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Topic 10: Genetics HL
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Saved by Darrell Sharp
on March 4, 2011 at 3:13:47 pm
Human Sex Chromosomes, Biology, McDougal Littell, 2008.
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Dihybrid Punnett Square, http://cccmkc.edu.hk/index.en.php
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10.1 Meiosis
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Modeling Meiosis Activity
Cartoon Guide to Genetics 1, 2, 3
Gonick, Larry, and Mark Wheelis. The Cartoon Guide to Genetics. New York, NY: Collins Reference, 2007. Print.
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10.1.1 Describe the behaviour of the chromosomes in the phases of meiosis.
10.1.2 Outline the formation of chiasmata in the process of crossing over.
10.1.3 Explain how meiosis results in an effectively infinite genetic variety in gametes through crossing over in prophase I and random orientation in metaphase I.
Meiosis Simulation
Meiosis II
http://www.biologycorner.com/APbiology/inheritance/10-1_meiosis.html
Chiasmata
Sadava, D. et al. Life: The Science of Biology, 8th ed
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DNA is replicated before meiosis starts
Meiosis I - key points: crossing over and independent assortment
Prophase I
- Chromosomes condense - "supercoiling"
- Homologous chomosomes make pairs
- Chiasmata form - positions where non-sister chromatids cross over (singular: chiasma)
- Nuclear membrane breaks down
- Spindle fibers attach to centromeres
Metaphase I
- Homologous pairs move to the middle of the cell
- Each pair lines up randomly - Mendel called this "independent assortment"
- Every time meiosis happens the pairs line up differently
Anaphase I
- Spindle fibers separate homologous pairs
Telophase I and Cytokinesis
- Spindle fibers break down
- Cytoplasm and plasma membrane are divided into two cells
- In some species, new nuclear membranes form
Result: two haploid cells
Meiosis II - key point: sister chomatids are separated
Prohpase II
- Spindle fibers attach to centromeres
Metaphase II
- Chromosomes line up individually in the middle of the cell
Anaphase II
- Spindle fibers separate suster chromatids
Telophase II and Cytokinesis
- Spindle fibers break down
- New nuclear membranes form
- Cytoplasm and plasma membrane are divided into two cells
Result: four unique haploid cells
Important Ideas
Crossing over and independent assortment create unique gametes.
- Because of crossing over, the four chromatids of homologous chomosome pairs are different
- Because of independent assortment, the result of meiosis is different every time it happens
- = genetic variation in gametes
Random fertilization happens during sexual reproduction - one random sperm and one random egg meet and create one unique offspring.
Sexual reproduction's most important characteristic is its ability to create genetic variation in offspring and increase the evolutionary success of the species.
Thanks meiosis and random fertilization!
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10.1.4 State Mendel’s law of independent assortment.
10.1.5 Explain the relationship between Mendel’s law of independent assortment and meiosis.
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Mendel observed the inheritance of many traits in his pea plants.
The inheritance of one trait was not connected to the inheritance of another trait.
For example, seed color could be yellow or green, and seed shape could be round or wrinkled.
The inheritance of seed color did not affect the inheritance of seed shape.
The offspring could have yellow and round seeds, yellow and wrinkled seeds, green and round seeds, or green and wrinkled seeds.
From these observations, Mendel wrote his Law of Independent Assortment which predicted the random assortment of homologous chromosomes during metaphase I of meiosis !!!!
Why can't I be more like Mendel?
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10.2 Dihybrid crosses and gene linkage
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Dihybrid crosses follow the inheritance pattern of two traits.
(Di- = two)
In the following cross, two traits of each parent are observed:
1) seed color: Y=yellow / y=green
2) seed shape: R=round / r=wrinkled
P1: Phenotypes = Yellow/Round X Green/Wrinkled
Genotypes = YYRR X yyrr
F1: Phenotypes = 100% Yellow/Round
Genotypes = 100% YyRr
Yellow/Round X Yellow/Round
YyRr X YyRr
F2: Phenotypes = 9 Yellow/Round
3 Yellow/Wrinkled
3 Green/Round
1 Green/Wrinkled
Genotypes =
9:3:3:1 ratio
http://cccmkc.edu.hk/index.en.php
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10.2.1 Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
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Practice Problems
Write the genotypic and phenotypic ratios for the following crosses:
Alleles
A = long wings
a = short wings
B = wide beak
b = thin beak
Crosses
AABB X aabb
aaBB X AAbb
AaBb X AaBb
AaBB X Aabb
Determine the genotype of a long winged, wide-beaked bird.
It could be AABB or AaBb. Use a testcross.
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10.2.2 Distinguish between autosomes and sex chromosomes.
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10.2.3 Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
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10.2.4 Define linkage group.
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10.2.5 Explain an example of a cross between two linked genes.
Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example
Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
Alleles are usually shown side by side in dihybrid crosses, for example, TtBb. In representing crosses involving linkage, it is more common to show them as vertical pairs, for example
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10.2.6 Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.
In a test cross of
the recombinants will be
and
In a test cross of
the recombinants will be
and
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10.3 Polygenic inheritance
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10.3.1 Define polygenic inheritance.
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10.3.2 Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.
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Topic 10: Genetics HL
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