1. CENTRO ESCOLAR SOLALTO
9th Pre-IB Biology
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NAME_____________________________________ Date: 08/18/10
Mendel's Laws of Heredity
Part 2 (Section 10.1)
Materials:
• Notebook
• “Biology – The Dynamics of Life” textbook
Instructions:
• Paste and complete today’s handout in your notebook
• Read pages 255 to 257 of your textbook and answer the following questions.
1. What did Mendel do with the tall and short pea plants he selected? What did
Mendel call the offspring of this cross?
He crossed them to produce new plants. Mendel referred to the offspring of
this cross as hybrids.
2. What are Mendel’s first experiments called? Why?
Mendel’s first experiments are called monohybrid crosses because mono means
“one” and the two parent plants differed from each other by a single trait.
3. Mendel selected a six-foot-tall pea plant and he cross-pollinated it with pollen from
a pea plant less than two feet tall. What did he discover after he planted the seeds
from this cross? What did he call these plants?
When Mendel planted the seeds from this cross, he found that all of the
offspring grew to be as tall as the taller parent. He called these plants first
generation.
4. Next Mendel allowed the tall plants from the first generation to self-pollinate.
After the seeds formed, he planted them. What did he discover this time? What
did he call this generation of plants?
Mendel found that three-fourths of the plants were as tall as the tall plants
in the parent and first generation. He also found that one-fourth of the
offspring were as short as the short plants in the parent generation. In other
words, tall and short plants occurred in a ratio of about three tall plants to
one short plant. He called these plants second generation plants.

2. 5. What nomenclature do geneticists use to refer to the parent generation? To the
first generation? To the second generation?
Parent generation = P1 / First generation = F1 / Second generation = F2
6. Mendel did similar monohybrid crosses with a total of seven pairs of traits. What
did he discover in every single case?
In every case, he found that one trait of a pair seemed to disappear in the F 1
generation; only to reappear unchanged in one-fourth of the F2 plants.
7. What are genes and where are they found?
Each organism has two factors that control each of its traits. These factors
are genes and they are located on chromosomes.
8. Genes exist in alternative forms. How do we call these different gene forms?
We call these different gene forms alleles.
9. How many alleles of the gene that determined its height did each of the pea plants
studied by Mendel have? Where is each one inherited from?
Each of Mendel’s pea plants had two alleles of the gene that determined its
height. One is inherited from the female parent and one from the male
parent.
10. In the F1 offspring of Mendel’s experiment with tall and short plants, only one trait
was observed. How do we call it?
The observed trait is called dominant.
11. In the F1 offspring of Mendel’s experiment with tall and short plants one trait
disappeared. How do we call it?
The trait that disappeared is called recessive.
12. When recording the results of crosses, it is customary to use the same letter for
different alleles of the same gene. How would you represent the allele for tallness
in pea plants? How would you represent the allele for shortness?
A capital letter is used for the dominant allele and a lowercase letter for the
recessive allele. The dominant is always written first. Thus, the allele for
tallness is written as T and the allele for shortness as t.

3. • Homework:
1. Please reproduce Figure 10.3 in your notebook.
The Dominant
Trait appears
in all F1
offspring
2.
Answer the Reading Check question found on page 257.
The rule of unit factors says that there are two different factors that
control each trait. The rule of dominance explains that one of the two factors
(dominant allele) governs a trait and the alternative form of the trait
(recessive) is only manifests itself if the dominant factor is not present.
3. Read the paragraph entitled “The law of segregation” found at the bottom of page
257 and summarize it in your notebook.
Mendel formulated the law of segregation to explain how the trait of
shortness reappeared in the F2 generation. He concluded that each tall plant in
the F1 generation carried one dominant allele for tallness and one unexpressed
recessive allele for shortness. Each plant received the allele for tallness from
its tall parent and the allele for shortness from its short parent in the P1
generation. The law of segregation states that every individual has two alleles
of each gene and when gametes are produced, each gamete receives one of
these alleles.