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Patterns of Inheritance
Chapter 9
Genetics
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The science of heredity.
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A distinct genetic makeup results in a distinct
set of physical and behavioral characteristics.
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The DNA you get from your parents determine
your physical characteristics.
History of Genetics
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Hippocrates (ancient Greek physician) proposed an
explanation called pangenesis.
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Particles called “pangenes” travel from each part of the
organism’s body to the egg or sperm and are then passed
to the next generation.
Also thought that changes that occur during an organism’s
life are passed on in this way.
Aristotle (384 B.C.) rejected this idea as simplistic,
saying that what is inherited is the potential to
produce body features rather than particles of the
feature themselves.
History of Genetics
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Biologists in the 19th century observed
inheritance patterns in plants and concluded
that offspring inherit traits from both parents.
The favored explanation for inheritance then
became the “blending” hypothesis.
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This is the idea that the hereditary materials
contributed by the male and female parents mix in
forming the offspring.
History of Genetics – Gregor Mendel
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Modern genetics began in the 1860’s when a
monk named Gregor Mendel experimented
with breeding garden peas.
With a history in mathematics, his research
implemented a great deal of statistics.
He stressed that the heritable factors (genes)
retain their individuality generation after
generation (no blending).
History of Genetics – Gregor Mendel
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He studied pea plants because they had
short generation times, they produced large
numbers of offspring, and they came in many
varieties.
Character: flower color, height, seed shape,
pod color, etc.
Traits (each variant for a character)
purple/white flower, tall/short height,
round/wrinkled seed, green/yellow pod color.
Characters
Traits
Mendel chose to
study 7 characters,
each of which
occurred in two
distinct forms.
History of Genetics – Gregor Mendel
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He could strictly control mating of pea plants.
Pea plants usually self-fertilize –pollen
grains (carrying sperm) released from the
stamens land on the egg containing carpel of
the same flower.
He could also cross-fertilize
– fertilization of one plant by
pollen from a different plant.
Cross fertilization
History of Genetics – Gregor Mendel
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He worked with plants until he was sure they
were a true-breed (one that produced
offspring all identical to the parent if selffertilized).
Example – parent plant had purple flowers,
and if self fertilized it would only produce
purple flowered plants generation after
generation.
History of Genetics – Gregor Mendel
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Once he had a true-breed, he then
investigated what would happen if he crossed
true-breeding varieties with each other.
Example – what offspring would result from
cross-fertilization of true-breeds?
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purple flowers x white flowers
This offspring of two different true-breeds is
called a hybrid.
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(the fertilization is called hybridization, or cross.)
History of Genetics – Gregor Mendel
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P generation – the true-breeding parental
plants.
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F1 generation – the hybrid offspring.
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(F stands for filial, Latin for “son”)
F2 generation – result self-fertilization of the
F1 plants.
History of Genetics – Gregor Mendel
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Mendel tracked and recorded the inheritance
of characters, & the results lead him to
formulate several ideas about inheritance.
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Let’s look at his monohybrid-cross (parent
plants differ in only 1 character).
History of Genetics – Gregor Mendel
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He crossed a true-breed purple flower with a truebreed white flower.
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He observed that the F1 generation were all purple flowers.
Self-fertilizing the F1 generation he found that the F2
generation had a ratio of 1:3. One white for every three purple.
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He concluded that the white trait did not disappear, and
that they MUST carry two factors for the flower color
character.
He called these alleles – alternative versions of a gene.
Homologous chromosomes
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Alleles reside at the same locus on
homologous chromosomes.
History of Genetics – Gregor Mendel
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Dominant allele – is always expressed if
present. (like a trump card)
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PP or Pp
Recessive allele – is only expressed if the
dominant allele is NOT present.
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pp
History of Genetics – Gregor Mendel
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A homozygous genotype has identical
alleles.
PP or pp
A heterozygous genotype has two different
alleles
Pp
Law of Segregation:
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A sperm or egg carries only
one allele for each inherited
character.
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because allele pairs separate
from each other during the
production of gametes.
Genetic composition & appearance
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Genotype – organism’s genetic makeup.
(Represented by letters)
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Phenotype – organism’s expressed or
physical traits.
Genetic composition & appearance
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Carriers – organisms that
are heterozygous, they carry
the recessive allele for a trait
but phenotypically only the
dominant trait is expressed.
Types of hybrid crosses
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Monohybrid cross – the parents differ in only
one character.
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Green or Yellow seeds.
Dihybrid cross – parents differ in two
characters.
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(Round or Wrinkled) and (Green or Yellow) seeds.
Law of Independent Assortment: For
Dihybrid Crosses
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Each pair of alleles segregates independently of
other pairs of alleles during gamete formation.
(Each trait is inherited independently of one
another.)
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Which means you could get your mom’s hair color and
not get her eye color (or vice versa). The two traits are
inherited INDEPENDENTLY of one another.
Law of Independent Assortment
Punnett Square
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Punnett squares are used to show the
probability of what genotypes the offspring
could have.
Test Cross
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Used to determine the genotype of a
unknown character.
Used to verify if organism is in fact a truebreed.
B = black. The dominant allele.
But, is it BB or Bb? This is unknown.
Test Cross
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Mate organism with unknown genotype, with
an organism that has a homozygous recessive
genotype.
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The appearance of the offspring reveals the
unknown genotype.