Transcript NB_ Meiosis & Genetics

Chapter 11- Genetics
Meiosis
Principles of genetics require:
-each organism inherits a single copy of
every gene from each parent
-during gamete formation, two sets of
genes must separate, so each gamete
contains just one set of genes
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Chromosome Number
Fruit fly example:
8 chromosomes total
4 came from male parent
4 came from female parent
Two sets are homologous
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Homologous chromosomes- paired
chromosomes having genes for the same
trait located at the same place on the
chromosome
Diploid- having two of each kind of
chromosome (2n); normal body cells
Diploid cells have two complete sets of
chromosomes/two complete set of genes
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Gametes of sexually reproducing organisms
have only a single set of chromosomes
Haploid- having one of each kind of
chromosome (n)
Fruit fly gametes would be n = 4
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Phases of Meiosis
How haploid cells (gametes) produced from
diploid cells (somatic cells)
Meiosis- cell division that results in a
gamete containing half the number (n) of
chromosomes of its parent (2n)
Involves two rounds of division:
-Meiosis I
-Meiosis II
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Phases of Meiosis
Meiosis I
Similar to mitosis
Four phases the same: prophase I,
metaphase I, anaphase I, and telophase I
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DifferencesProphase I:
Pairing of homologous chromosomes to
form tetrad (4 chromatids)
Crossing over- exchange of genetic
material between non-sister chromatids of
homologous chromosomes
Shuffles genes like a deck of cards
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Meiosis II
Second round of division (reduction)
NO chromosome replication
Phases just like meiosis I:
Prophase II, metaphase II, anaphase II,
telophase II
At end of division cycle, each cell contains
haploid # of chromosomes (n)
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Gamete Formation
In male animals, gametes produced through
meiosis called sperm
In some plants, pollen contains sperm cells
In female animals, only one cell produced is
made into a gamete
Cytokinesis is uneven: produces one egg
and 3 polar bodies
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Comparing Mitosis & Meiosis
Mitosis
Meiosis
2 identical diploid cells 4 genetically different
haploid cells
Multicellular
organisms: growth &
repair
Asexual reproduction
Formation of gametes
Sexual reproduction
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The Work of Gregor Mendel
Genetics- scientific study of heredity
Gregor Mendel- Austrian monk who
experimented using pea plants
Determined rules of inheritance
Fertilization- joining of male & female
gametes to form single cell
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Pea plants normally self-pollinating
Mendel used stock of true-breeding plants
(produced offspring identical to themselves)
Cross-pollinated pea plants with different
traits to study resulting offspring
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Genes & Dominance
Studied 7 different pea plant traits
Trait- a characteristic that can be passed on
to offspring
For each trait, studied a contrasting
character:
Ex. Seed color- green or yellow
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Original pair of plants = P generation
Offspring = F1 generation
Hybrid- offspring of parents that have
different forms of a trait
Results?
Every offspring had a character of only one
of the parents
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Two conclusions:
• Inheritance determined by factors passed
from one generation to the next
Gene- the functional units of inheritance
Each trait controlled by one gene with two
contrasting forms
Alleles- alternate forms of a gene
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Second conclusion called the principle of
dominance:
• Some alleles are dominant and others are
recessive
Dominant- an allele of a gene that is always
expressed (T)
Recessive- an allele of a gene that is
expressed only when the dominant allele
is not present (t)
Chapter 11- Genetics
Segregation
What had happened to recessive alleles
(traits) in plants?
Mendel allowed F1 plants to self-pollinate
Produced F2 generation which showed
recessive trait in 1 out of 4 plants (3:1 ratio)
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Dominant allele masked recessive trait in F1
generation
Reappearance in F2 generation showed that
at some point alleles became separated
Mendel suggested that they segregated
from one another during gamete formation
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Law of Segregation- states that every
individual has two alleles of every gene;
when gametes are produced, each gamete
receives one of these alleles
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Probability & Punnett Squares
Every time Mendel repeated a particular
cross, he obtained similar results
¾ plants showed dominant trait; ¼ showed
recessive trait
Mendel realized that probability could be
used to explain results
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Chapter 11- Genetics
Chapter 11- Genetics
Genetics & Probability
Probability- the likelihood that a particular
event will occur
Two possible outcomes for a coin flip:
heads or tails
Probability of each occurring is equal
Chance of heads is 1 in 2, or 50%
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Chance of heads coming up three in a row?
Each flip is an independent event:
Probability of three heads in a row½ x ½ x ½ = 1/8
Way alleles segregate is random, like a coin
flip
• Principles of probability can be used to
predict outcomes of genetic crosses
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Punnett Squares
Gene combinations from a given cross can
be determined using a Punnett square
Letters in Punnett square represent alleles
Gametes from each parent are shown along
one side and the top of the square
Used to predict and compare genetic
variations resulting from a cross
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Homozygous- if an organisms two alleles
for a trait are the same
Can be homozygous dominant (TT) or
homozygous recessive (tt)
Heterozygous- when the two alleles for the
same trait are different (Tt)
Homozygous = true-breeding for trait
Heterozygous = hybrid for trait
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Phenotype- the physical expression of the
genes; how the trait looks
Ex.Tall; short; yellow seeds; green seeds
Genotype- allele combination that an
organism contains
Ex.TT; Tt; tt
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Probability & Segregation
Probability predicts that if the alleles
segregate, then a 3:1 phenotypic ratio
should be seen
Each cross showed a 3:1 ratio
Mendel’s law of segregation proven correct
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Since probabilities predict averages:
the larger the number of individuals, the
closer the data comes to the probability
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Exploring Mendelian Genetics
Mendel further investigated:
Alleles segregatebut do alleles package together?
or are they independent of one another?
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Independent Assortment
Mendel followed two different genes through
two generations
Crossed purebred plants for seed color and
seed shape
Round, yellow seeds (RRYY)
with
Wrinkled, green seeds (rryy)
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Only provided hybrid plants needed for next
cross
All F1 plants had genotype RrYy
F2 generation showed 209 plants that had
phenotypes not found in the parents
So…
Alleles for different traits segregate
independently of one another
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Summary of Mendel’s Principles
• Inheritance is determined by individual
units called genes.
Genes passed from parents to offspring.
• When two or more forms (alleles) of a
gene exist, some forms may be dominant,
others recessive.
Chapter 11- Genetics
• In most sexually reproducing organisms,
each adult has two copies of each gene –
one from each parent.
These copies are segregated from one
another during meiosis (gamete formation)
• Alleles for different genes usually
segregate independently of one another
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Complex patterns of Inheritance
Important exceptions to Mendel’s principles
Some alleles are neither dominant nor
recessive
Many traits controlled by multiple alleles, or
multiple genes
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Incomplete dominance
Appearance of a third phenotype
Due to one allele being not completely
dominant over another
3rd phenotype somewhere in between the
two homozygous phenotypes
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Codominance
Expression of both
alleles
(ex. White bull
crossed with a red
cow yields a roan
calf)
Both alleles contribute
to the phenotype
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Multiple phenotypes from multiple alleles
• More common for multiple alleles to
control a trait in a population
• Only two alleles of a gene can exist within
the cell
• Multiple alleles for a single gene can be
found within a population
Examples:
Blood type- A, B, o
Rabbit coat color- C, cch, ch, c
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Polygenic Inheritance
• Traits that are determined through
expression of two or more genes
• Polygenic traits show wide range of
phenotypes
• Ex- eye color, skin color, height
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Genetics & the Environment
Characteristics of any organism not solely
determined by genes
Determined by interaction between genes
and environment
Genes provide plan, how plan goes also
depends on environment