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Genetics
The study of how traits are
inherited through the interactions
of different types of genes.
Heredity
• The passing of traits from parent to offspring
• Genes on chromosomes generally control an
organism’s form and function
• The different forms of a trait that a gene may
have are called alleles
• Alleles are separated during the process called
Meiosis, which is a component of sexual
reproduction
• From one parent, the alleles may be the same or
different for a particular trait and thus the
offspring may receive either allele in combination
with another allele from the other parent
Mendel-The Father of Genetics
• Austrian monk who served as the
gardener for his monastery.
• Became curious about the connection
between the color of a flower and the type
of seed produced from crossbreeding.
• Traced one trait through several
generations of pea plants.
• Used mathematical probability to explain
the results
Mendel’s Experiments
• Mendel crossed plant with different expressions
of a trait and found that the new plants all looked
like one of the parents.
• He called these hybrids because the received
different genetic information (alleles) for a trait
form each parent.
• He hand pollinated purebred short plants with
pollen from purebred tall plants and got all tall
plants so he called the tall form dominant and
the short form recessive because it disappeared,
or was “covered up.”
Genotype and Phenotype
• Genotype, or genetic makeup is either purebred
or hybrid. (3 possible types)
• Phenotype is the way the an organism looks and
behaves as a result of its phenotype.(2 possible)
• Purebred organisms have either two dominant
alleles or two recessive alleles.
• Hybrid organisms have one dominant and one
recessive allele.
• Purebred and Hybrid organisms with the
dominant allele will exhibit the dominant trait.
• Only purebred recessive organisms will exhibit
the recessive trait.
Dominant vs. Recessive
• Many incorrectly assume that Dominant
means stronger or more common.
• Dominant means that trait appears when
that particular allele is present.
• The law of dominance explains that one
trait (the dominant) is expressed in
homozygous and heterozygous conditions.
• The recessive trait is expressed only in the
homozygous condition.
Homozygous and Heterozygous
• Most cells have two alleles for every trait
• If the alleles are the same, the organism is
considered homozygous.
• If the alleles are different, the organism is
considered heterozygous.
• Homozygous = purebred
• Heterozygous = hybrid
Probability and Predictions
• Probability is a branch of mathematics
used to predict the chance of something
happening.
• Different possible outcomes will each have
a percentage chance of occurring which is
its probability.
• Mendel worked with nearly 30,000 plants
over 8 years so the percentages of
different possible outcomes yielded
reliable patterns of probability.
Punnett Squares
• Punnett squares use
T=tall
letters as a code to
t=short
represent alleles and
show different possible
combinations and
T
outcomes.
• Uppercase letters
represent dominant
alleles and lowercase
t
show recessive alleles
T
t
TT
Tt
Tt
tt
Making a Punnett Square
• In a Punnett square for predicting one trait, the
letters representing the two alleles from one
parent are written along the top of the grid with
one letter per section.
• Those of the second parent are placed down the
side of the grid, one per section.
• Each square of the grid is filled in with one allele
from each parent.
• The combinations in each square represent the
genotypes of possible offspring that parents can
produce.
• Their probability is the percentage of the whole
of each genotype.
More Punnett Square examples
T=tall
t=short
T
T
T=tall
t=short
T
t
t
Tt
Tt
t
Tt
tt
t
Tt
Tt
t
Tt
tt
Principles of Heredity
• Traits are controlled by alleles on
chromosomes.
• An allele’s effect is dominant or recessive
• When a pair of chromosomes separates
during meiosis, the different alleles for a
trait move into separate sex cells