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The Genetics of Inheritance
The Science of Heredity
• The scientific study of heredity is called
• GENETICS
The study of heredity started
with the work of Gregor Mendel
and his pea plant garden
The study of heredity started
with the work of Gregor Mendel
and his pea plant garden
Mendel noted that the size of pea
plants varied. He cross-bred
these pea plants to find some
surprising results.
Gregor Mendel
Pea Plant Experiment
• Gregor Mendel chose pea plants to conduct
genetic experiments because of their plant life
special properties. For example:
• When pollen fertilizes an egg cell, a seed for a
new plant is formed.
• Pea plants normally reproduce by selfpollination.
Gregor Mendel
Pea Plant Experiment
• What does it mean when pea plants are
described as being true-breeding (pure
breed)?
•If the plants are allowed to selfpollinate, they would produce
offspring identical to themselves.
Mendel’s cross between tall pea plants yielded all
tall pea plants. His cross between small pea plants
yielded all small pea plants.
X
=
X
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Mendels’ cross between tall pea plants and small pea
plants yielded all tall pea plants.
x
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Mendel then crossed these second generation tall pea
plants and ended up with 1 out 4 being small.
x
=
Gregor Mendel
Pea Plant Experiment
• Traits - Specific characteristics that vary from
one individual to another
• Hybrids-The offspring of crosses between
parents with different traits
• Genes - Chemical factors that determine traits
• Alleles - The different forms of a gene
Mendel’s work led him to the understanding
that traits such as plant height are carried in
pairs of information not by single sets of
information.
-Carrying the information are chromosomes.
-Chromosomes are made up of sections
called genes.
-Genes are made up of DNA
Why do we look the way we do?
• All organisms are made of cells.
• Inside of these cells, a chemical called
DNA contains the “instructions” that
control the way we look.
DNA
DNA: the instruction manual
• The segments in the DNA that carry the
“instructions” are called GENES.
• Every individual has two of every gene.
• There is more than
one version of
each gene. The
different versions
are called alleles.
Genetics
There are three basic kinds of genes:
– Dominant - A gene that is always
expressed and hides others
– Recessive - An organism with a
recessive allele for a particular form of a
trait will NOT always exhibit that form.
– Codominant - Genes that work
together to produce a third trait
Dominant Alleles
State the principle of dominance:
• The trait will always be expressed mask the
other gene.
– Dominant -Expressed regardless of what paired
with
– Dominant -Expressed with a capital letter (T)
Recessive Alleles
– Recessive Allele-A gene that is only
expressed when a dominant gene isn’t
present
– Recessive-Physically expressed when paired with
another recessive allele
– Recessive-Expressed with a lower case letter (t)
Codominant Allele
• Codominant - Genes that work together to
produce a third trait
– In your genetic smiley baby it was the ears and
nose color
• Red (RR)
• Orange (RY)
• Yellow (YY)
Incomplete Dominant Traits
Blood types:
Flower color:
In incomplete dominance, the dominant allele does
not cover up the expression of the recessive allele.
Genotype & Phenotype
•
The observable traits of an organism is called the phenotype.
# Students w/ free lobes
# Students w/ attached lobes
• The genotype is the combination of all of the different alleles
that make up an individual
Allele Types
Definitions
• Organisms that have two identical alleles of a
particular trait (TT or tt) Homozygous
• Organisms that have two different alleles of a trait
(Tt) Heterozygous
• Physical characteristic of an organism Phenotype
• Genetic makeup of an organism (Tt) Genotype
Allele Types
•
•
•
•
Homo - Same
Hetero - Opposite
Pheno – Physical
Geno - Genetic
Gene Expression
Is the following sentence true or false?
- Plants with the same phenotype always have
the same genotype.
• False
The likelihood that a particular event will occur
is called_______.
• Probability
Probability
Circle the letter of the probability that a single
coin flip will come up heads.
• a. 100 percent
• b. 75 percent
• c. 50 percent
• d. 25 percent
Probability
• Why can the principles of probability be used
to predict the outcomes of genetic crosses?
• The way in which the alleles segregate is
completely random, like a coin flip.
Genetics
Punnet Square - A tool we use for predicting
the traits of an offspring
– Letters are used as symbols to designate genes
– Capital letters are used for dominant genes
– Lower case letters are used for
recessive genes
– Genes always exist in pairs
Punnett Squares
• A diagram that is used to predict the
genotype and phenotype of the
offspring based on the genotype of the
parents.
• Chart of possible combinations
of the outcomes of alleles that
may result from genetic cross
p
It’s a probability
problem!
Pp
pp
p
P
Pp
Pp
p
pp
pp
½ purple pea plants; ½ white pea plants
Punnett Squares
• Male parent goes across the top of the square
• Female parent goes across the left side of
square
Practice
• Draw a Punnett square - 4 small squares in the
shape of a window. Write the possible gene(s) of
male parent across the top-Father is black fur
(dominant) homozygous
• and the gene(s) of the female parent that is blond
fur recessive homozygous along the left side of
the Punnett square.
• Complete a Punnett square to show the possible
gene combinations for the F1 offspring.
Punnett Square- practice
Complete a Punnett square to show the
possible gene combinations for the F2
offspring.
Probability and Segregation
• b. The F2 ratio of black fur to pink fur
produced in a cross between two hybrid Black
fur (Bb) is 3 black for every 1 pink.
• c. observe that about 3⁄4 of the F2 offspring
showed the dominant trait.
Probability
• Is the following sentence true or false?
• Probabilities predict the precise outcome of
an individual event.
• False
Punnett Squares
1. What is the probability that this couple will
have a child with brown eyes?
2. What is the probability that
this couple will have 3
children with brown eyes?
Bb
Bb
BB=
Bb=
bb=
More Punnett Squares
There are 10 guinea pigs in a litter. Five of the guinea pigs have long hair, and five
have short hair. Long hair is dominant and short hair is recessive. What are the
genotypes of the parents?
Genotype
for long hair:
Genotype
for short hair:
Applications of math in genetics
Selective breeding—The selection of
certain seeds or animals for
reproduction so that the next
generation has the same desirable
traits.