BACKGROUND:
Genetics is the study of heredity or the passing of traits
from parents to offspring. Offspring can inherit dominant traits or recessive
traits. A dominant trait is one that prevents another trait from appearing. A
recessive trait is one that does not appear when a dominant trait is present.
A pure trait (homozygous) is one that is made up of all dominant traits or all
recessive traits. A hybrid trait (heterozygous) is a trait that is made up of
a combination of dominant and recessive traits.
A gene is the unit of inheritance which is passed from parents
to offspring. Genes occur in pairs in chromosomes inside the nucleus of a
cell. There are dominant genes for dominant traits and recessive genes for
recessive traits. Dominant genes mask recessive genes when paired. Gregor
Mendel's experiments on hybridizing garden peas was the first recorded
experiment on plant breeding. Mendel successfully studied the inheritance of
unit characters. He also kept accurate records of how the characters
reappeared in the offspring of selected parents. Mendel was also the first to
control pollination techniques. These procedures are understood today, but in
1865 when Mendel's work was done, this was revolutionary. Mendel's work laid
unnoticed for 35 years until European plant breeders rediscovered his work.
Mendel selected garden peas for his experiment. He took pollen
from a dwarf growing pea and dusted it on a tall growing variety. The seeds
resulting from this cross pollination were collected and planted the following
season. All the plants that grew from these seeds were tall. Mendel reversed
the situation and dusted the dwarf peas with pollen from the tall variety, and
this resulted in offspring that were all tall.
Mendel then allowed the tall plants to self pollinate and
found that 3/4 were tall plants and 1/4 were dwarf. From this result Mendel
concluded that the expression of a given character was dominant, while the
other character was recessive.
PROCEDURE:
- Make an overhead of "Mendel's Garden Peas" to
illustrate Mendel's result. It is important to go over this graph carefully
with students because they will need to understand this for the lab. Capitol T
is the dominant and small t is recessive. The dominant character will make the
plants tall, while only two recessives together will form a dwarf.
- After students understand how to use the grid, go to the
following web site and have the students breed their own hybrid pea with this
interactive pea experiment.
http://www.sonic.net/~nbs/projects/anthro201/exper/
