Lab 5
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Lab 5: Mendelian Gene c
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Introduc on
In 1866, Gregor Mendel, an Austrian Monk, published a paper en tled “Experiments in plant hybridiza-
on”. It went largely unno ced un l 1900 when it was rediscovered and subsequently became the
basis for what we now refer to as Mendelian Gene cs.
Mendel was the Þrst to recognize:
Inherited characters are determined by speciÞc factors (now recognized these as genes).
These factors occur in pairs (genes).
When both alleles of a gene are the same they are said to be homozygous, while if they are di erent
they are said to be heterozygous. When gametes form, these factors segregate so that each gamete
contains only one allele for each gene. Remember, alleles reside on the chromosomes that are divid-
ing. These original observa ons lead to what we now refer to as The law of segrega on and the law of
independent assortment.
Figure 1: Law of Segrega on
Concepts to explore:
Gregor Mendel
Law of segrega on
Homozygous
Heterozygous
Independent assortment
Dominant vs. recessive
Incomplete dominance
Co-dominance
Genotype
Phenotype
Monohybrid cross
Dihybrid cross
Punne square
Lab 5: Mendelian Gene cs
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The law of segrega on states that during
meiosis, homologous (paired) chromo-
somes split (Figure 1). The law of inde-
pendent assortment states that during
meiosis, each homologous chromosom
has an equal chance of ending up in ei-
ther gamete, and alleles for individual
genes segregate with the chromosomes
on which they are located (Figure 2).
Using corn as an example (Figure 2):
The large chromosome has the gene for kernel color (Y = yellow, y = blue).
The small chromosome has the gene for kernel texture (S = smooth (green); s = wrinkled (red)).
When a dominant allele is present, that characteris c is expressed, regardless of the second allele. In
this case both the Yy and YY o spring will be yellow.
A recessive allele is only expressed when both alleles are recessive. In this case only the yy combina-
on is blue. The dominant allele is always represented by capital le ers, while the recessive is repre-
sented by lower case le ers.
Genotype refers to the combina on of alleles for a par cular trait. Phenotype refers to the appear-
ance of that combina on of alleles. In our example, the genotype of the diploid cell is Yy, Ss, while the
phenotype is Yellow and Smooth.
Figure 2: Law of Independent Assortment
Figure 3: Monohybrid Cross
Punne Square F1
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Alleles can exhibit incomplete dominance and co-dominance. An example of incomplete dominance is
the cross of two plants, one with red ßowers and one with white, whose o spring have pink ßowers.
In the case of codominance, the same cross would result in red and white striped ßowers.
If we know the genotype of two parents we can predict both the genotype and phenotype of their o –
spring using a Punne Square. A monohybrid cross is a cross between two parents (P), looking at a
single gene (Figure 3). In this example, both parents are pure breeding (homozygous); one for the yel-
low color and one for the blue color. This cross can be shown as a Punne Square (Figure 4), with each
parent (P) contribu ng a single gamete. The o spring (F1) are determined adding the gamete of eac
parent (P) (Row and Column). The F1 genotypes are all Y , y; with yellow phenotypes. The cross of the
(F1) genera on, known as the F2 genera on, is shown in Figure 5. The Punne square can also predict
the F1 for mul ple genes
Y Y
y Y y Y y
y Y y Y
Figure 4: Punnet square of a monohybrid cros
(F1)
Parent 1
Parent 2
Figure 5: Punnet square of a monohybrid cross
(F2)
Y y
Y Y Y Y y
y Y y y y
Parent
Parent 2
Y s Y S y S y s
Y s Y Y s s Y Y S s y Y S s y Y s s
Y S Y Y s S Y Y S S y Y S S y Y s S
y S Y y s S Y y SS y y S S y y s S
y s Y y s s Y y S s y y S s y y s s
Figure 6: Punnet square of a dihybrid cross (F1)
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Using our corn example, let’s look at two genes (color and texture), also known as a dihybrid cross. In
this example we use two parents that are heterozygous for both traits (Figure 6), using the gametes we
already iden Þed in (Figure 2).
The F2 phenotypes are:
Yellow & Smooth: 9
Yellow & Wrinkled: 3
Blue & Smooth: 3
Blue & Wrinkled: 1
Experiment 1: Punne Square Crosses
Procedure
low, and y = blue)
Y Y and Y y
Y Y and y y
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cross above.
Experiment 2: Monohybrid Crosses
Procedure
beads and yellow beads into the beaker and mix them around.
The beaker contains beads that are either yellow or blue.
These colors correspond to the following traits for kernel color:
Yellow (Y) vs. Blue (y)
This is the genotype of individual #1, record this informa on. Do not put these
beads back into the beaker.
Repeat this for individual #2. These two genotypes are your parents for the next
genera on. Set up a Punne square and determine the genotypes and pheno-
types for this cross.
Repeat this process 4 mes (5 total).
Materials
Blue beads
Yellow beads
100mL Beaker
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Ques ons
es? How much in the o spring?
in the o spring?
spring of your Þve crosses?
you expect the ra o of phenotypes to be in the o spring?
o spring? Why or why not?
what this means.