Genetics
Mendel’s
Garden Pea Plant Experiment
In
this activity, you will examine inheritance in a small population of garden pea
plants. When Mendel made experimental
crosses with his pea plants, he noted the following results…
|
PARENTS |
OFFSPRING FIRST FILIAL (F1) GENERATION |
GRANDCHILDREN SECOND FILIAL (F2)
GENERATION |
|
Yellow X
green seeds |
All yellow |
6,022
yellow : 2,001 green |
|
Smooth X
wrinkled seeds |
All smooth |
5,474
smooth : 1,850 wrinkled |
|
Green pod X
yellow pod |
All green |
428 green :
152 yellow |
|
Long stem X
short stem |
All long |
787 long :
277 short |
|
Axial
flowers X terminal flowers |
All axial |
651 axial :
207 terminal |
|
Inflated
pods X constricted pods |
All
inflated |
882
inflated : 299 constricted |
|
Red flowers
X white flowers |
All red |
705 red :
224 white |
We will
explore the inheritance of flower color in this lab. As we can see from Mendel’s results above,
red flower color is the dominant allele and white flower color is the recessive
allele. Thus, if a flower inherited
red/red alleles (was homozygous for the flower color trait), or if a flower
inherited red/white alleles (was heterozygous for the flower color trait, the
flower would have a phenotype of red flowers.
On the other hand, the only way a garden pea plant could have a
phenotype of white flowers is if it had a genotype of white/white.
Exploring
Segregation (the production of gametes – egg & sperm)
In
this exercise, we are going to imagine that we crossed a pure line red flowered
pea plant with a white flowered pea plant.
This cross produced 10 hybrid pea plants that were red flowered, but
carried the recessive allele. So, from
the hybrid F1 (hybrid) generation of plants, we will have a total of
10 alleles for red flower color and 10 alleles for white flower color. (Each plant had one allele for red & one
allele for white. If we have 10 total
plants, that means we have 10 red alleles and 10 white alleles.)
We
are going to simulate this situation by placing 10 white beads (representing
the white flower allele) & 10 red beads (representing the red flower
allele) into a cup. We will draw one
bead at a time from the cup, without looking, to produce a "gamete"
& then return the bead to the gene pool.
Repeat the process of picking a bead out of the cup & returning it
for the number of times indicated by your instructor. Each time, record in Table 1 whether the
chosen bead is red or white. When you
have completed your table, add your results to those of other members of the
class in the table on the chalkboard.
|
|
RED
|
WHITE |
|
Your Total |
|
|
|
Your
percentage |
|
|
|
Class Total |
|
|
When we are drawing beads to “make
gametes”, we have a one in two (or 50%) chance of drawing a white bead and a one
in two (or 50%) chance of drawing a red bead.
Let's imagine that we drew 200 times at random from the cup. We would expect to form 100 gametes with the
(white) allele and 100 gametes with the (red) allele.
Does one
always get exactly the fraction expected in gamete production? _____
What could
one do to get closer to the expected ratio?
______________________________
___________________________________________________________________________
Do a larger
number of choices (the pooled data of the class) more closely approach what is
expected? ____
In a heterozygous
garden pea plant with one allele for red flower color and one allele for white
flower color, what fraction of the gametes should contain the red flower
allele? ________
Imagine that
we had a red flowered garden pea plant.
How could we determine if this plant were heterozygous or homozygous?_____________________________________________________________
___________________________________________________________________________
___________________________________________________________________________
In this exercise, we will work in
pairs. Each partner will represent one
hybrid F1 generation garden pea plant capable of producing gametes with
red flower (red) alleles and gametes with white flower (white) alleles. One partner will produce the eggs (by drawing
one bead from the cup) and the other partner will produce the sperm (by drawing
one bead from the cup). We will use the
cup with 10 red beads and 10 white beads, as before. Each time you & your partner draw, you
are simulating the union of one egg and one sperm, resulting in the formation
of one offspring. You and your partner
should each pick one bead from the cup as often as your instructor
indicates. Return the two beads to the
cup after each pick & record the red/red, red/white, and white/white
combinations in Table 2. Again, the
class results will be tabulated on the chalkboard. Since in the offspring it usually makes no
difference whether the one allele (white) comes from the male parent or form
the female parent, the two different red/white or white/red combinations are
recorded together.
Table 2.
"Genotypes" of F2 Generation
|
|
RED /RED
|
RED / WHITE |
WHITE / WHITE |
|
Your
Results |
|
|
|
|
Your
percentage |
|
|
|
|
Class
Totals |
|
|
|
In the procedure on the previous
page, you & your partner each have a 50% chance of drawing a red bead and a
50% chance of drawing a white bead. Every
time you both draw, there are four possible outcomes.
