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In the late 1800's, Gregor Mendel was a monk in an Austrian monastery which
grew peas for food. They had yellow peas and green peas. One day Gregor Mendel
crossed the yellow peas with the green peas, but only yellow peas grew. Later
he allowed the new yellow peas to self-fertilize. (Remember plants have both
the pollen and the pistil, and they can fertilize themselves.) This time Mendel
got many yellow peas and a few green peas. This is a basic genetics experiment.
The phenotype is what you see or the outward appearance. In the previous
example, the yellow or the green color was the phenotype of the peas.
The genotype is the part on the DNA that you can't see or the genetic
part. In our example, we would say that the genotype of the yellow peas can be
YY or Yy, and the genotype of the green peas would be yy.
A dominant trait is one that can mask another trait. Yellow peas are
dominant because they mask the green peas in the first part of our experiment.
Dominant genes are represented by a capital letter; usually the first letter of
the dominant trait. So yellow peas can be YY or Yy.
The recessive trait is the trait that is masked. Green peas are masked
by the yellow color. The recessive genes are represented by a lower case
letter. We use the lower case of the dominant, yy, so that we know that green
is the recessive of the yellow.
A homozygous genotype is one that has two identical copies of the gene.
Examples of homozygous genotypes are YY or yy.
A heterozygous genotype is one that has two different copies of the
gene; for example Yy.
The P1 generation is the parental generation.
The F1 generation is the first filial generation or the children.
The F2 generation is the second filial generation or the grandchildren
(grandplants?).
When Mendel first experimented by crossing the yellow peas with the green peas
(P1), he got all yellow peas (F1). He then self-fertilized the new yellow peas
(F1) and got many yellow and a few green peas (F2).
So let's see if we can follow genetically what Mendel did. In the P1
generation, he crossed a homozygous dominant yellow pea plant with a homozygous
recessive green pea plant.
__________ x __________
What is the genotype of a homozygous dominant yellow pea plant?
a. YY
b. Yy
c. yy
d. yellow
Last updated: 12-30-01.