Genetics: The Science of Heredity Chapter 3

Genetics: The Science of Heredity
Chapter 3
Section 1: Gregor Mendel
• A young priest from Europe, became a ____________ and cared for hundreds of pea
• Became curious about why some of the plants had different ____________
characteristics, or ____________
• He noticed that some plants were ____________ and some were ____________
• He also noticed some had ____________ seeds while some had ____________
Mendel observed that some pea plants had different ____________ that their parents
The ____________ of traits from parents to offspring is called ____________
The study of heredity is ____________
Mendel’s Peas
• Mendel chose pea plants to study because many of their traits exist in only
____________ forms: for example, height is either tall or short, not medium
• Mendel developed a way to cross-pollinate, or “____________” pea plants
• He removed pollen from a flower on one plant and ____________ it on a flower of a
second plant
Mendel’s Experiments
• To study ____________ of traits, Mendel decided to cross plants with ____________
forms of a trait…
• For example, he crossed tall plants with short plants to see what the ____________
would be
• He always started his experiment with ____________ plants
• A purebred plant is one that always produces ____________ with the same form of a
____________ as the ____________ (purebred short peas always produce short
First Experiment
• In his first experiment, Mendel crossed ____________ tall plants with purebred short
plants (“P generation”)
• He called the offspring of the “____________” the first filial, or the “F1 generation”
(Filial means ____________ in Latin)
• To his surprise, all of the F1 generation plants were tall even though one of the
parents were short
Second Experiment
Mendel let the F1 generation plants ____________
These offspring were known as the ____________
Mendel was surprised to see the ____________ were both ____________
This occurred even though all of the F1 parents were____________!
Mendel discovered about ____ of the plants were tall and ____ were short
Other Traits
Mendel also did experiments to check for:
Seed____________, seed____________, seed ____________ color,
pod____________, pod____________, and flower ____________
Dominant and Recessive Alleles
Factors that control each trait exist in ____________ …one from the ____________
parent and one from the ____________
One factor in a pair can ____________ the other factor
In the F1 generation, the ____________ factor hid the short factor
Today, factors that control traits are called ____________
Different forms of genes are called ____________
In Mendel’s work, each pea plant inherits a ____________ of two alleles from its
– Either:
• Two alleles for ____________ stems
• Two alleles for ____________ stems
• One of each
• Individual alleles control the ____________ of traits
• Some alleles are ____________
– A trait that always shows up in the organism when the allele is present
• Some alleles are ____________
– These are____________, or covered up, when the dominant allele is present
– A trait controlled by a ____________ allele will only show up if the organism does
not have the dominant trait
Understanding Mendel’s Crosses
• Here is how Mendel’s results work:
• The P generation tall plants (____________) had two alleles for tall stems
• The purebred short plants had two ____________ for short stems
• The ____________ received one tall and one short allele
• The F1 plants are called ____________ (they have different alleles for the trait)
• All the F1 plants are tall because the ____________ allele (tall) covers the recessive
allele (short)
• When the F1 plants (hybrids) were crossed with each other, some inherited two
____________ alleles (these became tall plants)
• Some inherited one dominant and one ____________ allele (these were tall)
• Some inherited two ____________ alleles (these were short!)
Symbols in Genetics
____________ are used to represent traits in today’s world
A dominant allele is represented by a ____________ letter (T)
A recessive allele is represented by a ____________ letter (t)
TT would mean two ____________ alleles
tt would mean two ____________ alleles
Tt would mean one ____________ and one ____________ allele
Mendel’s Contribution
• In 1900, three scientists rediscovered Mendel’s work
• Many of Mendel’s discoveries are still used today
• Because of his important work, Gregor Mendel is known as the “________________”
Section 2: Probability
Probability is the ____________ that a particular event will occur
If you flip a coin… the probability of landing on ____________ is 50%, and the
probability of landing on ____________ is 50%
Principles of Probability
• There are three ways to express probability
• As a____________: 1 to 2
• As a____________: 50%
• As a ____________: ½
• The laws of probability predict what ____________ occur.
• If you flip a coin 10 times you ____________ get 5 heads and 5 tails, or you
____________ get 1 head and 9 tails…
Mendel and Probability
• Remember…when Mendel crossed two ____________ plants (Tt), ¾ were tall and
¼ were short
• Mendel stated that the ____________ of producing a tall plant was 3 in 4, and the
probability of producing a short plant was 1 in 4
• Mendel was the first scientist to recognize that the principle of ____________ can be
used to predict the results of ____________ crosses
Punnett Squares
A ________________________ is a chart that shows possible combinations of alleles
that can result from a genetic cross
They are also used to determine the probability of a particular ____________
TT = 25%
Tt = 50%
tt = 25%
An organisms phenotype is its ____________ appearance
Phenotypes are ____________ traits
An example:
Pea plants can have one of two different phenotypes for stem height: ____________
or ____________
These are physical characteristics of something!
A genotype is the genetic makeup or the ____________ combinations
TT, Tt, tt
An organism that has two ____________ alleles for a trait is ____________ (TT, tt)
An organism with ____________ alleles for a trait is ____________ (Tt)
Section 3: Chromosomes and Inheritance
In humans, each body cell (skin, blood, liver, etc.) has ____________ (or 23 pairs)
Sex cells (sperm and egg) contain ____________
Walter Sutton, an American geneticist, discovered this
He concluded that genes are carried from parents to their offspring on____________.
This is known as the “Chromosome Theory of ____________”
Meiosis is the process by which the number of chromosomes is ____________ by
half to form sex cells (sperm and egg)
During meiosis the chromosome pairs separate and are distributed to two
____________ cells.
The sex cells that are created have only ____________ as many chromosomes as
the other cells in the organism.
In body cells of humans, there are 23 pairs (46 total) chromosomes
Chromosomes are made of many ____________ joined together like beads on a
Each gene controls a particular ____________
Section 4: DNA Connection
• The main function of genes is to control the production of ____________ in cells
• Proteins help to determine the____________, ____________, and other traits
• ____________ is a major component in chromosomes
• A = Adenine
• T = Thymine
• G = Guanine
• C = Cytosine
• Remember, these four bases form the “____________” of the DNA ladder
• Any ____________ that occurs in a gene or chromosome is called a ____________
• Mutations can cause a cell to produce an ____________ protein during protein
• If this happens, an organism’s phenotype (____________) will be different from what
it should have been
• If a mutation happens in a ____________ cell, only that cell is affected
• If a mutation happens in a ____________ cell, offspring can be affected
Effects of Mutations
Some mutations can be harmful
– These mutations can ____________ an organism’s chance of survival
Some mutations can be helpful
– These mutations can ____________ an organism’s chance of survival
Some can be neither