Download Give Peas a Chance Mendel Concluded: But There`s More. . .

3/3/11
Mendel Concluded:
• There must be some sort of "particles" or "elements"
inside the pea cells that make them look the way they do.
Give Peas a Chance
– Others had proposed that inheritance was a sort of "blending"
of traits. But Mendel found no evidence for that.
• The trait of purple flower color is dominant to the trait of
white flower color, which is said to be recessive—
because in a hybrid, the purple "element" somehow
covers up the white "element".
• However, the white "elements" can be passed on—
although hidden, they may reappear in future
generations.
by
MC Doc W
The same was true for the other six pairs of traits that
Mendel studied. . .
DOMINANT
RECESSIVE
purple flowers
white flowers
round seeds
wrinkled seeds
yellow seeds
green seeds
inflated pods
constricted pods
unripe pods green
unripe pods yellow
flowers along stems flowers at ends of stems
tall plant
dwarf plant
But There's More. . .
• Mendel carefully counted the number of pea
plants in each generation.
• When he crossed a true-breeding purple-flowered
pea plant with a true-breeding white-flowered pea
plant (P), all the offspring were purple-flowered
(F1).
• BUT. . . when he crossbred F1 plants, his F2
generation included 705 purple-flowered plants
and 224 white-flowered plants, making 929 in all.
• This is very close to a ration of 3:1—three purpleflowered plants for every white-flowered plant.
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Mendel observed the same thing in his other F2
crosses: there were always almost exactly three
plants with the dominant trait for every one recessive
(at least if you counted a large number of plants). DOMINANT
RECESSIVE
RATIO
5474 round seeds
1850 wrinkled seeds
2.96 : 1
6022 yellow seeds
2001 green seeds
3.01 : 1
705 purple-flowered
224 white-flowered
3.15 : 1
882 inflated pods
299 constricted pods
2.95 : 1
428 green pods
152 yellow pods
2.82 : 1
651 w/side flowers
207 w/end flowers
3.14 : 1
787 tall plants
277 short plants
2.84 : 1
The white-flowered plants in the F2 generation only
produced more white-flowered plants, for as long as
Mendel kept raising more generations (F3, F4, . . . .) X
X
It got even worse!
Mendel looked
more closely at the
plants in the F2
generations, and
tried letting them
self-fertilize. . . One-third of the purple-flowered plants in the F2
generation were also true-breeding, producing nothing
but more purple-flowered plants. 2
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But two-third of the purple-flowered plants in the F2
generation were not true-breeding—they produced purpleflowered and white-flowered plants in the same 3:1 ratio! But two-third of the purple-flowered plants in the F2
generation were not true-breeding—they produced purpleflowered and white-flowered plants in the same 3:1 ratio! Mendel reasoned thus:
Mendel reasoned thus:
• There are "elements" inside the pea cells
that determine what traits the peas have. – We now call these genes.
• A gene may exist in several forms called
alleles. – In all the examples I've shown you, each gene
has two alleles: purple / white flowers, green /
yellow peas, etc. But a gene may (and often
does) have one, three, four, or more alleles.
More about that later!
• Each pea plant has two copies of each of its
genes. • These two copies may be of the same allele,
or each may be a different allele.
– When a plant's two copies of a gene are both
the same allele, we call it homozygous.
– When a plant's two copies of a gene are
different alleles, we call it heterozygous.
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Mendel reasoned thus:
• An allele may be dominant or recessive. (There
are a few other possibilities, but we'll look at them
later; don't get sidetracked now!)
• A dominant allele will cover up the existence of a
recessive allele, if one is present.
– Example: The F1 pea plants had one dominant purple
allele and one recessive white allele—and all looked
purple; the purple allele "masked" the white allele.
• “Dominant” doesn’t mean “better” or “superior”
or anything like that.
Mendel reasoned thus:
• We can represent dominant and recessive
alleles of the same gene by capital and
lower-case letters.
– For example, we can call the dominant flower
color allele P (purple), and the recessive flower
color allele p (white)
– The alleles that any given plant is carrying can
thus be represented by a pair of letters. A whiteflowered plant must have the allele combination
pp, but a purple-flowered plant could have the
combination PP or Pp.
Mendel's first crosses looked like this. Each of the F1 plants
got one dominant (purple) allele from one parent, and one
recessive (white) allele from the other parent.
X
X
X
Here's what
Mendel's crosses
looked like in the
second generation.
Each F1 plant could
pass on either the P
allele or the p allele
—but not both.
Mathematically,
there are four
possible outcomes.
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To make this a
little easier to
visualize, we
can use a
calculating
device invented
after Mendel's
death by G. H.
Punnett, known
as the Punnett
square. Start by
drawing a
square, like
so. . .
Now: Each plant
contributes one
of its pair of
genes to each of
its offspring. The
purple-flowered
pea plant can
only pass on a
purple allele. . .
so you write in
the alleles that
that parent can
pass on, at the
top of the
square.
The whiteflowered pea
plant can only
pass on a white
allele. . . so write
in the alleles that
that parent can
pass on, at the
left of the
square.
Now, fill in the
square down and
across. This
gives you the
possible
genotypes of the
offspring.
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100% of the
offspring get one
P allele from one
parent and one p
allele from the
other. In other
words, the
genotype of all
the offspring is
Pp. Since P is
dominant to little
p, the phenotype
of all the
offspring is
"purple flowers."
Now, watch how
to set this up for
the F2
generation. Take
two of the F1
heterozygous
purple-flowered
peas, and
crossbreed them.
Each parent may
contribute either
a P allele or a p
allele.
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Now, fill in the
square down and
across. This
gives you the
possible
genotypes of the
offspring.
Three out of four
of the offspring
will have the
purple-flowered
phenotype. One
out of four will
have the whiteflowered
phenotype. This
3:1 ratio is what
Mendel observed
in his crosses.
What's more, 1
out of 4 plants
will be a
homozygous or
true-breeding
purple (genotype
PP); 2 out of 4
will be
heterozygous
purples
(genotype Pp);
and 1 out of 4
will be a
homozygous
white (genotype
pp).
This comes in
handy when you
start looking at
crosses in which
two pairs of
genes are
involved.
Consider
crossing a plant
with yellow,
smooth peas and
a plant with
wrinkled, green
peas.
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Mendel
established that
the allele for
yellow peas is
dominant to the
allele for green
peas, and the
allele for round
peas is dominant
to the allele for
wrinkled peas.
So here are the
genotypes.
Each parent
contributes one
of each pair of
alleles that it
has. In this case,
there is only one
possibility: The
yellow, smooth
parent can only
contribute the Y
and S alleles,
and the green,
wrinkled parent
can only
contribute the y
and s alleles.
This sets up a
4x4 Punnett
square, but all of
the F1 offspring
have one Y and
one S from one
parent, and one y
and one s from
the other. All
have the
genotype YySs,
and all have the
phenotype
"smooth yellow
peas".
But things get
fun when you
cross two of the
F1 plants!
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Each parent
contributes one
of each pair of
alleles that it
has. Each parent
can thus
contribute any
one of four
possible allele
combinations:
YS, Ys, yS, or
ys.
Fill in the
Punnett square
down and across,
and you get
this. . .
Now, if you
determine the
phenotypes, you
get:
9 yellow smooth
3 yellow wrinkled
3 green smooth
1 green wrinkled
out of every 16
offspring.
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