Chapter 10, Genetics
History of Genetics
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Genetics is the science of heredity. It is the branch of
science dealing with the way traits are inherited from
parents to offspring.
Gregor Mendel is often called the father of genetics,
because he was the first person to discover how traits
are passed from parents to offspring.
In the late 1800's, Mendel, an Austrian monk and plant
breeder, conducted experiments which led to the
discovery of genetics.
Gregor Mendel
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Mendel chose pea
plants for his studies.
The pea plants Mendel
studied were TrueBreeding, which
means they can
consistently produce
offspring with only
one form of a trait.
For example color;
yellow or green.
Usefulness of Pea Plants
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Another useful
characteristic of pea
plants is their ability to
self-fertilize.
Self-Fertilization
occurs when a male
gamete within a
flower combines with
a female gamete on
the same flower.
Self-Fertilization and Cross-Pollination
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The pea plants that Mendel
grew could also be CrossPollinated.
By cross pollinating,
Mendel was able to control
which plants bred by
transferring the male
gamete from one plant to
the female gamete of a
different plant.
The Experiments
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In every one of his experiments, Mendel was very
careful and detailed in his data collection, how he
controlled his experiments and how he prevented
certain plants from self-fertilizing.
The first generation of pea plants that were truebreeding for a particular trait were referred to as the
Parent Generation or P generation.
The resulting offspring produced from crossing
different varieties of plants from the P generation
produced the First Filial or F1 generation.
The Experiments
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After Mendel obtained his plants from the F1
generation, he allowed the offspring from the F1
generation to self-fertilize.
The offspring that resulted from self-fertilizing the F1
generation were referred to by Mendel as the Second
Filial or F2 generation.
Mendel conducted seven separate experiments. Each
time he tested separate traits independently.
What were some of the truebreeding traits Mendel tested?
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Flower color
Flower position
Seed color
Seed shape
Pod shape
Pod color
Stem length
What were the results of his
experiments?
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In every one of his crosses from the P generation, the
F1 generation always yielded only one trait. The other
trait always disappeared in the F1 generation.
What about the F2 generation?
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When the F1
generation
was allowed to
self-fertilize,
the trait that
disappeared in
the F1,
reappeared in
the F2 1 out of
4 times.
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The offspring consistently
produced a 3:1 ratio.
So what did Mendel conclude about his
experiments with pea plants?
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Mendel observed that there always seemed to
be two different ways that a trait can be
expressed.
He predicted that some unknown factor
caused these varieties to occur.
He called these unknown factors Alleles.
An allele can be defined as an alternative form
of a single gene.
Dominant and Recessive Alleles
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Mendel called the
alleles that showed up
in the F1 generation, the
Dominant Alleles.
Mendel called the
alleles that were hidden
in the F1 generation the
Recessive Alleles.
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Mendel determined that
dominant alleles masked
recessive alleles.
What is the result of the allele combinations?
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Homozygous Dominant (Pure)-both alleles found on
homologous chromosomes are dominant. The result
is the dominant trait is expressed.
Homozygous Recessive (Pure)-both alleles found on
homologous chromosomes are recessive. The result
is the recessive trait is expressed.
Heterozygous (Hybrid)-both the dominant and
recessive alleles are present in the genes for that trait.
The dominant trait usually masks the recessive trait.
How are alleles represented?
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Usually the first letter of
the dominant gene is
used to express the
allele when written.
If the allele is dominant,
the letter is capitalized.
Example (G-green)
If the allele is recessive,
the letter is lowercase.
Example (g-yellow)
Genotype vs. Phenotype
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When discussing genetics, the terms genotype and
phenotype are frequently used.
An organism's Genotype is the combination of alleles
for a particular trait (homozygous dominant (GG),
homozygous recessive (gg), or heterozygous (Gg)).
An organism's Phenotype is the trait that is being
expressed as a result of the genotype.
For example, a person's genotype for eye color might
be BB, Bb, or bb. The person's phenotype would be
either brown, hazel, or blue eyes.
Genotypic and Phenotypic Ratios
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Notice the ratio between
genotype and phenotype
are not always the same.
In this example, the
genotypic ratio is 1:2:1.
The phenotypic ratio is 3:1.
Genotype vs. Phenotype
Genotype vs. Phenotype
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In reality, eye color is
much more complex. We
will discuss this genetic
pattern in more detail in
chapter 11.
Mendel's law of segregation
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Mendel predicted that something must occur when
gametes are formed that causes the alleles to separate.
We now know that during meiosis, the homologous
pairs (alleles) separate (during anaphase I).
Amazingly, Mendel's predictions were right, without
even knowing what meiosis was. Mendel called this
principle the Law of Segregation.
Mendel's laws of segregation and independent
assortment
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Mendel also
predicted that
alleles pair up
randomly
during the
formation of
gametes.
He called this
the Law of
Independent
Assortment.
What is a Monohybrid Cross?
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A Monohybrid Cross is when fertilization occurs
between two individuals with heterozygous
genotypes when only one trait is being crossed.
For example, Pp x Pp or Gg x Gg.
Organisms with a heterozygous genotype are
referred to as Hybrids.
A normal Monohybrid cross results in a 3:1
Phenotypic ratio in the offspring.
What is a Dihybrid Cross?
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A Dihybrid Cross is when fertilization occurs between
two individuals with heterozygous genotypes when
two traits are being crossed.
For example, RrYy x RrYy
A typical dihybrid cross usually results in a 9:3:3:1
phenotypic ratio in the offspring.
In this case, 9/16 of the offspring are dominant for
both traits, 3/16 of the offspring are dominant for
one trait and recessive for the other, and 1/16 of the
offspring are recessive for both traits.
Punnett Squares and Probability
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In the early 1900's, a doctor by the name of Reginald
Punnett developed a method for studying genetics
by using diagrams called punnett squares.
Punnett squares can be used to predict the
probability that certain traits will be expressed in
offspring when parental genotypes are known.
Punnett squares can also be used to predict
genotypic and phenotypic ratios of offspring, and
they to determine the genotypes of the parents by
analyzing the phenotypes of the offspring.
Monohybrid Crosses with Punnett Squares
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Mendel's laws of segregation and independent assortment can
be demonstrated in a simple punnett square.
Probability can help predict the outcome of which alleles will
combine by random chance.
Monohybrid Cross (Test Cross)
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Analyzing the
phenotypic
ratios of the
offspring can
also help
determine the
genotypes of
the parents.
This is known as
a Test Cross.
Dihybrid Crosses with Punnett Squares
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When
analyzing
two traits at
the same
time, we
see a
9:3:3:1
Phenotypic
Ratio
This is
called a
Dihybrid
Cross.
Genetic Recombination
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The new combination of genes produced by crossing
over and independent assortment is called Genetic
Recombination.
The total number of recombinations of genes due to
just independent assortment alone (not counting
crossing over) can be calculated using the formula 2n,
where n represents the number of chromosome pairs.
In humans that number is 223 (possible # of male
gametes) x 223 (possible # of female gametes) = 7.04 x
1013 or roughly 70 trillion.
What is Polyploidy?
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Polyploidy is the occurrence of one or more extra sets
of chromosomes.
A triploid organism, like a triploid trout, is represented
as 3n. The advantage would be a sterile organism that
would grow larger and faster because less energy is
being used to produce gametes.
Polyploidy occurs in some animals but it is always
lethal in humans.
About a third of all flowering plants are polyploid.
Wheat and oats are 6n and sugar cane is 8n. Polyploid
plants often have increased size and health.