mendelian Genetics

1. Mendelian Genetics

2. Gregor Johann 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. • 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.

4. • With Abbot Napp’s
encouragement, Mendel
studied heredity in peas,
carefully choosing traits that
did not appear to blend.
Collected data from 1856 -
1865.
• Mendel’s 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.

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

6. • 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.

7. • 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
Mendel’s work still stands.

8. • A scientific law is an evidence-based
description of a natural phenomenon in
a given set of circumstances.
• Mendel’s three Laws of Heredity
describe what Mendel observed in
patterns of inherited traits.
Mendel’s Laws

9. Three Laws of Heredity
• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

10. Vocabulary
• Character –heritable feature
• Trait – each variant for a character
• True-breeding – plants that self-pollinate all
offspring are the same variety
• Monohybrid cross – a cross that tracks the
inheritance of a single character
• P generation – (parental) true-breeding
• F1- (first filial) offspring of P generation
• F2 – (second filial) offspring from F1 cross

11. Vocabulary (continued)
• Allele- alternate version of a gene
• Dominant allele – expressed in the
heterozygote
• Recessive allele – not expressed in the
heterozygote
• Homozygote – pair of identical alleles for a
character
• Homozygous dominant- BB
• Homozygous recessive - bb
• Heterozygote – two different alleles for a
character (Bb)
• Genotype – genetic makeup
• Phenotype – appearance of an organism

13. • Pea plants have several advantages for genetics.
• Pea plants are available in many varieties with distinct
heritable features (characters) with different variants
(traits).
• Another advantage of peas is that Mendel had strict
control over which plants mated with which.
• Each pea plant has male
(stamens) and female
(carpal) sexual organs.
• In nature, pea plants typically
self-fertilize, fertilizing ova
with their own sperm.
• However, Mendel could also
move pollen from one plant
to another to cross-pollinate
plants.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14. • In a typical breeding experiment, Mendel would
cross-pollinate (hybridize) two contrasting,
true-breeding pea varieties.
• The true-breeding parents are the P
generation and their hybrid offspring are the
F1 generation.
• Mendel would then allow the F1 hybrids to self-
pollinate to produce an F2 generation.
• It was mainly Mendel’s quantitative analysis of
F2 plants that revealed the two fundamental
principles of heredity: the law of segregation
and the law of independent assortment.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

15. 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.

16. X
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?
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.

17. The offspring of a purple-flowered pea plant and a white-
flowered pea plant all have purple flowers. The purple trait
is dominant. Why?
true-breeding,
purple-flowered
plant
First-generation
offspring (F1)
Parental
generation (P)
pollen
pollen
cross-fertilize
true-breeding,
white-flowered
plant
RR rr
Rr

18. 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.
1/4 white
Second-
generation
offspring (F2)
First-
generation
offspring (F1)
3/4 purple
X
Rr Rr
RR Rr Rr rr

19. 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.
RR or Rr rr
genotypes:
phenotype purple white
Same letter,
different case =
same gene,
different allele

20. Solving problems involving dominance
Dexter has freckles. So
does his wife, Darla.
Their son, Derek has no
freckles. Is having
freckles a dominant or a
recessive trait?
Dexter
freckles
Darla
freckles
Derek
no freckles

21. 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.

22. gametes
homozygous parent
A A A A
In modern terms, the homozygous parents in the P
generation can pass one one kind of allele to their
offspring.
Homologous chromosomes
gene

23. 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
A a A a
Homologous chromosomes
gene

24. The genotypes can be represented with letters, which
symbolize the alleles: capital for dominant alleles, small
case for recessive.
all p sperm and eggs
all P sperm and eggs
purple parent
white parent
pp
PP P P
p p
+
+

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

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

27. P
p
sperm
eggs
P
p
1/4
1/4
1/4
1/4
1/2
1/2
1/2 1/2
PP Pp
pP pp
Pp
self-fertilize
A Punnet square is one
way to predict the
outcome of a cross by
showing all the possible
combinations of all the
possible gametes.

28. • If the blending model were correct, the F1
hybrids from a cross between purple-flowered
and white-flowered pea plants would have pale
purple flowers.
• Instead, the F1 hybrids
all have purple flowers,
just a purple as the
purple-flowered
parents.
2. By the law of segregation, the two alleles
for a characters are packaged into separate
gametes
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 14.2

29. • When Mendel allowed the F1 plants to self-
fertilize, the F2 generation included both
purple-flowered and white-flowered plants.
• The white trait, absent in the F1, reappeared
in the F2.
• Based on a large
sample size, Mendel
recorded 705
purple-flowered F2
plants and 224
white-flowered F2
plants from the
original cross.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 14.2

31. Law of Segregation - the two alleles for each
character segregate during gamete
production

33. 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?

34. 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?

35. • 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.
Law of Independent Assortment

36. Dominant form Recessive form
Trait
Seed
shape
Seed
color
Pod
color
Pod
shape
Flower
color
Flower
location
Plant
size
tall
(1.8 to
2 meters)
dwarf
(0.2 to 0.4
meters)
constricted
purple white
green
green
yellow
wrinkled
smooth
at leaf
junctions
at tips of
branches
inflated
yellow
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).

37. meiosis II
meiosis I
pairs of alleles on homologous
chromosomes in diploid cells
chromosomes
replicate
orienting like this
or like this
replicated homologues
pair during metaphase
of meiosis I,
independent assortment produces four equally
likely allele combinations during meiosis
S
Y
s
y
S S
S
S
S
S
S S S S
Y
Y
Y
Y
Y
Y
Y
Y Y
Y
s s
s s s s
s
s
s
s
y
y
y
y
y
y
y y y y
Traits carried on
separate
chromosomes sort
independently of one
another during gamete
formation.
Notice that each gamete receives ONE s-bearing and
ONE y-bearing chromosome from the original cell.

38. meiosis II
meiosis I
chromosomes
replicate
orienting like this
or like this
replicated homologues
pair during metaphase
of meiosis I,
independent assortment produces four equally
likely allele combinations during meiosis
S
Y
s
y
S S
S
S
S
S
S S S S
Y
Y
Y
Y
Y
Y
Y
Y Y
Y
s s
s s s s
s
s
s
s
y
y
y
y
y
y
y y y y
Now consider this in terms of
genotypes:
Genotype of this
parent (for these two
traits) is SsYy
Genotypes of the
gametes that this parent
can produce are:
SY sy Sy sY
Meiosis puts ONE S-
bearing and one Y-
bearing chromosome in
each gamete.

39. SY
SY
sY
sY
Sy
Sy
sy
sy
SSYY SsYY
ssYY
ssyY
SsyY
SSYy SsYy
Ssyy
SSyy
SSyY
sSYY
sSyY
sSYy ssYy
ssyy
sSyy
1/4
1/4
1/4
1/4
1/4 1/4 1/4 1/4
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
1/16
eggs
sperm
SsYy
self-fertilize
seed shape seed color phenotypic ratio
(9:3:3:1)
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 green
=
×
1/4 1/4 1/16
wrinkled green wrinkled yellow
=
×
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.

40. Solving dihybrid crosses with Mendelian (dominant-
recessive) inheritance.
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?

41. • Mendel’s Laws were good descriptions
of what he observed in the peas and
other plants he worked with.
• New knowledge accumulated since
Mendel’s 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.
Laws: “proven” forever?

42. 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.