Transcript Mendel and Meiosis - Bishop Ireton High School

Mendel and Meiosis
Genetics
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Study of heredity
Gregor Mendel
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Monk
Studied pea plants- easy to grow,
observable traits, easy to manipulate
Plants also reproduce sexually-produce
gametes(sex cells)
Selected to breed tall plants(Parents-P)
with short plants. Made hybrids
Hybrid- offspring of parents with different
traits(F1)
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Next he bred the offspring with
each other to get 2nd
generation(F2)
P-parents
F1-you
F2-your kids
What did Mendel figure out?
Mendel’s Findings
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Each organism has 2 factors that
control each trait. (GENES) 1 from
mom,1 from dad
Genes exist in different forms called
alleles-sort of like choices.
Ex. Eye color- 2 choices or
alleles
BLUE or BROWN
Rule of Dominance
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One allele is dominant over the
recessive allele.
Ex. Plant height- Tall allele(T) is
dominant over short allele(t)
T-Dominant(Upper case)
t-recessive (lower case)
Law of Segregation
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Every individual has 2 alleles for each
trait(1from mom,1 from dad)
When that individual produces
gametes(sex cells) each sex cell will only
have 1 of the alleles for each trait
Ex. Plant could be TT or Tt or tt for its allelic
combination but it would only donate one to a
gamete.
TT- T or T
Tt- T or t
tt- t or t
Law of Independent Assortment
Each allele is inherited
independently of other alleles
 Ex. Pea seed color and shape are
inherited independantly of each
other so you can have:
Smooth green peas , Wrinkled green peas
Smooth yellow peas ,Wrinkled yellow peas
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The more alleles for a trait leads to
more possible combinations.
Genetics vocabulary
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Phenotype- how the trait looks(observe)
Ex- tall, short,blue eyes, brown eyes
Genotype- allelic combination
Ex. TT,Tt,tt BB,Bb,bb
Homozygous- both alleles are the same
Ex. TT,tt BB,bb
Heterozygous- allele combination is
different
Ex. Tt Bb
Punnett Squares
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We can use a punnett square to
predict the results of breeding
between 2 parents. Sometimes our
predicted results do not match the
actual results.
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T
t
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TT
Tt
T
t
Heterozygous
Cross
Phenotype ratio-3:1
Tt
tt
Predicted results:
Genotype
25% TT
50% Tt
25% tt
Genotype ratio –
1:2:1
Phenotype
75% Tall
25% short
More Complex Alleles
 Incomplete
Dominance- neither allele is
dominant.
Red flowers X White flowers =Pink
Flowers
 Co
dominance- both alleles expressed
ex Black chicken X White chicken=
Checkered chicken
Multiple alleles
more than 2 possible alleles but
there can only be 2 in each
individual.
Ex.Pigeon color- grey ,black, white,
brown
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Polygenic inheritance
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many genes determine phenotype.
Ex. Skin color and height
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We know that mitosis produces
identical cells.
In humans each body cell has 46
chromosomes(23 pairs)
If we joined 2 cells each having 46
chromosomes what do we get?
A cell with 92 chromosomes-TOO
MANY!!!
What to Do?
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Somehow we need to have a cell
with ½ the number of
chromosomes(1/2 x 46=23) so that
if 2 cells joined together each
having 23 chromosomes we end up
with the magic number of 46! (2 x
23)
All organisms have a set number
of chromosomes. They get one
of each from their parents.
Humans have 46 or 23 pairs. We
number the chromosomes 1 to
23.
 We have a # 1 from mom, a # 1
from dad, #2 from mom,#2
from dad, etc.The chromosome
of each pair are called
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HOMOLOGOUS CHROMOSOMES
Cells that have the pairs of their
chromosomes like body cells are
called DIPLOID
 Diploid cells are said to be 2N
where N= 23 in humans.
 Cells that have only 1 of each
chromosome like gametes are
called HAPLOID
 Haploid cells are 1N
 So in humans our N =23, peas
(7), Fruit fly(4)
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So how do we get a cell that has
the correct number of chromosomes
to be the gamete or sex cell (
haploid cell)?
MEIOSIS- produces gametes
that are 1N
23 + 23 =
46
sperm + egg = zygote
(offspring)
Meiosis
Process of making gametes,
sex cells 0r egg/sperm
Humans have 46 chromosomes(23
pairs) that we get from our parents.
Each pair is called
a homologous pair.
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Remember
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Diploid cells contain 2 of each chromosome
2N- body cells
Haploid cells contain 1 of each
chromosome
1N- sex cell
N = # of pairs of chromosomes
2 Stages of Meiosis
Meiosis I
Diploid cell
2N
Meiosis II
4 Haploid cells
4 1N
copy
46
split
92
46
46
23
23
23
23
Interphase
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Chromosomes replicate
46
92
copy
Prophase I
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Dna coils
Spindle fibers form
Homologous chromosomes pair up
with their sister chromatid
All 4 together called TETRAD
TETRAD
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Ends of
chromosomes are
sticky
This is where some
of the genetic
material
exchanges places
on its homologue.
This is how we get
genetic variation.
CROSSING OVER
Metaphase I
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Centromere
attaches to the
spindle fibers
Tetrads line up in
the middle
Anaphase I
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Homologous
pairs split
Telophase I
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Spindle breaks up
Pairs at opposite
ends
Cell splits
End up with cell
with 46
chromosomes
Not done yet- split
again to get to
magic #23!
Meiosis II-2nd division
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Prophase II- spindle reforms
Metaphse II- line up in middle
Anaphase II- move apart
Telophase II – 2 nuclei at opposite
ends
4 Haploid cells at end of Meiosis IIeach has 1 copy of each
chromosome.
How do we all look so different?
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Crossing over
2 23= 70 trillion different combinations
Nondisjunction- homologous pairs fail to
separate during prophase I. One cell will
have an extra chromosome. Ex. Down’s
Syndrome has an extra #21 chromosome.
Gamete can have 1 less- Ex. Turner’s
Syndrome has only 1 X chromosome
instead of 2