Segregation
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In
Mendel’s first experiment, he first established pure lines of tall and short
garden pea plants, by allowing them to self-pollinate for several
generations. He then cross-pollinated
these plants by first placing pollen from the short plants onto the stigmas
of flowers on tall plants. By
examining the first generation (F1), he was able to determine the
law of dominance. He knew that all
the plants in the F1 generation that were tall were hybrid for the
traits he was looking for.
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In Mendel’s first experiment, he first established pure lines of tall
and short garden pea plants, by allowing them to self-pollinate for several
generations. He then cross-pollinated
these plants by first placing pollen from the short plants onto the stigmas
of flowers on tall plants. By
examining the first generation (F1), he was able to determine the
law of dominance. He knew that all
the plants in the F1 generation that were tall were hybrid for the
traits he was looking for. Mendel
next began to cross tall, hybrid plants, which appeared no different from
pure tall plants, and view the results of that hybridization (the F2
generations). The result of this
experiment was that there were three times as many tall plants as there were
short one. This led Mendel to his Law
of Segregation: “When large numbers
of hybrids are crossed, the factors segregate and then recombine to produce
dominant and recessive offspring in a ration of 3:1.” Because Mendel was before the discovery of
DNA, he correctly attributed the transmission of traits to “factors,” which
he could not describe, but did know their function. Today, these factors would be known as alleles.
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It is
much easier to understand the results of Mendel with the use of a Punnett
square. In order to create such a
square to compare the possible results of one cross with respect to one
trait, the two alleles donated by one partner are placed above a box, while
the two alleles of the other are placed on the side.
In the
square above, the “T” represents the dominant tall gene, which creates a tall
plant, while the “t” represents the recessive tall gene, or the phenotype
short. In the case of this cross, the
phenotype would be all tall, as only tall plants would be created, while the
genotype equals 100& hybrid.
These are the result obtained by Mendel in his first experiment. When he crossed the hybrid plants to
create the F2 generation, he created a cross like the one below
and created the F2 generation.
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This time,
the phenotype of the offspring is 75% tall, 25% short and the genotype of the
offspring is 25% homozygous tall, 50% heterozygous tall, and 25% homozygous
short. Because the allele for a short
plant is recessive, it will only appear when it is the only gene
present.
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