2. • 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
3. Three Laws of Heredity
• Law of Dominance
• Law of Segregation
• Law of Independent Assortment
4. 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.
5. 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.
6. 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
7. 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
8. 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 rrgenotypes:
phenotype purple white
Same letter,
different case =
same gene,
different allele
9. 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
10. 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.
11. gameteshomozygous 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
12. 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.
gametesheterozygous parent
A a A a
Homologous chromosomes
gene
13. 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
+
+
14. 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
15. 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.
17. 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?
18. 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?
19. • 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
20. Dominant form Recessive formTrait
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
greenyellow
wrinkledsmooth
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).
21. 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
YY YY
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.
22. 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
YY YY
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.
23. SY
SY
sY
sY
Sy
Sy
sy
sy
SSYY SsYY
ssYY
ssyY
SsyY
SSYy SsYy
SsyySSyySSyY
sSYY
sSyY
sSYy ssYy
ssyysSyy
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 9/16smooth yellow smooth yellow=
3/4 1/4 3/16smooth green smooth green=
1/4 3/4 3/16wrinkled yellow wrinkled green=
1/4 1/4 1/16wrinkled 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.
24. 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?
25. 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?
26. • 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?
27. 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.
