Download Gregor Mendel

Gregor Mendel
“The Father of
Genetics”
In 1865, a plump, middle-aged monk gave a lecture to a
small group of scientists. The monk explained his
experiments on how pea plants inherit their traits. Even
though his findings answered questions that had puzzled scientists for many
years, Gregor Mendel was ignored.
Mendel wanted to understand how traits were passed from parent to offspring.
His hypothesis was the same as everyone else’s at that time, that heredity was a
blending process, just like you mix two colors of paint to get a third color.
Gregor used pea plants because of their distinct traits.
Stem
Flower
Seed
shape
Seed color
Seed coat
color
Pod shape
Pod color
Tall
Along
stem
Top of
stem
Round
Yellow
Colored
Puffy
Green
Wrinkled
Green
White
Rippled
Yellow
Short
A plant that always produces plants having a certain trait is called pure for that
trait. For example, a pea plant that is pure for yellow seeds will always
produce plants with yellow seeds. Mendel started his experiments with the
pure forms of each trait. He crossed, or bred, the two pure forms of each
trait.
To test the height trait, Gregor used pollen from a pure tall plant. He
brushed the pollen onto a pure short plant. Then he planted the seeds that
were produced by those short plants. According to the theory, when he
“blended” a tall plant and a short plant, he should get a medium plant. Right?
Wrong! He only got tall plants!
This made the scientist more curious. What happened to the traits of the
short plant? He then used the pure short plants’ pollen and brushed the
pollen onto the pure tall plants. Same result; tall plants!
Knowing the old theory was now wrong, Mendel searched for answers. He
took the tall offspring that he had created and pollinated them with each
other. This time they were not all the same! After counting all the types of
plants, Gregor found that ¾ of the plants were tall, and ¼ of the plants were
short. Again…no medium plants. So what did he conclude?
First, he concluded that for each trait, there were two heredity factors. An
offspring received one factor for each trait from each parent. We now call
these factors, genes.
Second, Mendel concluded that for every pair of traits, one form was stronger
than the other. He called the traits that were stronger the dominant traits. He
called the weaker traits the recessive traits. So when he crossed a pure tall
plant with a pure short plant, the trait tall was dominant, because that is
showed up if both traits were available. So how did he end up with some
short plants?
The tall plants he created had genes from a tall plant and genes from a small
plant. They were no longer pure; they were hybrid. So, as they were crossed
with other hybrid plants, some of the plants received a short gene from both
plants. If there are two short genes, or recessive genes, then the plant will be
short!
These traits work the same way in animals and plants. We have dominant
and recessive traits. For example, black hair is dominant over blonde hair,
curly hair is dominant over straight hair, dimples are dominant over no
dimples, and being able to roll your tongue is also dominant!
Some years later, a scientist by the name of Reginald
Punnett was able to show Mendel’s work through a tool
called the Punnett square. In this, you are able to
predict the probability of what the offspring will look
like. To do this, you show the genes for one
parent’s trait on the top of the square, and the genes
for the second parent’s traits on a side of the square.
Then start matching up the genes, one from each
parent according to the row and column. Dominant
traits are shown with a capital letter, and recessive
traits are shown with a lowercase letter. Practice some of these on your own
and see how you do!