Download Genetics: The Science of Heredity

Genetics: The Science of Heredity
Table of Contents
Mendel’s Work
Probability and Heredity
The Cell and Inheritance
The DNA Connection
Genetics: The Science of Heredity
Heredity, Traits and Genetics
*Heredity is the passing
of traits from parent to
offspring
*A trait is a characteristic
of an organism, such as
hair color, eye color, etc.
*Genetics is the scientific
Trait Quiz
study of heredity.
Genetics: The Science of Heredity
The Father of Modern Genetics
*Gregor Mendel outlined the
basic laws of heredity.
*Mendel was an Austrian monk
born in 1822.
*Experimented with garden
pea plants
*Presented the first paper on
genetic research in 1866 titled,
“Experiments with Plant Hybrids”
Genetics: The Science of Heredity
Mendel’s Experimental Garden
Mendel chose garden pea plants for his
experiments. Why?
Genetics: The Science of Heredity
*Pea plants usually self-pollinate,
producing purebred plants.
*Mendel found a way to cross-pollinate
them so he could mix traits.
*Easy to grow large numbers of plants.
*Many different traits such as height,
seed color, seed shape, pod color, pod
shape, and flower color can be easily
studied.
Genetics: The Science of Heredity
wrinkled seeds
Side or end flowers
Genetics: The Science of Heredity - Mendel’s Work
How did he do it?
Gregor Mendel crossed pea plants that had different traits.
The illustrations show how he did this.
Details of Mendel’s First
Experiment
Mendel’s first step was to cross pure
tall plants with pure short plants.
What were his results?
X
The first generation of offspring were
all tall.
P generation
X
How can this be?
F1
generation
Genetics: The Science of Heredity
Mendel’s Explaination
In pea plants, tall traits overpower
short traits. Each plant, although it
looked tall, was hiding a factor for
short height.
Will the short trait reappear in the
next generation?
To find out, Mendel now crossed tall
plants of the first generation with each
other.
x
What were his results?
In the second generation, the offspring
were both tall and short. Tall
outnumbered short 3 : 1
X
F1 generation
F2
generation
Genetics: The Science of Heredity - Mendel’s Work
In all of Mendel’s crosses, 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 fourth of the plants.
Genetics: The Science of Heredity
Mendel’s Conclusion
Tall is a dominant factor for height
since it overpowers or hides the short
factor.
Short is a recessive factor for height
since it appears to disappear if the
tall factor is present.
We now call these “factors” alleles.
Genetics: The Science of Heredity - Mendel’s Work
Dominant and Recessive Alleles
Mendel studied several traits in pea plants.
Genetics: The Science of Heredity
Traits, Genes and Alleles
Sometimes the words "gene",
"allele" and "trait" seem to be used
to mean the exact same
thing. Although they all are closely
related, there is a difference between
genes, alleles, and traits.
Genetics: The Science of Heredity
A trait is a characteristic of an organism, such as
tall height.
A gene is a small section of DNA on a chromosome
that controls a trait. (Mendel didn’t know about
chromosomes).
The different forms a gene may come in are its
alleles (example—short or tall are the alleles for the
gene that controls the height of a plant).
Gene
Allele
Trait
Homologous Chromosomes
Tall
Genetics: The Science of Heredity
Mendel showed that organisms have two
alleles for every trait. (One that we get
from mom and one that we get from dad).
He also showed that these alleles can be
either dominant or recessive.
Capital letters are used to represent
dominant alleles and lower case letters
are used to represent recessive alleles.
Genetics: The Science of Heredity
Dominant traits will be seen even if there
is only one dominant allele in the pair.
TT = tall plants
Tt = tall plants
Recessive traits will only be seen when
both recessive alleles are present in the
pair.
tt = short plants
This explains the 3:1 ration of tall to
short plants observed by Mendel.
Genetics: The Science of Heredity
Heterozygous and Homozygous
An individual with two alleles for a trait
that are exactly the same (TT or tt) is
said to be homozygous or “pure” for
that trait.
An individual with two different alleles
for a trait (Tt) is said to be
heterozygous or “hybrid” for that trait.
Genetics: The Science of Heredity
Genotypes and Phenotypes
Genotype is an organism’s genetic
make-up: it includes hidden traits and is
represented by letters:
Examples: TT Tt or tt
Phenotype is the physical trait that you
see in an organism: it is a description
Examples: tall plants or short plants
Genetics: The Science of Heredity - Probability and Heredity
Phenotypes and Genotypes
Genetics: The Science of Heredity
Remember, Mendel did not know
how factors were passed from parent
to offspring.
In 1903, Walter Sutton discovered
that Mendel’s “factors” were actually
genes carried on chromosomes.
These genes are passed to the
offspring in the sperm and egg cells
produced during meiosis.
Genetics: The Science of Heredity
Sutton discovered this while studying
the development of sperm and egg
cells in grasshoppers.
This is now known as the
chromosome theory of inheritance.
