Download 1-2-13 Genetics PPT - Madison County Schools

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.
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.

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 – sperm
carrying pollen grains 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 he
has 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

Mendel tracked and recorded the inheritance
of characters, & the results lead him to
formulate several ideas about inheritance.

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 3:1 (3 purple for every 1
white).
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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

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:
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Each pair of alleles segregates independently
of other pairs of alleles during gamete
formation.
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.