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Topic 10 – Genetics and Evolution

Meiosis
Meiosis is the reduction division producing four haploid daughter cells, each genetically different
...
During meiosis I, homologous
chromosomes separate, producing two haploid daughter cells
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Prophase I:
-

Chromosomes begin to condense
Homologous chromosomes pair up (producing a bivalent)
Nuclear envelope disappears
Crossing over may occur

Metaphase I:
-

Chromosomes line up on the equator of the cell
Random orientation occurs
Spindle fibres attach to centromeres of homologous chromosomes

Anaphase I:
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Homologous pairs are separated
One chromosome of each pair is pulled to the opposite ends of the cell as spindle fibres
shorten / contract

Telophase I:
-

Chromosomes reach poles of the cell and the cell starts to divide
Chromosomes uncoil
Nuclear membrane reforms and cytokinesis (cytoplasmic cleavage) occurs

Prophase II:
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Chromosomes consisting of two chromatids condense
Nuclear envelope disappears
Spindle fibres form at right angles to previous one

Metaphase II:
-

Chromosomes line up on equator of the cell
Centromeres attach to spindle fibres

Anaphase II:
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Centromeres separate and chromatids are pulled to opposite poles

Telophase II:
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Chromatids reach opposite poles
Nuclear envelope reforms
Cytokinesis occurs

Crossing over is the swapping of genetic material (alleles) between chromatids of homologous pairs
that produces new combinations of genes in a gamete
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Synapsis occurs when bivalents are formed
after a pair of homologous chromosomes pair up tightly
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Inheritance and Dihybrid Crosses
When gametes are formed, the separation of one pair of alleles between daughter cells is independent
of the separation of another pair of alleles (Mendel’s Law of Independent Assortment)
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When two genes on different chromosomes are investigated together we draw a dihybrid cross
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Gene Linkage
Exceptions to Mendel’s Law are called gene linkage
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e
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A linkage group is any two genes found on the same
chromosomes
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Individuals with new characteristics can be produced, which are called recombinants and are formed
by crossing over
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Topic 10 – Genetics and Evolution

Polygenic Inheritance
Polygenic inheritance produces characteristics that are controlled by many pairs of genes and the
number of possible phenotypes is very large
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Calculating Chi Squared
Expected value = (sum of observed values / sum of the ratio) x ratio
e
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(sum of observed / (9+3+3+1)) x 9, 3, or 1
Observed
Expected
O-E
Degrees of freedom = number of categories – 1

(O-E)2

(O-E)2 / E

Gene Pools and Speciation
A gene pool is all the genes and their different alleles in an interbreeding population
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Evolution always involves a change in allele frequency over time in a population’s gene pool
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Selection pressure is a change in environmental conditions
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The normal
distribution curve will shift because
the individuals with this phenotype
are more likely to survive and pass on
their alleles
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Extreme phenotypes are
selected against so are less likely to pass their alleles
onto the next generations
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Topic 10 – Genetics and Evolution

Disruptive Selection
Extreme types are selected for and intermediates are selected against
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A species is a group of individuals that belong to the same
gene pool and are capable of interbreeding to produce fertile offspring
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g
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Punctuated equilibrium is when speciation occurs quickly and abruptly as a result of
a mutation
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2
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4
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6
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Original single population of a species
The population is split into two due to environmental isolation
Each population shows variation due to mutations
Different environments have different selection pressures
There is a change in the frequency of different alleles in the two separate populations
Eventually the allele differences are so great that the two populations can no longer breed
Two different species develop (reproductive isolation)

Reproductive Isolation:
1
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Some insects, e
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cicada only breed
every thirteen and seventeen years
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2
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Geographical
- Two populations occupy different environments that are separated by some physical barrier,
e
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rivers, roads, or mountains
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This is when chromosome numbers are multiples
of a basic number, e
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allium has 32 chromosomes
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An advantage of polyploidy is that it can produce crops with
a much greater yield

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