Download Mendelian 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.
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The DNA you get from your parents determine
your physical characteristics.
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.
Mendel’s Experiment
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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
Mendel’s Experiment
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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.
Mendel’s Experiment
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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.)
Mendel’s Experiment
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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.
Mendel’s Results
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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).
Mendel’s Results
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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.
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** Very important to know this term
** Important Notes To Remember
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Gene – enough DNA to make 1 protein
(which would in turn control 1 trait)
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You inherit 23 chromosomes from your mom and
23 from your dad. These 23 chromosomes from
each pair up to form homologous chromosomes
(on these chromosomes will be hundreds of
genes).
Allele – the 2 versions of each gene that you
receive (1 from each parent)
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Homologous chromosomes – pairs of
chromosomes that are the same size, shape,
and carry the same genes (get 1 from mom
and 1 from dad)
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Alleles reside at the same locus (location) on
homologous chromosomes.
Terms Mendel use to describe his
findings
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Dominant allele – is always expressed if
present. (like a trump card)
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PP or Pp
**Note: This does not mean that the dominant allele is better or
even more frequent. It simply means that the dominant allele will
mask or cover up the recessive allele.
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Recessive allele – is only expressed if the
dominant allele is NOT present.
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pp
Examples of human traits that are
dominant / recessive
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Tongue rolling
Attached earlobes
Widows peak
Hitchhiker’s thumb
Terms Mendel use to describe his
findings
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A homozygous genotype has identical
alleles.
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PP or pp
A heterozygous genotype has two different
alleles
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Pp
Genetic composition & appearance
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Genotype – organism’s genetic makeup.
(Represented by letters – Each letter represents 1 allele)
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Phenotype – organism’s 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.
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Note that being a carrier and
being heterozygous are the
same thing
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 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.
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Which stage of meiosis does
this occur in?
This means you will have a
50% chance of getting
either of your parents
alleles.
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.
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** Note – In all of our somatic (body) cells, we have 2
alleles for each chromosome. During meiosis we
separate these chromosomes (and thus the alleles)
when making our gametes. (Law of segregation)
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Show this on the Punnett square above
Using Punnett Squares
Alleles
Parents
Offspring
(zygotes)
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Rules:
1.
2.
3.
4.
5.
Separate alleles from each parent (it doesn’t matter if mother or father goes
on top)
Pull the alleles from up top down and the alleles from the side over to show
the possibilities of all of the zygotes that could be created from this cross.
Each box represents a 25% chance that this particular zygote will be formed.
Genotype ratio – Homozygous dominant : Heterozygous : Homozygous
recessive
Phenotype Ratio – Dominant : Recessive
Practice Questions
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Use your whiteboards to answer the following
questions.
1.
2.
3.
4.
5.
6.
What is meant by the P, F1, and F2 generations?
What are genes, alleles, and homologous
chromosomes?
Difference between homozygous and heterozygous?
Difference between genotype and phenotype?
Define: Law of segregation, Law of independent
assortment
Complete the monohybrid cross on the board and
give the genotype / phenotype ratios. (We will work
on dihybrid crosses tomorrow)
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.