their son, Derek has no freckles. Is having
freckles a dominant or a recessive trait?
This suggests that the childs phenotype is
recessive, and the parents are both carriers
(heterozygous).
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• Fred and Wilma both have pointed (widows peak)
  hairlines. Their daughter, Pebbles, has a smooth
  hairline. Which is dominant smooth or pointed
  hairline? How do you know?
• Penelope has small, attached earlobes. Armando
  has larger, free earlobes. All four of their
  children have large, free earlobes. Which is
  dominant free or attached earlobes? How do you
  know?

D A I L Y W O R K
23
Pebbles has a smooth hairline. Bam-bam has a
pointed hairline, but one of his parents has a
smooth hairline. If Pebbles and Bam-bam were to
have kids, what are the odds that their child
would have a pointed hairline?

1. 100
2. 75
3. 50
4. 25

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Law of Segregation

• Each individual has a pair of factors controlling
  each trait, one inherited from each biological
  parent.
• During the formation of gametes (sex cells) these
  two factors separate. Only one ends up in each
  sex cell.

25
In modern terms, the homozygous parents in the P
generation can pass on one kind of allele to
their offspring.
gametes
homozygous parent
gene
A
A
A
A
Homologous chromosomes
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The heterozygous parents of the F1 generation
have two alleles for the gene in question, and
can pass one or the other, but not both, to their
offspring.
gametes
heterozygous parent
gene
A
a
A
a
Homologous chromosomes
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The genotypes can be represented with letters,
which symbolize the alleles capital for dominant
alleles, small case for recessive.
purple parent
PP
P
P

all P sperm and eggs
white parent
pp
p
p

all p sperm and eggs
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When the gametes join to produce the F1
generation, all offspring of homozygous dominant
and homozygous recessive parents are
heterozygous.
F1 offspring
gametes of parents
sperm
eggs
P
p

Pp
or
p
P

Pp
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gametes from F1 plants (Pp)
F2 offspring
The heterozygous F1 individuals can put either a
dominant OR a recessive allele in each of their
gametes.
sperm
eggs
P

P
PP
p

P
Pp
p

P
Pp
p
p

pp
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A Punnet square is one way to predict the outcome
of a cross by showing all the possible
combinations of all the possible gametes.
Pp self-fertilize
p
eggs
P
1/2
1/2
P
1/2
1/4
1/4
PP
Pp
sperm
p
1/2
Professor Reginald Punnett
1/4
pp
1/4
pP
31
Solving single-gene (monohybrid) crosses with
Mendelian (dominant-recessive) inheritance.
Tomato fruit color can be red or yellow. a. A
red tomato plant is crossed with a yellow tomato
plant, and all the offspring have red tomatoes.
Which trait is dominant? b. If two of the
resulting hybrid red tomato plants are crossed,
what will be the ratio of phenotypes in the
offspring?
If all of the F1 offspring are red, then red must
be the dominant trait.
Its probable that the genotypes of the parents
were RR (red) and rr (yellow).
RR produces these gametes
R
rr produces these gametes

r
Rr
All offspring are red and heterozygous
32
Solving single-gene (monohybrid) crosses with
Mendelian (dominant-recessive) inheritance.
Tomato fruit color can be red or yellow. a. A
red tomato plant is crossed with a yellow tomato
plant, and all the offspring have red tomatoes.
Which trait is dominant? b. If two of the
resulting hybrid red tomato plants are crossed,
what will be the ratio of phenotypes in the
offspring?
All of the F1 offspring were red and had the
genotype Rr.
An Rr parent can produce these gametes
The other Rr individual it is crossed with can
produse these gametes.
R
r


Rr
R
RR
r
Rr
rr
The offspring of this cross are
33
A person is heterozygous (Aa) for Trait A. What
alleles for that trait can that person put in
their gametes?

1. Aa
2. AA
3. Aa
4. A OR a

34
D A I L Y W O R K

• Flat feet are caused by a recessive allele (f).
  Both Bert and his wife Betsy have normal feet,
  but their first child, Binky, has flat feet.
• What are the genotypes of Bert, Betsy, and Binky?
• What are the odds of Bert and Betsys next child
  having flat feet?

35
D A I L Y W O R K

• Prominent chin dimples are inherited by a
  dominant allele (D). Herman has a prominent chin
  dimple. So does his father, but not his mother.
  Hermans wife, Honoria, has no chin dimple. What
  is the probability that Herman and Honorias
  first child will have a prominent chin dimple?
  How do you know?

36
D A I L Y W O R K

• Leslie has a long palmar muscle (third tendon in
  the middle of the wrist). Her twin brother Louis
  does not. Neither do their parents. Leslies
  husband, Lamont, has the long palmar muscle. So
  do their twin sons, Larry and Lance. Is the long
  palmar muscle dominant or recessive? How do you
  know? (It may help to make a family tree for this
  problem.)

37
Law of Independent Assortment

• When genetic factors segregate in the gametes,
  they segregate independently of one another. A
  dominant allele for one trait does not guarantee
  inheritance of a dominant allele for a different
  trait.

