Gregor Mendel

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Gregor Mendel
Monk at St Thomas Abbey in Brno of the Czech
Republic studied garden peas from
1856-63 published his 'paper' in 1866 pretty
much ignored until 1900 that year 3 scientists
independently came to many of the same
ideas his experiments were special because 1)
Quantitative 2) carefully documented 3)
elegantly designed
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Mendels Peas
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Mendels Peas
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Mendelian Genetics
phenotype observable characteristic ie.
something that can be seen
x
green yellow ratio
F1 generation 227 0 F2 generation 593 193
3.07 dominant phenotype seen in the first
(F1) generation recessive phenotype not seen in
the first generation, but re-appears later in
this case, green is dominant, yellow is recessive
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Mendelian Genetics
Importantly, Mendel went another step further
looking at F3 plants self-cross using the
pollen from one plant to fertilize a flower on
the same plant
193 crosses 193 yellow offspring only 0
green pea pods
593 crosses 201 green offspring only
392 green and yellow F3 31 ratio green yellow
true breeding able to pass on a given phenotype
to all of its offspring
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Mendelian Genetics
Mendel used his peas and the number of each type
produced from each mating to come up with his
rules for inheritance 1) true breeding plants
must have only 1 'determinant' or allele 2)
plants that give rise to offspring with 2 traits
must have at least 2 different alleles 3)
each plant (mother and father) gives the
offspring one of their alleles 4) each trait, or
gene, controls one characteristic gene a piece
of DNA that codes for a protein which determines
some particular trait of the plant (ie. gene
for round or smooth peas) allele a particular
FORM of a gene that gives a phenotype for some
trait (ie. purple vs. white flowers) one gene
can have more than 2 alleles (sometimes MANY more)
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Mendelian Genetics
gamete reproductive cell (sperm, egg, pollen,
etc) zygote fertilized egg generated when 2
gametes come together Punnett Square way of
figuring out all of the possible phenotypes
from any 2 parents
gametes from one parent
gametes from the other
all possible offspring
homozygous when an individual has only 1 allele
and can form only one possible type of
gamete-- this individual is true breeding
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Mendelian Genetics
heterozygote when an individual contains 2
different alleles and can therefore make 2
different types of gametes in this example,
assume that A represents the allele for a green
pod
a represents the allele for a yellow pod
capital letters represent dominant traits,
lower case recessive heterozygotes show the
dominant trait (ie. green) because they have
one of these alleles 3 of the 4 possible
offspring have an A, and are therefore green 1 of
the 4 possible offspring have 2 a's, and is
therefore yellow 31 ratio of dominant to
recessive phenotypes is what Mendel found
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Mendelian Genetics
Smooth peas (S) is dominant over wrinkled peas
(s). If a heterozygote pea plant is mated to a
homozygous recessive pea plant A) what is the
phenotype and the genotype of a heterozygote pea
plant? B) what is the phenotype and genotype of a
homozygous recessive plant? C) Draw the Punnett
Square for this mating D) what percentage of the
offspring should be wrinkled.
A. heterozygotes have one of each allele (Ss),
and are smooth because smooth is
dominant. B. homozygous recessive have 2
recessive alleles, so (ss) C. S s D. to be
wrinkled, the plants must s Ss ss be ss. From
the Punnett Square, 2 of s Ss ss 4 possibilites
give homozygous recessives, so 50 are
wrinkled.
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Mendelian Genetics
Mendel's Principle of Segregation each parent
has 2 alleles of a given gene, and will
randomly pass on 1 of those alleles to each
offspring
Principle of Independent Assortment segregation
of any pair of alleles is independent of other
pairs in the formation of gametes consider a pea
plant that is heterozygous for green pods and
also is heterozygous for purple flowers
(purple is dominant over white) A dominant
allele for green pea pods note that
flower color is a a recessive allele for
yellow pea pods different gene than
pod V dominant allele for purple flowers
color, and therefore gets v recessive
allele for white flowers a different
letter!
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Mendelian Genetics
each gamete has to have 1 letter A and one V, but
can be either allele each allele goes randomly
to each gamete in reality, each gamete has
thousands of different alleles, one for each
gene in the organism ie. AaBbCcDdEeFfGgHhIiJj
is heterozygous for 10 genes!! each parent then
contributes 1allele of each gene to their
offspring to give each zygote 2 alleles of
each gene
VV AA
VV Aa
Vv AA
Vv Aa
VV Aa
VV aa
Vv Aa
Vv aa
Vv AA
Vv Aa
vv AA
vv Aa
Vv Aa
Vv aa
vv Aa
vv aa
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Genes and Chromosomes
DNA is the substance that makes up genes and are
found as chromosomes each parent passes one
chromosome of each type onto their
offspring each chromosome contains hundreds or
thousands of genes
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Genes and Chromosomes
because chromosomes are what get physically
separated when forming gametes, genes on the
same chromosomes usually get segregated
together this would mean that each person could
only get 1 complete set of genes on each
chromosome from the grandmother or
grandfather recombination crossing over between
2 copies of the same chromosome happens when
gametes form allows traits from both the
grandmother AND the grandfather to go into the
child
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Genes and Chromosomes
grandparents
grandparents gametes
parents
parents gametes after recombination
child is a mixture of both parents and
grandparents recombination is random-- other
children will have different parts of each
chromosome than siblings
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we can follow traits as they get inherited from
grandparents to parents to children pedigree
diagram showing how a trait is inherited over
generations
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Genes and Chromosomes
mutation inheritable change in the DNA molecule
can be generated by many things sunlight,
radiation, random error mutations get passed
along from parents to offspring if a mutation
is recessive THERE IS NO PHENOTYPE recessive
mutations show nothing wrong and people appear
normal if 2 recessive individuals have children,
then there is a possibility of having a child
who is homozygous recessive and has the
phenotype genetic screening looking for known
human mutations that can cause a disease
can be important because it lets parents
determine if a child between them may have a
nasty disease
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Genes and Chromosomes
sickle cell anemia recessive allele that
generates deformed red blood cells that die
faster and can't carry as much O2
common for those with african ancestors
heterozygotes (those with only 1 allele) are
resistant to malaria sometimes one copy of an
allele is good, but 2 copies of the same
allele are bad
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Genes and Chromosomes
sex chromosome common way of determining sex for
organisms in humans, X and Y
chromosomes women have 2 X chromosomes men have
1 X and 1 Y chromosome -- exception to the 2
allele rule
men only have 1 X can't be heterozygotes one
bad copy disease women have 2 Xs- can be
heterozygotes rarely show disease baldness,
hemophilia, color blindness are all
X-linked genes (ie. on X)
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Genes and Chromosomes
most colorblind people can't tell the difference
between red and green can you read the number
hidden in the image below colorblindness is a
recessive allele that usually show up in men
women are carriers of colorblindness carrier
heterozygote for a particular recessive trait--
they are completely normal, but their offspring
may show a recessive trait