1. Partner 1 might draw a red bead. Partner 2 might draw a red bead. This would produce a homozygous dominant (or
red flowered) pea plant. The probability
of this happening can be obtained by multiplying the chance of the first event
occurring by the chance of the second event occurring. In other words…
(50% chance
of Partner 1 drawing red)(50% chance of Partner 2 drawing red) = 25% chance
of offspring inheriting red / red, or both alleles for the red flower
condition.
2. Partner 1 might draw a red bead. Partner 2 might draw a white bead. This would produce a heterozygous pea plant
that would be red flowered. The
probability of this happening is…
(50% chance
of Partner 1 drawing red)(50% chance of Partner 2 drawing white) = 25% chance
of offspring inheriting red / white, or one allele for the red flower condition
and one allele for white flower condition.
3. Partner 1 might draw a white bead. Partner 2 might draw a red bead. This would produce a heterozygous pea plant
that would be red flowered. The
probability of this happening is…
(50% chance
of Partner 1 drawing white)(50% chance of Partner 2 drawing red) = 25% chance
of offspring inheriting white / red, or one allele for the white flower
condition and one allele for red flower condition.
Note that
outcomes 2 & 3 can be combined because it doesn't matter whether the pea
plants inherit the allele for red flowers from the mother or the father, it still produces a heterozygous pea plant that
would be red flowered.
Thus, the
chance of producing a heterozygous offspring is 25% + 25% = 50% chance of
red / white
4. Partner 1 might draw a white bead. Partner 2 might draw a white bead. This would produce a homozygous white
flowered pea plant. The probability of
this happening is…
(50% chance
of Partner 1 drawing white)(50% chance of Partner 2 drawing white) = 25%
chance of offspring inheriting white / white, or both alleles for
the white flower condition.
Imagine
that we drew 200 times (representing the production of 200 F2 pea
plants). We could predict that 25% or 50
of the offspring would inherit red / red, 50% or 100 of the offspring would
inherit red / white, and 25% or 50 of the offspring would inherit white /
white. This would give us a genotypic
ratio of 50 red / red : 100 red / white : 50 white
/ white. We could simplify this as 1
homozygous red flowered : 2 heterozygous red flowered
: 1 homozygous white flowered.
In
the above situation, we know that red flowered is dominant to white
flowered. Thus, of the 200 offspring we
created, we would expect 150 to be red flowered and 50 to be white
flowered. This would give us a phenotypic
ratio of 150 red : 50 white. We could simplify this as 3 red : 1 white.
Using the genotype data from Table 2,
transfer the allele combination to Table 3 that represents the phenotypes of
our bunnies.
|
|
RED
FLOWERED (red / red or red / white) |
WHITE
FLOWERED (white /
white) |
|
Your
Results |
|
|
|
Class
Results |
|
|
What ratio of
red flowered to white flowered did you and your partner get? _______________
How does the
expected F2 flower color ratio compare to the data you obtained here?________
___________________________________________________________________________
How does the
pooled data of the class compare to your data for the expected F2
ratio? ______
___________________________________________________________________________
Is the dominant
or recessive trait more frequent in an F2 generation? ____________________
___________________________________________________________________________
1. Alleles control an inherited
characteristic & exist in individuals in pairs (You inherit one member of
the pair from your father & one member of the pair from your mother). The two alleles of a pair are the same in homozygous
individuals (e.g. the pure line white flower plants are homozygous for the
character state of white flower color.
Their allele pair is white/white).
The two alleles of the pair differ in heterozygous individuals
(e.g. the hybrid F1 generation inherited a red allele from one
parent and a white allele from the other parent. Thus their allele pair is red/white). The genotype is the allele combination
that produces a character state. The
genotype of the hybrid F1 generation is red/white. The phenotype
is the visible, physical trait. The
phenotype of the hybrid F1 generation is red flowers.
2. LAW OF DOMINANCE: Whenever the two alleles of a pair in a given
individual differ, only one, the dominant one will be expressed. In the hybrid F1 generation, the
plants inherited one red allele and one white allele. Yet, all of these plants appeared to be red
flowered. Thus red is the dominant
allele (the allele that indicates the appearance of heterozygotes). One allele is said to be dominant over
another if a heterozygous individual for that allele has the same appearance as
an individual homozygous for it. The
white flower allele is the recessive allele (an allele whose phenotype
effects are masked in heterozygotes by the presence
of a dominant allele).
3. LAW OF SEGREGATION OF ALLELES: When the gametes (egg & sperm) are formed
by an individual, only one member of each allele pair is included in a
gamete. Recall that gametes are haploid. When the hybrid F1 generation
plants produce gametes, each gamete will receive only one allele for flower
color. So, an egg (or a sperm) will have
an allele for red or an allele for white, but not both. When the egg unites with the sperm during fertilization,
the sperm will carry one allele for flower color, restoring the allele pair and
the diploid condition.
4. LAW OF INDEPENDENT ASSORTMENT: All of the possible kinds of gametes that can
be formed, will be formed in equal proportions. Alleles for different traits are inherited
independently of each other if they are located on different homologous gene
pairs.