Download Mendelian Genetics

Mendelian Genetics
Chapter 12, part 1
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.
• 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.
• 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.
•
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.
•
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.
• 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.
• 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.
Mendel’s Laws
• 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.
Three Laws of Heredity
• Law of Dominance
• Law of Segregation
• Law of Independent Assortment
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.
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.
The offspring of a purple-flowered pea plant and a whiteflowered 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
Offspring of the F1 generation (the hybrids) may be purpleflowered if they inherit at least one factor for purple flowers,
or may be white flowered if they inherit the white factor from
both parents.
Rr
RR
Rr
X
Rr
Rr
Firstgeneration
offspring (F1)
rr
Secondgeneration
offspring (F2)
3/4 purple
1/4 white
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.
genotypes:
RR or Rr
phenotype
purple
The white-flowered trait is
recessive because an
individual must inherit two
copies of the white allele (r)
to show the white
phenotype.
rr
white
Same letter,
different case =
same gene,
different allele
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
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.
In modern terms, the homozygous parents in the P
generation can pass one one kind of allele to their
offspring.
homozygous parent
gametes
gene
A
A
Homologous chromosomes
A
A
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.
heterozygous parent
gametes
gene
A
a
Homologous chromosomes
A
a
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
When the gametes join to produce the F1 generation, all
offspring of homozygous dominant and homozygous
recessive parents are heterozygous.
gametes of parents
sperm
P
F1
offspring
eggs
+
p
Pp
P
Pp
or
p
+
gametes from
F1 plants (Pp)
sperm eggs
F2
offspring
P
+
P
PP
P
+
p
Pp
p
+
P
Pp
p
+
p
pp
The heterozygous F1
individuals can put
either a dominant OR a
recessive allele in each
of their gametes.
Pp
self-fertilize
1/2 P
eggs 1/2 p
1/2 P
sperm
A Punnet square is one
way to predict the
outcome of a cross by
showing all the possible
combinations of all the
possible gametes.
1/4 PP
1/4 Pp
1/4 pP
1/4 pp
1/2 p
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?
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?
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.
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).
Seed
color
Pod
shape
Pod
color
Dominant form
Recessive form
smooth
wrinkled
yellow
green
inflated
constricted
green
yellow
purple
white
at leaf
junctions
at tips of
branches
tall
(1.8 to
2 meters)
dwarf
(0.2 to 0.4
meters)
Flower
color
Flower
location
Plant
size
S
Y
pairs of alleles on homologous
chromosomes in diploid cells
s
y
chromosomes
replicate
Traits carried on
separate
chromosomes sort
independently of one
another during gamete
formation.
S
Y
s
y
replicated homologues
pair during metaphase
of meiosis I,
orienting like this
or like this
S
y
s
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
Y
s
Y
S
s
y
y
S
y
s
y
s
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.
Now consider this in terms of
genotypes:
Genotype of this
parent (for these two
traits) is SsYy
Meiosis puts ONE Sbearing and one Ybearing chromosome in
each gamete.
S
Y
s
y
chromosomes
replicate
S
Y
s
y
replicated homologues
pair during metaphase
of meiosis I,
orienting like this
or like this
S
y
s
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
Y
s
Y
S
s
y
y
S
y
s
y
s
Y
independent assortment produces four equally
Genotypes of the
likely allele combinations during meiosis
gametes that this parent SY
sy
Sy
sY
can produce are:
Y
SsYy
self-fertilize
This Punnet square shows a
cross between two pea
plants which are
heterozygous for two traits.
1/4 sy
1/4 SY
1/16 SSYY 1/16 SSYy 1/16SsYY 1/16 SsYy
sperm
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.
1/4 SY
eggs
1/4 Sy 1/4 sY
1/4 Sy
1/16 SSyY 1/16 SSyy 1/16 SsyY 1/16 Ssyy
1/4 sY
1/16 sSYY 1/16 sSYy 1/16 ssYY 1/16 ssYy
1/4 sy
1/16 sSyY 1/16 sSyy 1/16 ssyY 1/16 ssyy
seed shape
seed color
3/4 smooth 
phenotypic ratio
(9:3:3:1)
3/4 yellow = 9/16 smooth yellow
3/4 smooth 
1/4 green = 3/16 smooth green
1/4 wrinkled 
3/4 yellow = 3/16 wrinkled green
1/4 wrinkled 
1/4 green = 1/16 wrinkled yellow
Solving dihybrid crosses with Mendelian (dominantrecessive) 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?
Solving dihybrid crosses with Mendelian (dominantrecessive) 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?
Laws: “proven” forever?
• 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.
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.