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6.1 Two groups of Cells
1) Somatic Cells – body
cells
Can NOT pass DNA on to
offspring
2) Gametes – sex cells
Develop from germ cells in
reproductive organs
DNA IS passed on to offspring
Somatic Cells
Are diploid
Full set of
chromosomes
Diploid number
=2n
Ex: 2n = 46
(humans)
= 23 pairs
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Sex Cells (Gametes)
Egg (female)
Sperm (male)
Haploid
Contain half the number of chromosomes
NO pairs!
Haploid number = n
Ex: n = 23 in humans
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Fertilization
Fusion of egg and
sperm forms a zygote
Nuclei fuse and form
one
= diploid cell
1/2 DNA from each
parent
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Chromosome Numbers
Each organism has a characteristic #
Not related to complexity
Consistent within a species
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Homologous Chromosomes
= a pair of
chromosomes
One from each
parent
Similar in:
size
shape
genetic information
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Types of Chromosomes
Autosomes
Chromosomes that do
not determine gender
Sex chromosomes
Determine the sex of
individual
Only two sex
chromosomes
X and Y
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Risk of Down Syndrome
Increases with the
mother’s age
All eggs present in
ovaries at birth
Can accumulate
damage
Women over 35
Advised to get prenatal
testing
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Nondisjunction
Gametes form but
chromosomes fail to
separate properly
One gamete gets
both chromosomes
Now 2 in one gamete
and 0 in other
Mitosis (Review)
Type of cell division
Divides the nuclear
material
Maintains
chromosome #
Forms diploid cells
Forms somatic cells
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Meiosis
From Greek meioun = to
make smaller
Type of cell division
Divides nuclear material
Cuts chromosome # in
half
Forms haploid
reproductive cells
Gametes
Spores
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What’s the difference????
Homologous
chromosomes vs. sister
chromatids?
Chromosome = name
given during anaphase of
Mitosis or anaphase of
Meiosis II
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Homologous Chromosomes
Two separate chromosomes
One from mom, one from dad
Very similar to each other
Same features and functions
Some instructions may be slightly
different
Same genes
NOT exact copies!
Divided in Meiosis I
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Sister Chromatids
Duplicated chromosomes that
are attached by a centromere
Exact copies of each other
Separated in anaphase of
Meiosis II
OR
Separated in anaphase of
Mitosis
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Before Meiosis . . .
Remember – DNA has already been
copied!!!
Just like what happens before Mitosis!
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Meiosis has 2 Stages:
Meiosis 1
Prophase 1
Metaphase 1
Anaphase 1
Telophase1 and cytokinesis
Meiosis 2
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2 and cytokinesis
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Meiosis 1
Prophase 1
Chromosome become visible
Nuclear envelope breaks down
Centrioles move
Spindle fibers appear
Homologous chromosomes pair
Crossing over occurs
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Prophase 1
UNLIKE mitosis, homologous
chromosomes line up next to each other
Called tetrads
Process = synapsing
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Crossing Over
Increases genetic variation (diversity) . . .
Why is this important in the long haul?
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Anaphase 1
Homologous chromosomes
separate
Move to opposite poles by
spindle fibers
Each chromosome still
composed of 2 chromatids
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Telophase 1 and Cytokinesis
Chromosomes gather at poles
Cytoplasm divides
2 new cells formed
One chromosome from each pair in each
*Chromosomes do NOT replicate bw
meiosis 1 and meiosis 2
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Result of Meiosis 1
Homologous
chromosomes
separated
Two NON-identical
daughter cells
formed
Chromosome # cut
in half
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Ponder . . .
Why are chromosomes NOT replicated between
meiosis I and meiosis 2 ?
Think about it . . .
Keep thinking . . .
Got it!
Makes sense, doesn’t it?!
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Telophase 2 and cytokinesis
Nuclear envelope reforms
Spindle breaks down
Cytokinesis occurs
Two daughter cells formed for each cell
Total: 4 haploid cells
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Thinker. . .
What is the major difference between
metaphase I and metaphase II?
Metaphase I = pairs of chromosomes
line up. Metaphase II = chromosomes
are NOT paired
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Spermatogenesis
Forms sperm
Cells move quickly
Small
Compact
Flagellum
Main contribution = DNA
Yields 4 haploid sperm
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Oogenesis
Forms Ova (eggs)
Begins before birth
DNA, cytoplasm,
organelles, etc
Yields 4 cells
One egg
3 polar bodies
smaller, break down, die
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Meiosis: The Continuation of Life
Meiosis - The Continuation of Life - 11:48
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Basic Vocabulary
Trait
Inherited characteristics
Heredity
The passing on of traits from parents to
offspring
Genetics
The study of heredity
Cross
Mating of two organisms
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6.3 How it all Began. . .
