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Genetics and Heredity
Transcript of Genetics and Heredity
The F1 Offspring In one experiment, Mendel crossed purebred tall plants with purebred short plants. Scientists today call these parent plants the parental generation, or P generation. The offspring from this cross are the first filial generation,or the F1 generation.
Dominate and Recessive Alleles
is the passing of physical characteristics from parents to offspring.
Each different form a characteristic, such as your height, hair color, or eye color is called a
How does probability help explain the results of genetic crosses?
What is meant by genotype and phenotype?
What is codominance?
Probability is a number that describes how likely it is that an event will occur.
The principles of probability predict what is likely to occur, not necessarily what will occur.
If you toss a coin, what is
the chance that it will land
heads up? Tails up?
of an event, written P(event), is the measure of how likely the event is to occur. A simple event has a single outcome.
Genetics and Heredity
A new organism begins to form when egg and sperm join in the process called
organism is the offspring of many generations that have the same trait. For example, purebred short pea plants always come from short parent plants.
organisms has two different alleles for a trait.
In the mid nineteenth century, a priest named
Gregor Mendel tended a garden in a central
European monastery. Mendel's experiments in
the garden would one day revolutionize the
study of heredity.
Who is Gregor Mendel?
EQ: What controls the
inheritance of traits in organisms?
Demonstrated that the
inheritance of certain traits
in pea plants follows particular
patterns. Now referred to as the laws of Mendelian inheritance.
Mendel experimented with thousands of pea plant to understand the process of heredity. Today, Mendel's discoveries form the foundation of genetics, the scientific study of heredity.
The F2 Offspring When the
plants in the F1 generation
were full-grown, Mendel allowed
them to self-pollinate.Surprisingly,
the plants in the F2 (second filial) generation were
a mix of tall and short plants. The shortness trait
had reappeared,even though none of
the F1 parent plants were short.
Mendel counted the tall and
short plants. About three
fourths of the plants were
tall, while one fourth
Today, scientists use the word gene for the factors that control a trait. Alleles are the different forms of a gene. The gene that controls stem height in peas, for example, has one allele for tall stems and one allele for short stems. Each pea plant inherits two alleles from its parents— one allele from the egg and the other from the sperm. A pea plant may inherit two alleles for tall stems, two alleles for short stems, or one of each.
Genes and Alleles
An organism’s traits are controlled
by the alleles it inherits from its parents.
Some alleles are dominant, while other
alleles are recessive. A dominant allele is
one whose trait always shows up in the
organism when the allele is present. A recessive allele, on the other hand, is hidden whenever
the dominant allele is present. A trait
controlled by a recessive allele will
only show up if the organism
does not have the dominant
Symbols for Alleles
Geneticists use letters to represent
alleles. A dominant allele is represented by
a capital letter. For example, the allele for tall stems is represented by T. A recessive allele is represented by the lowercase version of the letter. So, the allele for short stems would be represented by t. When a plant inherits two dominant alleles for tall stems, its alleles are written as TT. When a plant inherits two recessive alleles for short stems, its alleles are written as tt. When a plant inherits one allele
for tall stems and one allele for short
stems, its alleles are written
The probability of an event is a ratio that compares the number of favorable outcomes to the number of possible outcomes.
P(event) = no. of favorable events
-- -- -- -- -- -- -- -- -- -- -- --
no. of possible events
The Punnet Square
A tool that applies the laws of probability to genetics is a Punnet square. A Punnet square is a chart that shows all the possible combination of alleles that can result from a genetic cross.
In combined alleles in the boxes of the Punnet square represent all the possible combinations in the offspring. In a genetic cross, the allele that each parent will
pass on to its offspring is based on probability.
is an organism’s genetic makeup, or allele combinations.
When an organism has two identical alleles for a trait, the organism is said to be homozygous for that trait. If the organism has different alleles for a trait is is heterozygous for that trait.
is an organism’s physical appearance. How it looks, the visible traits.
For all of the traits in peas that Mendel studied, one allele was dominant while the other was recessive. This is not always the case. In an inheritance pattern called codominance, the alleles are neither dominant nor recessive.
As a result, both alleles are expressed in the offspring. Codominant alleles are written as capital letters with superscripts to show that neither is recessive.
Mendel reached several conclusions on the basis of his experimental results. He reasoned that individual factors, or sets of genetic “information,” 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.
Finally, one factor in a pair can mask, or hide, the other factor. The tallness factor, for example,
masked the shortness factor.
Dominant Traits Recessive Traits
Probability is a measure between 0 and 1:
impossible unlikely as likely as not likely certain
0 0.25 0.5 0.75 1
0% 25% 50% 75% 100%