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Mendel’s Experiments:
For thousands of years farmers and herders have been selectively breeding their plants and
animals to produce more useful hybrids. It was a hit or miss process since the actual ways physical
characteristics were inherited were unknown. At this time, scientists believed that offspring’s traits came
from a blending of parents’ characteristics. For example, they thought that a dog with black fur and a dog
with white fur would produce a dog with grey fur.
Think
1. How did scientists think offspring’s traits were determined?
2. Do you think these scientists were correct? Why or why not? How would you test their belief?
Gregor Mendel was a 19th century monk who set out to explore the relationship between traits
and heredity scientifically. He chose to work with pea plants, which were relatively easy to use for his
experiments. He studied the pattern of traits passed from one generation of pea plants to the next. The
pea traits Mendel analyzed included seed color (yellow vs. green), and seed surface (wrinkled vs.
smooth). A summary of Mendel’s findings is shown below:
Stem Length
Pea Color
Seed Form
Pod color
Original cross
(Parental)
tall x short
yellow x green
round x wrinkled
green x yellow
Generation two
offspring (F1)
All tall
All yellow
All round
All green
Generation three
offspring (F2)
787: 277
6,022: 2,001
5,474: 1,850
428: 152
(tall: short)
(yellow: green)
(round: wrinkled)
(green: yellow)
Think
1. Carefully examine the data shown in Table 1: Mendel’s Results. What do you notice about the generation
two offspring? If you were Mendel, what would you conclude from these results? Explain
Mendel noticed that his results contradicted scientists’ belief that offsprings’ traits came from a
blending of parents’ characteristics. For example, he found that generation 2 flowers from the purple x
white cross were all purple (as opposed to light purple – a blend of purple and white). He also noticed
that one version of each trait seemed to take over (or dominate) the other version. For example,
generation 2 flowers from the purple x white cross were all purple (as opposed to half purple and half
white).
Think
1. Carefully examine the data shown in Table 1: Mendel’s Results. What do you notice about the
generation three offspring? If you were Mendel, what would you conclude from these results? Explain
As he looked at his data, Mendel also noticed an interesting relationship in the third generation.
He found that all of the traits in the third generation appeared in a 3:1 ratio. Based on this analysis, he
proposed that:

Each trait that appears in the second generation is the dominant version of the characteristic.
The trait that is “hidden” in the second generation is called recessive.

Every plant has two copies of the gene for each characteristic. These two copies are called alleles

Each pea plant receives only one allele for each characteristic from each parent and ends up
with two alleles of its own.
Think
1. Look at the information presented in Mendel’s Results. Based on these results, which allele for each
trait is a) dominant? b) recessive? What evidence supports your claim?
Through his observations, Mendel established three principles or laws of inheritance
According to the law of dominance, if a dominant allele is present, then the dominant trait will
show up in the physical appearance.
According to the law of segregation, for any particular trait, the pair of alleles of each parent
separate and only one allele passes from each parent on to an offspring. Which allele in a parent's pair of
alleles is inherited is a matter of chance.
According to the law of independent assortment, different pairs of alleles are passed to
offspring independently of each other. For example, a pea plant might inherit its mother’s purple flower
color and its father’s wrinkled seeds. This means that it is possible for a child to inherit a combination of
genes that neither parent had.
In your own words……
1. What does the law of dominance mean?
Think
1. What does the law of segregation mean?
2. What does the law of independent assortment mean?
Make a tree map for Mendel’s 3 laws
Use the below lines to write a short biography about Mendel and his
contribution to modern day genetics including his 3 laws. Think: Why is he called
the “Father of Genetics?”
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