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Chromosomes replicate in interphases before meiosis
Meiosis follows period of interphase with cell cycle phases of G1, S, and S2; in the S phase, DNA is
replicated so each chromosome has two chromatids

Initially, chromosomes condense and are visible as two chromatids called sister chromatids

Pairing of the sister chromatids, also called synapsis, occurs where they align beside each other; this
combination is called a tetrad (because of four chromatids) or a bivalent (because of two pairs)

In many eukaryotic cells, a protein based structure forms in between the bivalent called synaptonemal
complex

Crossing over is the exchange of DNA material between non-sister homologous chromatids
During prophase I, breaks within the DNA occur; then, a set of chromatids “invade” a homologous
sequence on a different set of chromatids and bind in the region of the breaks
Once the crossing over occurs, the first set of chromatids continue to adhere even after crossing over;
this connection point is called chiasmata or a chiasma

Chiasmata formation between non-sister chromatids in a bivalent can result in an exchange of alleles
2 consequences of chiasmata formation:
Increased stability of bivalents at the chiasmata
Increased genetic variability if crossing over occurs
Results in an exchange of DNA between maternal and paternal chromosomes; can decouple linked
combinations of alleles and lead to independent assortment
Can occur multiple times and between different chromatids within the same homologous pair

Discrepancies in Mendelian rations
Some discrepancies were found between observations and Mendel’s principle of independent
assortment
Bateson and Punnett conducted crosses with sweet peas; the first generation’s results were as the ones
predicted; however, the second generation had a far larger number of parental phenotypes and a much
smaller number of non-parental phenotypes (recombinants)
Bateson and Punnett realized results did not conform to Mendel’s principle but didn’t find explanation;
Morgan observed similar discrepancies in fruit flies
His theory of sex linkage led him to develop a theory of gene linkage that accounted for the higher than
expected number of parental phenotypes and also the notion of crossing over to explain the
recombinants

Crossing over produces new combinations of alleles on the chromosomes

The red and blue diagrams represent
homologous structures; they have the same
length, the same centromere position and the
same gene content but with different alleles
They have to first replicate in prophase, then the
sister chromatids form a homologous structure
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Homologous chromosomes separate in meiosis
Differences between meiosis I and meiosis II
Sister chromatids remain associated with each other in meiosis I
Homologous chromosomes behave in a coordinated fashion in prophase during meiosis I
Homologous chromosomes exchange DNA leading to genetic recombination during meiosis I
Meiosis I is a reduction division in that it reduces the chromosome number by half
The processes that result in the creation of genetic variety of gametes occur in meiosis I
In anaphase I, the bivalent/tetrad is separated and each set of chromatids is sent to one of the two cells
Independent assortment of genes is due to the random orientation of pairs of homologous
chromosomes in meiosis
When Mendel’s work was rediscovered, the mechanism for independent assortment of unlinked genes
was identified
In the grasshopper, Brachystola magna, it was observed that homologous chromosomes pair up and
then separate but the poles to which they move depends on which way the pair is facing, which is
random
One pair’s orientation does not affect the orientation of another pair
If an organism is heterozygous for a gene, then in the cells one chromosome will have one allele and the
other chromosome will hold another allele for the gene
During meiosis the orientation of the pair of chromosomes will determine which allele will go to which
pole; there is a fifty percent chance that the other allele for this gene will go to the other pole; the
chance of both alleles coming to the same pole is 25%
Sister chromatids separate in meiosis II
After meiosis I, daughter cells enter meiosis II without passing through interphase

Meiosis II is similar to mitosis in that the replicated chromosomes is separated into chromatids; sister
chromatids are separated but they are likely to be non-identical chromatids due to crossing over

Unlinked genes segregate independently as a result of meiosis
Segregation- separation of two alleles of every gene that occurs during meiosis
Independent assortment- observation that the alleles of one gene segregate independently of the alleles
of other genes; the theory that the alleles of two genes pass into gametes without influencing each
other
Genes on different chromosomes are un-liked and segregate independently due to meiosis; genes on
the same chromosome are linked and don’t segregate independently
Morgan’s discovery of non-Mendelian ratios in Drosophila
Morgan bred thousands of flies and found one fly that had white eyes; then mated that with a red eyed
fly and received a bunch of flies with red eyes but only three with white
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Chi-square tests are used to determine whether the different between a observed and expected
frequency distribution is statistically significant
In a chi-squared test, you have two hypotheses; one rejects the deviation from expected figures as
sampling error, and the other one suggests a reason for the deviation
Steps:
1
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Calculated the expected frequencies using the expected ratio and total number of individuals
3
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Find the critical region using a table and your degrees of freedom while looking at the
significance level of 0
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∑

(𝑜𝑏𝑠−𝑒𝑥𝑝)2
𝑒𝑥𝑝

is the formula used for the chi squared tes
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Compare the calculated value with the critical value; if the value is in the critical region or larger
than the critical value, then you can reject your hypothesis which assumes that it’s a sampling

error; if there value is under the critical region, there is evidence that you can assume it to be a
sampling error
The null hypothesis is the one that assumes there is a sampling error; the alternate hypothesis is the one
that gives an alternate explanation for what’s going on
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If
speciation occurs due to geographical isolation then the speciation is called allopatric speciation
Cichlids (fishes) are separated due to annual fluctuations in water levels leading to isolation of
populations; then they are subject to different selective pressures; during rain season, when
recombined they may not be able to reproduce due to isolation, thus new species is formed

Sometimes isolation of gene pools occurs within the same geographic area, this is termed sympatric
speciation
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When one species may flower, the others may be withered or not yet mature
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The absence of the
intermediate forms was described as
imperfections in the fossil record

Speciation can occur abruptly
Punctuated equilibriums- long periods of relative stability in a species are punctuated by periods of rapid
evolution; gaps in the fossil records may not be gaps, but show that there was no long sequence of
intermediate forms
Allotropic speciation and the opening of new niches can lead to rapid speciation; it is much more
common in organisms with short generation times like prokaryotes and insects

Polyploidy can lead to speciation
A polyploid organism is one that has more than two sets of homologous chromosomes; can result from
hybridization events between different species
There are also polyploids whose chromosomes originate from the same ancestral species; can occur if
chromosomes duplicate in prep for meiosis, but it doesn’t occur
The result is a diploid gamete that when fused with a haploid gamete produces a fertile offspring; the
polyploid has become reproductively isolated from the original population; it can self-pollinate or mate
with other polyploids; sympatric speciation has occurred

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