3/3/11 1 Give Peas a Chance by MC Doc W Mendel Concluded: • There must be some sort of "particles" or "elements" inside the pea cells that make them look the way they do. – 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. 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 (F 1 ). • BUT. . . when he crossbred F 1 plants, his F 2 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 purple- flowered plants for every white-flowered plant.
Transcript
3/3/11
1
Give Peas a Chance
by MC Doc W
Mendel Concluded:
• There must be some sort of "particles" or "elements" inside the pea cells that make them look the way they do. – 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.
The same was true for the other six pairs of traits that Mendel studied. . .
generations, and tried letting them self-fertilize. . .
X
X
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, . . . .)
One-third of the purple-flowered plants in the F2 generation were also true-breeding, producing nothing
but more purple-flowered plants.
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But two-third of the purple-flowered plants in the F2 generation were not true-breeding—they produced purple-flowered 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 purple-flowered and white-flowered plants in the same 3:1 ratio!
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!
Mendel reasoned thus: • 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 white-flowered 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
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.
X
X
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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 white-flowered 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 white-
flowered 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