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Dominant form
Recessive form
Trait
Seed shape
All organisms have multiple inheritable traits
controlled by genes. Each trait is inherited
independently of the others. A pea plant may, for
example, have yellow seeds (dominant) but white
flowers (recessive).
wrinkled
smooth
Seed color
green
yellow
Pod shape
constricted
inflated
Pod color
green
yellow
Flower color
purple
white
Flower location
at leaf junctions
at tips of branches
Plant size
tall (1.8 to 2 meters)
dwarf (0.2 to 0.4 meters)
39
S
pairs of alleles on homologous chromosomes in
diploid cells
s
Y
y
chromosomes replicate
Traits carried on separate chromosomes sort
independently of one another during gamete
formation.
replicated homologues pair during metaphase of
meiosis I,
y
Y
S
S
orienting like this
or like this
s
s
y
Y
meiosis I
S
S
y
Y
Y
s
s
y
S
S
y
s
s
y
Y
Y
meiosis II
S
S
S
S
s
s
s
s
y
y
y
y
Y
Y
Y
Y
independent assortment produces four
equally likely allele combinations during meiosis
Notice that each gamete receives ONE s-bearing
and ONE y-bearing chromosome from the original
cell.
40
Now consider this in terms of genotypes
S
s
Y
y
Genotype of this parent (for these two traits) is
SsYy
chromosomes replicate
replicated homologues pair during metaphase of
meiosis I,
y
Y
S
S
orienting like this
or like this
s
s
y
Y
Meiosis puts ONE S-bearing and one Y-bearing
chromosome in each gamete.
meiosis I
S
S
y
Y
Y
s
s
y
S
S
y
s
s
y
Y
Y
meiosis II
S
S
S
S
s
s
s
s
y
y
y
y
Y
Y
Y
Y
Genotypes of the gametes that this parent can
produce are
independent assortment produces four
equally likely allele combinations during meiosis
SY
sy
Sy
sY
41
This Punnet square shows a cross between two pea
plants which are heterozygous for two
traits. Again, the Punnet square represents all
possible combinations of the gametes that the
plants can donate to their offspring. They must
put one copy of a gene for each trait in their
gametes.
SsYy self-fertilize
eggs
SY
sY
Sy
sy
1/4
1/4
1/4
1/4
SY
1/4
SsYY
SSYy
SsYy
SSYY
1/16
1/16
1/16
1/16
Sy
1/4
SsyY
Ssyy
SSyy
SSyY
1/16
1/16
1/16
1/16
sperm
sY
1/4
ssYY
sSYY
sSYy
ssYy
1/16
1/16
1/16
1/16
sy
1/4
ssyY
sSyY
ssyy
sSyy
1/16
1/16
1/16
1/16
seed shape
seed color
phenotypic ratio (9331)
3/4
3/4
9/16
smooth
yellow
smooth yellow


3/4
1/4
3/16
smooth
green
smooth green


1/4
3/4
3/16
wrinkled
yellow
wrinkled yellow


1/4
1/4
1/16
wrinkled
green
wrinkled green


42
Solving dihybrid crosses with Mendelian
(dominant-recessive) inheritance.
Begin with the parents genotypes. We will assume
that the two traits are on different chromosomes.
Pea plants can be tall (T) or short (t) and
produce purple (R) or white (r) blossoms. a. A
pure-breeding tall plant with purple flowers
(TTRR) is crossed with a pure-breeding short
plant with white flowers (ttrr). What will the
offspring look like? b. If two of the hybrid (F1)
plants are crossed, what offspring can they
produce?
A TTRR parent can produce these gametes
A ttrr parent can produce these gametes
TR

tr
TtRr
The offspring of this cross are all tall with
purple flowers.
43
Solving dihybrid crosses with Mendelian
(dominant-recessive) inheritance.
The hybrid offspring all have the genotype TtRr
Pea plants can be tall (T) or short (t) and
produce purple (R) or white (r) blossoms. a. A
pure-breeding tall plant with purple flowers
(TTRR) is crossed with a pure-breeding short
plant with white flowers (ttrr). What will the
offspring look like? b. If two of the hybrid (F1)
plants are crossed, what offspring can they
produce?
A TtRr parent can produce these gametes
TR
tr
Tr
tR
The other TtRr parent produces the same gametes




TR
TTRR
TTRr
TtRR
TtRr
Tr
TTrR
TTrr
TtTr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Tttr
ttRr
ttrr
Phenotypes in the F2 generation
44
A person is heterozygous for Traits A and B
(AaBb). What is ONE possible combination of those
alleles that can go in the persons gametes?

1. AaBb
2. AABBaabb
3. AB
4. Aa

45
D A I L Y W O R K

• A pea plant with purple (P) flowers and yellow
  (Y) seeds is crossed with a pea plant with white
  flowers (p) and green seeds (y). All of the
  plants in the next generation have purple flowers
  and yellow seeds. If two plants from this hybrid
  generation are crossed, what will their offspring
  look like? Do a dihybrid Punnet square to predict
  the outcome.

46
D A I L Y W O R K

• A gardener has a tall tall (T) pea plants with
  smooth (S) seeds. She crosses it with a plant
  that is also tall and with smooth seeds, BUT she
  knows this plant is the offspring between a
  pure-breeding tall plant with smooth seeds plant
  and a pure-breeding short plant with wrinkled
  seeds. What will the the results of the
  pure-breeding tall/smooth plant and the hybrid
  plant?

47
Laws proven forever?

• Mendels Laws were good descriptions of what he
  observed in the peas and other plants he worked
  with.
• New knowledge accumulated since Mendels time has
  refined his ideas. While his laws still hold true
  in some instances, there are many exceptions that
  we will explore in the next presentations.

48
Recap

• Genes may have multiple alleles, such as dominant
  and recessive alleles.
• Chromosomes, which carry genes, separate from one
  another during gamete formation.
• Chromosomes sort independently of one another
  during gamete formation, but each gamete gets ONE
  of each kind of chromosome.