Science Explorer CH. 3
Generics: The Science of Heredity

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.

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 examples-
• plant stems are either short or tall.
• Produce large amounts of offspring in one generation.

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.

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.

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.

• 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.

• 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.

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.

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.

• 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.

• Homework p.85 All

Probability and Genetics
• Principles of Probability
– Probability- is the likelihood that a particular event
will occur.
– The laws of probability predict what is likely to occur,
not necessarily what will occur.
– However, the more tosses you make, the closer your
actual results will be to the results predicted by
probability.
– When you toss a coin more than once, the results of
one toss do not affect the results of the next toss.

Mendel and Probability
• Mendel realized that he could use probability
to predict the type of pea plants that will be
produced when he crossed certain types of
plants.
– When Mendel crossed two hybrid pea plants
together, he found that ¾ of the F1 generation
would be tall stem and ¼ of F1 will be short stem.
• Mendel was the first scientist to recognize
that the principles of probability can be used
to predict the results of genetic crosses.

Punnett Squares
• Punnett Squares-is a chart that shows all the
possible combinations of alleles that can
result from a genetic cross.
• Geneticists use Punnett squares to show all
the possible outcomes of a genetic cross and
to determine the probability of a particular
outcome.

Using a Punnett Square
• Can use the Punnett to calculate the
probability that offspring with a certain
combination of alleles will result.
• When you cross two hybrids together the
punnett square will result in a 3:1 ratio.

Phenotypes and Genotypes
• Phenotype- is its physical appearance, or its
visible traits.
– Pea plants can have one of two different
phenotypes for stem height- short or tall.
• Genotypes- is its genetic makeup, or allele
combinations.
– Homozygous- an organism that has two identical
alleles for a trait.
– Heterozygous- an organism that has two different
alleles for a trait.

Codominance
• For all of the traits that Mendel studied, one
allele was dominant while the other was
recessive. This is not always the case. For
some alleles, an inheritance pattern called
codominance exists.
• Codominance- the alleles are neither
dominant nor recessive.
– All the codominant alleles are written as capital
letters with superscripts- FB.

Chromosomes and Inheritance
• Chromosome Theory of Inheritance- genes
are carried from parents to their offspring on
chromosomes.
• Genes are located on chromosomes.
• Grasshoppers have 24 chromosomes.
– The fertilized egg gets 12 chromosomes from the
female and 12 from the male parent.

Meiosis
• Meiosis- is the process by which the number of
chromosomes is reduced by half to form sex cells-
sperm and egg.
• During meiosis, the chromosome pairs separate and
are distributed to two different cells. The resulting sex
cells have only half as many chromosomes as the other
cells in the organism.
• When sex cells combine to produce offspring, each sex
cell will contribute half the normal number of
chromosomes. Thus, the offspring gets the normal
number of chromosomes- half from each parent.

Chromosomes
• Different animals have different numbers of
chromosomes.
– Humans- have 23 pairs or 46 chromosomes.
– Dogs- have 39 pairs or 78 chromosomes.
– Silkworms- have 28 pairs or 56 chromosomes.

Genetic Code
• Today scientists know that the main function
of genes is to control the production of
proteins in the organism’s cells. Proteins help
determine the size, shape, and many other
traits of an organism.
• Amino Acids- are the building blocks of
proteins.

How Cells Make Proteins
• Protein Synthesis- is the production of
proteins.
• During protein syntheses, the cell uses
information from a gene on a chromosome to
produce a specific protein.
– Takes place on the ribosomes in the cytoplasm of
the cell.

The Role of RNA
• Before protein synthesis can take place, a
“messenger” must first carry the genetic code
from the DNA inside the nucleus into the
cytoplasm. This genetic messenger is called
ribonucleic acid or RNA.
• RNA molecule almost always looks like only
one side, or strand, of the ladder.

• Messenger RNA- copies the coded message
from the DNA in the nucleus, and carries the
message into the cytoplasm.
• Transfer RNA- carries amino acids and adds
them to the growing protein.

Translating the Code
• The first step is for a DNA molecule to “unzip”
between its base pairs. Then one of the
strands of DNA directs the production of a
strand of messenger RNA. To form the RNA
strand, RNA bases pair up with the DNA bases.
Instead of thymine, however, uracil pairs with
adenine. The messenger RNA then leaves the
nucleus and attaches to a ribosome in the
cytoplasm.

Mutations
• Mutations- is any change in a gene or
chromosome.
• Organism’s traits, or phenotype, will be
different from what it normally would have
been.

Types of Mutations
• Some mutations are the result of small changes in an
organism’s hereditary material, such as the substitution
of a single base for another.
– This can change hair color.
• Other mutations may occur when chromosomes don’t
separate correctly during meiosis.
– Genetic disorders
• If a mutation occurs in a body cell, such as a skin cell,
the mutation will affect only the cell that carries it. If,
however, a mutation occurs in a sex cell, the mutation
can be passed on to an offspring and affect the
offspring’s phenotype.

The Effects of Mutations
• Some of the changes brought about by
mutations are harmful to an organism. Other
mutations, however, are helpful, and still
others are neither harmful or helpful.
– Mutation is harmful to an organism if it reduces
the organism’s chance for survival and
reproduction.
– Whether a mutation is harmful or not depends
partly on the organism’s environment.

• Homework
• Page 100 1-4
• Page 106 1-4