1. Science Explorer CH. 3
Generics: The Science of Heredity

2. Mendel’s Work
• Gregor Mendel- was a young priest, taught at
local high school and cared for monastery’s
garden.
– Wondered why the pea plants had different physical
characteristics.
• Traits- a characteristic that an organism can pass on to its
offspring through its genes.
• Heredity- the passing of traits from parents to offspring.
– His work formed the foundation of genetics, the
scientific study of heredity.

3. Mendel’s Peas
• Mendel spent over ten years experimenting
with the Pea plants.
• Made a wise decision picking the Pea plants.
• Many of the traits only exist in two forms.
– For example-
• plant stems are either short or tall.
• Produce large amounts of offspring in one generation.

4. Mendel’s Experiment
• Mendel started the experiment by using
purebred plants.
– Purebred- is a plant that always produce offspring
with the same form of a trait as the parent.
• Short produces short and tall produces tall.
• Used opposite forms of purebred plants.
– First experiment Mendel, bred a purebred short with
a purebred tall pea plant.
• He called the parent plants parental generation or P
generation.
• The offspring from this cross was called first filial generation,
or F1 generation.

6. Other Traits
• In addition to stem height, Mendel studied six
other traits in garden peas:
– Seed shape, Seed color, Seed coat color, Pod
shape, Pod color, and Flower position.
• Just like in the stem experiment, only one
form of the trait appeared in the F1
generation. However, in the F2 generation the
“lost” form of the trait always reappeared in
about one forth of the plants.

8. Dominant and Recessive Alleles
• Mendel reasoned that individual factors must
control the inheritance of traits in peas.
– The factors that control each trait exist in pairs.
– The female parent contributes one factor, while
the male parent contributes the other factor.
• Mendel reasoned that one factor in a pair can
mask, or hide, the other factor.
– The tallness factor, for example, masked the
shortness factor in the F1 generation.

9. • Genes- a segment of DNA on a chromosome
that codes foe a specific trait.
• Alleles- the different forms of a gene.
– Each pea plant inherits a combination of two
alleles from its parents- either two alleles for tall
stems, two alleles for short stems, or one of each.
• Individual alleles control the inheritance of
traits. Some alleles are dominant, while other
alleles are recessive.

10. • Dominant allele- is one whose trait always
shows up in the organism when the allele is
present.
• Recessive allele- is masked, or covered up,
whenever the dominant allele is present.
– A trait controlled by a recessive allele will only
show up if the organism does not have a
dominant allele.
– Only pea plants that inherits two recessive alleles
for short stems will be short.

11. Understanding Mendel’s Crosses
• When you mate a long stem and a short stem
pea plant together, you will get a long stem
offspring with the F1 generation. With the F2
generation you will get ¾ tall stem and ¼ will
be short stem pea plant.
• Hybrids- an organism that has two different
alleles for a trait; an organism that is
heterozygous for a particular trait.

12. Symbols in Genetics
• Geneticists today use standard short hand
method.
– The dominant allele will be represented by a
capital letter.
– The recessive allele is represented by a lower case
letter.
– When both alleles are represented such as a
hybrid there will be one capital letter and one
lower case letter.

13. • Mendel presented his findings in 1866. Most
of the scientist that read the paper said he
over simplified the process of heredity. Others
didn’t even bother reading his findings.
• It wasn’t until 34 years later that his findings
was considered valid. Many of Mendel’s
principles discovered still stand today. That is
why he is called the father of Genetics.

14. • Homework p.85 All