Gregor Mendel – 1860s
Austrian monk
“Father of Genetics”
Studied pea plants
Pisum sativum
Developed rules to predict
patterns of heredity
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Mendel’s Background
Mathematician
Child of peasants
Knew a lot about agriculture
Studied theology
Became a priest
Went to U of Vienna
Studied science and math
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Mendel’s Background
Mendel repeated
experiments of T. A. Knight
British farmer
Crossed pea plants
Mendel counted offspring
Analyzed data
Mendel’s work rediscovered
in 1900
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Useful Features in Pea Plants
Small
Grows easily
Mature quickly
Lots of offspring
Traits occur in two forms
Enclosed flowers
Tend to self-pollinate
Can control crosses
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Mendel’s Experiment
Monohybrid cross
Cross one pair of contrasting traits
Ex:
Tall x Short
Purple x White
Round x Wrinkled
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Mendel’s Experiment
Used Purebreds = True
breeding
Homozygous
Always produce offspring
with same characteristics
P (parental) generation
First 2 individuals in a cross
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Mendel’s Experiment
F1 generation
First filial
Offspring of P
generation
Showed just one
form of the trait
Filial = “Son”
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Mendel’s Experiment
F2 generation
Second filial
generation
Offspring of the
F1 generation
Showed both forms
of the trait again
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Mendel’s Law of Segregation
Individuals get two copies of “heritable factors” (genes)
Alternative versions of genes (alleles)
One from each parent
Dominant or recessive
Gametes carry only one allele for each gene
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Gregorian Chant . . .
Mendel song 3:30
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6.4 Alleles
Different versions of gene
Everyone has 2 for each trait
One from each parent
Together they code for expression of gene (trait)
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Recessive Alleles
Not expressed if dominant allele is
present
Shows up only if both alleles are
recessive
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Genotype
Genetic makeup of organism
Actual alleles inherited
Letters represent alleles
Capital letters = dominant
Lower case = recessive
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Homozygous Genotype
Two similar alleles
Two dominant = two capital letters
Two recessive = two lower case letters
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Heterozygous Genotype
Two different alleles
One dominant
One recessive
One capital letter &
one lower case letter
Dominant always
written 1st
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6.5 Punnett Squares
Grid system for
predicting outcome
of a cross
Considers all
possible gamete
combinations
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Punnett Squares
Combine alleles
“Multiply” and
fill boxes in
Shows all
Possible
genotypes of
offspring
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Practice Punnett Squares
Determine genotypes
and phenotypes
Monohybrid cross
One trait crossed
Ex:
Pure short pea plant x
hybrid pea plant
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Practice
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Cross:
homozygyous
recessive individual
with blue eyes with
heterozygyous
individual with brown
eyes
Probability
Likelihood that an event
will happen
Predicts average
number of occurrences
Practice:
Determine the ratios
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Testcross
• Used to figure out
genotype of an
organism
• Must cross
unknown organism
with homozygous
recessive
individual
• WHY?!!!
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Law of Independent Assortment
Mendel’s 2nd law
Inheritance of one trait
does not influence the
inheritance of any other
Alleles of different
genes separate
independently during
meiosis
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6.6 Genetic Variation
Rapid because of
meiosis
Key contributors:
1) independent
assortment
2) random
fertilization
3) crossing-over
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1) Independent assortment
Random
distribution of
homologous
chromosomes
during meiosis
Determined by
chance
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1) Independent Assortment
# of different gametes possible from one original human cell:
◦ 223 (~8 million) !WOWZERS! That is a lot of
possibilities!
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2) Random Fertilization
Zygote formed bw random
gametes
Fertilization by random
sperm
Now outcome: 223 x 223
> 70 trillion
Human couple can produce a
child w 1 of ~70 trillion
possible combos
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3)Crossing Over
Results in Recombination
Mixing of parental alleles
DNA exchanged during prophase 1
# of possibilities is nearly unlimited
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Genetic Linkage
Genes close together tend to be
inherited together
Genes far apart sort independently
Allows scientist to calculate distance
bw genes
Create genetic map
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