Genetics: The Science of Heredity - The Cell and Inheritance
Chromosomes
Click the Video button to watch a movie about chromosomes.
Genetics: The Science of Heredity - The Cell and Inheritance
A Lineup of Genes
Chromosomes are made up
of many genes joined
together like beads on a
string.
The chromosomes in a pair
may have different alleles
for some genes and the
same allele for others.
Genetics: The Science of Heredity
When a pair of chromosomes
separate during meiosis, the different
alleles for a trait move into separate
sex cells.
Ee = parent genotype
ee = parent genotype
EE = parent genotype
Genetics: The Science of Heredity
The genotype of the offspring will be
determined by which sperm cell fertilizes
which egg cell.
E
e
Ee
Ee = genotype of
offspring
Genetics: The Science of Heredity
Punnett Squares
Punnett squares can be used to
show the possible ways that genes
can combine at fertilization
Genetics: The Science of Heredity - The Cell and Inheritance
A Punnett square is actually a way to
show the events that occur at
meiosis.
Genetics: The Science of Heredity
Monohybrid Cross
A Punnett Square that shows only
one trait is known as a Monohybrid
Cross.
Genetics: The Science of Heredity
Example: TT
x tt
T
T
t
Tt
Tt
t
Tt
Tt
The only possible
combination for the
offspring of these two
parents is one tall
allele (T) and one
short allele (t). All
offspring will be hybrid
(heterozygous) tall.
Genotype = Tt
Phenotype = tall
Genetics: The Science of Heredity
Let’s look at another example:
There are 3 combinations
Tt x Tt
T
t
T
t
TT
Tt
Tt
tt
for the offspring of these
two parents. They occur
in the following ratios:
Pure (homozygous) tall
TT = 25%
Hybrid (heterozygous) tall
Tt = 50%
Pure (homozygous) short
tt = 25%
Genetics: The Science of Heredity
What do you actually see?
75% tall plants and 25 % short plants.
Genetics: The Science of Heredity
LET’S PRACTICE!!
Monohybrid
Genetics
Problems
Genetics: The Science of Heredity
Mendel’s Law of Independent Assortment
Mendel discovered
that it was possible
to study more
than one trait at a
time. Example:
There are 4 possible outcomes for
the gametes. One letter does not
affect the selection of the other.
Genetics: The Science of Heredity
Dihybrid Crosses show two traits:
Example: Rr Yy
x
Rr Yy
R = round seeds
Y = yellow seeds
r = wrinkled seeds
y = green seeds
RY Ry
rY
ry
RY
Ry
RRYY RRYy
RrYY
RrYy
RRYy RRyy
RrYy
Rryy
rY
ry
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
9 = round/ yellow
3 = round/ green
3 =wrinkled/ yellow
1 = wrinkled/ green
Genetics: The Science of Heredity
Codominance
Codominance occurs when neither
allele in the pair is dominant over the
other.
Example:
Pure Red Flowers X Pure White Flowers
Result:
All Pink Flowers
Genetics: The Science of Heredity
X
RR
RW
WW
RW
RW
RW
Genetics: The Science of Heredity
Punnett Squares for codominance
do not use lower case letters, since
neither trait is dominant.
Example: RR X WW
R
R
W
RW
RW
W
RW
RW
All offspring are
PINK
Genetics: The Science of Heredity - Probability and Heredity
Codominance
Another example of codominance shows each
trait appearing together rather than a blending.
Genetics: The Science of Heredity
Mutations
Without correctly coded proteins, an
organism can’t grow, repair, or maintain
itself.
Sometimes mistakes happen in the
genetic code.
Any permanent change in a gene or
chromosome of a cell is called a
mutation.
Genetics: The Science of Heredity
Mutations can:
*be harmless, beneficial, or harmful
*happen randomly or be caused by an
outside factor (x-rays, radiation,
chemicals etc.)
*add variety to a species over time
Genetics: The Science of Heredity
Three Types of Mutations
*Substitution—one base pair is
substituted for another.
*Deletion—one base pair is omitted
*Addition—one base pair is added
Genetics: The Science of Heredity - The DNA Connection
Mutations
Mutations can cause a cell to produce an incorrect protein
during protein synthesis. As a result, the organism’s trait, or
phenotype, may be different from what it normally would
have been.
Genetics: The Science of Heredity - Mendel’s Work
Data Sharing Lab
Click the PHSchool.com button for an activity about sharing
data for the Skills Lab Take a Class Survey.
Genetics: The Science of Heredity - Probability and Heredity
Links on Probability and Genetics
Click the SciLinks button for links on probability and genetics.
Genetics: The Science of Heredity - The Cell and Inheritance
Links on Meiosis
Click the SciLinks button for links on meiosis.
Genetics: The Science of Heredity - The DNA Connection
Protein Synthesis Activity
Click the Active Art button to open a browser window and
access Active Art about protein synthesis.
Genetics: The Science of Heredity - The DNA Connection
Protein Synthesis
Click the Video button to watch a movie
about protein synthesis.